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ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA USING DRAINAGE-BASIN AND CHANNEL-GEOMETRY CHARACTERISTICS

By David A. Eash_______________________________

U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 93-4062

Prepared in cooperation with theIOWA HIGHWAY RESEARCH BOARDand the HIGHWAY DIVISION of theIOWA DEPARTMENT OF TRANSPORTATION(Iowa DOT Research Project HR-322)

Iowa City, Iowa 1993

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Robert M. Hirsch, Acting Director

For additional information write to:

District Chief U.S. Geological Survey Rm. 269, Federal Building 400 South Clinton Street Iowa City, Iowa 52244

Copies of this report can be purchased from:

U.S. Geological Survey Books and Open-File Reports Federal Center, Bldg. 810 Box 25425 Denver, Colorado 80225

ii ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA

CONTENTS

Page

Abstract...........................................................................................................^^Introduction....................................................................................................................................!

Purpose and scope.............................................................................................................2Acknowledgments .............................................................................................................3

Flood-frequency analyses of streamflow-gaging stations in Iowa...............................................3Development of multiple-regression equations............................................................................6

Ordinary least-squares regression...................................................................................6Weighted least-squares regression..................................................................................8

Estimating design-flood discharges using drainage-basin characteristics................................^

Geographic-information-system procedure................................................................... 10Verification of drainage-basin characteristics............................................................... 13Drainage-basin characteristic equations....................................................................... 16Example of equation use-example 1 .............................................................................18

Estimating design-flood discharges using channel-geometry characteristics.......................... 19

Channel-geometry data collection.................................................................................. 19Channel-geometry characteristic equations.................................................................. 22

Analysis of channel-geometry data on a statewide basis................................. 22Analysis of channel-geometry data by selected regions................................... 22Comparison of regional and statewide channel-geometry equations.............. 24

Examples of equation use examples 2-4....................................................................... 27

Application and reliability of flood-estimation methods............................................................ 32

'" Limitations and accuracy of equations ..........................................................................32Weighting design-flood discharge estimates .................................................................34

Calculation of estimates ....................................................................................35Example of weighting example 5.....................................................................35

Weighting design-flood discharge estimates for gaged sites ........................................35

Calculation of estimates ....................................................................................36Examples of weighting-examples 6-7............................................................... 36

Estimating design-flood discharges for an ungaged site on a gaged stream...............37

Calculation of estimates ....................................................................................37Example of estimation method-example 8 ......................................................38

Summary and conclusions...........................................................................................................41References......................................................................................................:.......................... ....42

CONTENTS iii

CONTENTS-Continued

Page

Appendix A. Selected drainage-basin characteristics quantified using a geographic-information-system procedure..............................................................................45

Appendix B. Techniques for manual, topographic-map measurements of primarydrainage-basin characteristics used in the regression equations....................... 49

Appendix C. Procedure for conducting channel-geometry measurements ..............................53

ILLUSTRATIONS

Page

Figure 1. Map showing location of streamflow-gaging stations used to collectdrainage-basin data.............................................................................................4

2. Map showing location of streamflow-gaging stations used to collect channel-geometry data and regional transition zone......................................... 5

3. Graph showing example of a flood-frequency curve........................................... 7

4. Map showing four geographic-information-system maps that constitute a digital representation of selected aspects of a drainage basin........................ 11

5. Map showing distribution of 2-year, 24-hour precipitation intensity for Iowa and southern Minnesota........................................................................... 13

6. Block diagram of a typical stream channel ......................................................20

7. Photographs showing active-channel and bankfull reference levels at six streamflow-gaging stations in Iowa.................................................................. 21

8. Graphs showing relation between 2-year recurrence-interval discharge and channel width for bankfull and active-channel width regression equations ............................................................................................................28

9. Graphs showing relation between 100-year recurrence-interval discharge and channel width for bankfull and active-channel width regression equations ............................................................................................................29

10. Graph showing bankfull cross section for Black Hawk Creek at GrundyCenter.................................................................................................................31

iv ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA

TABLES

Page

Table 1. Comparisons of manual measurements and geographic-information- system-procedure measurements of selected drainage-basin characteristics at selected streamflow-gaging stations ................................... 14

2. Statewide drainage-basin characteristic equations for estimating design- flood discharges in Iowa..................................................................................... 17

3. Statewide channel-geometry characteristic equations for estimating design-flood discharges in Iowa.........................................................................23

4. Region I channel-geometry characteristic equations for estimating design- flood discharges in Iowa outside of the Des Moines Lobe landform region..................................................................................................................25

5. Region II channel-geometry characteristic equations for estimating design- flood discharges in Iowa within the Des Moines Lobe landform region .........26

6. Statistical summary for selected statewide drainage-basin and channel- geometry characteristics, and for selected regional channel-geometry characteristics at streamflow-gaging stations in Iowa.................................... 33

7. Comparisons of manual measurements made from different scales of topographic maps of primary drainage-basin characteristics used in the regression equations ..........................................................................................50

8. Flood-frequency data for streamflow-gaging stations in Iowa........................ 57

9. Selected drainage-basin and channel-geometry characteristics for streamflow-gaging stations in Iowa ..................................................................80

TABLES v

CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply By To obtain

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2) - 2.590 square kilometer

foot per mile (ft/mi) 0.1894 meter per kilometer

mile per square mile (mi/mi2) 0.621 kilometer per square

kilometer

cubic foot per second (ft3/s) 0.02832 cubic meter per second

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929

a geodetic datum derived from a general adjustment of the first-order level nets of the United States

and Canada, formerly called Sea Level Datum of 1929.

vi ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA

ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA USING DRAINAGE-BASIN AND CHANNEL-GEOMETRY

CHARACTERISTICS

By David A. Eash

ABSTRACT

Drainage-basin and channel-geometry multiple-regression equations are presented for estimating design-flood discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years at stream sites on rural, unregulated streams in Iowa. Design-flood discharge estimates determined by Pearson Type-Ill analyses using data collected through the 1990 water year are reported for the 188 streamflow-gaging stations used in either the drainage-basin or channel-geometry regression analyses. Ordinary least-squares multiple-regression techniques were used to identify selected drainage-basin and channel-geometry characteristics and to delineate two channel-geometry regions. Weighted least- squares multiple-regression techniques, which account for differences in the variance of flows at different gaging stations and for variable lengths in station records, were used to estimate the regression parameters.

Statewide drainage-basin equations were developed from analyses of 164 streamflow- gaging stations. Drainage-basin characteristics were quantified using a geographic-information- system procedure to process topographic maps and digital cartographic data. The significant characteristics identified for the drainage-basin

^equations included contributing drainage area, relative relief, drainage frequency, and 2-year, 24-hour precipitation intensity. The average standard errors of prediction for the drainage- basin equations ranged from 38.6 to 50.2 percent. The geographic-information-system procedure expanded the capability to quantitatively relate drainage-basin characteristics to the magnitude and frequency of floods for stream sites in Iowa and provides a flood-estimation method that is independent of hydrologic regionalization.

Statewide and regional channel-geometry regression equations were developed from analyses of 157 streamflow-gaging stations. Channel-geometry characteristics were measured

onsite and on topographic maps. Statewide and regional channel-geometry regression equations that are dependent on whether a stream has been channelized were developed on the basis of bankfull and active-channel characteristics. The significant channel-geometry characteristics identified for the statewide and regional regression equations included bankfull width and bankfull depth for natural channels unaffected by channel ization, and active-channel width for stabilized channels affected by channelization. The average standard errors of prediction ranged from 41.0 to 68.4 percent for the statewide channel-geometry equations and from 30.3 to 70.0 percent for the regional channel-geometry equations.

Procedures provided for applying the drainage-basin and channel-geometry regression equations depend on whether the design-flood discharge estimate is for a site on an ungaged stream, an ungaged site on a gaged stream, or a gaged site. When both a drainage-basin and a channel-geometry regression-equation estimate are available for a stream site, a procedure is presented for determining a weighted average of the two flood estimates. The drainage-basin regression equations are applicable to unregu lated rural drainage areas less than 1,060 square miles, and the channel-geometry regression equations are applicable to unregulated rural streams in Iowa with stabilized channels.

INTRODUCTION

Knowledge of the magnitude and frequency of floods is essential for the effective manage ment of flood plains and for the economical planning and safe design of bridges, culverts, levees, and other structures located along streams. Long-term flood data collected from a network of streamflow-gaging stations operated in Iowa are available for hydrologic analysis to compute design-flood discharge estimates for the gaged sites as well as for ungaged sites on the gaged streams. Techniques are needed to estimate design-flood discharges for sites on all

INTRODUCTION 1

ungaged streams in Iowa because most such stream sites in the State have no flood data available, particularly sites on smaller streams.

Flood runoff is a function of many interrelated factors that include, but are not limited to climate, soils, land use, and the physiography of drainage basins. Previous investigations for Iowa (Schwob, 1953, 1966; Lara, 1973, 1987) have been limited to the types of basin characteristics that can be investigated as potential explanatory variables for the development of multiple-regression flood- estimation equations because many of the flood-runoff factors are difficult to measure. Previous investigations defined hydrologic regions to account for factors affecting flood runoff that were difficult to measure directly. The hydrologic regions were delineated on the basis of physiographic differences of broad geographic landform regions. However, two major limitations are encountered when using the hydrologic-region method to estimate flood discharges for ungaged sites. First, it is difficult to weight flood estimates for drainage basins located in more than one hydrologic region or located near the boundaries of hydrologic regions because the boundaries are not well defined. Regional boundaries are transitional zones where the physiographic characteristics of one hydrologic region gradually merge into another. Second, because large hydrologic regions may contain drainage basins with physiographies that are anomalous to the region in which they are located, it is difficult to correlate their physiographic differences to another hydrologic region, or to weight their flood estimates. Quantitative measurements of basin morphology to determine appropriate regional equations for drainage basins are not applicable for resolving these regional limitations. As a result, flood estimates for some ungaged sites become very subjective.

To address the need to minimize the subjectivity encountered in applying regional flood-estimation methods, a study using two different flood-estimation methods was made by the U.S. Geological Survey in cooperation with the Iowa Highway Research Board and the Highway Division of the Iowa Department of Transportation. Two new flood-estimation methods for Iowa, which are presumed to be independent from each other, were used in this

study. An advantage in developing flood- frequency equations using two independent flood-estimation methods is that each method can be used to verify the results of the other, and the estimates obtained from each method can be used to calculate a weighted average.

Methods are now available to more easily quantify selected morphologic and climatic characteristics for a large number of drainage basins. A geographic-information-system (GIS) procedure developed by the U.S. Geological Survey uses topographic maps and digital cartographic data to quantify several basin characteristics that typically were not quantified previously. This GIS procedure expands the capability to relate drainage-basin characteristics to the magnitude and frequency of floods for stream sites in Iowa and provides a flood-estimation method that is independent of hydrologic regionalization.

Measurements of channel-geometry characteristics have been used to estimate the magnitude and frequency of floods in investigations conducted by Fields (1975), Webber and Roberts (1981), Parrett and others (1987), Hedman and Kastner (1977), and Osterkamp and Hedman (1982). These investigations have shown that measurements of specific channel-geometry characteristics provide a reliable method £or estimating flood discharges because channel cross-sectional characteristics are assumed to be a function of flow volume and sediment-load transport (Pickup and Rieger, 1979, p. 41; Osterkamp, 1979, p. 2).

Purpose and Scope

The purpose of this report is to: (1) define equations for Iowa that relate measurable drainage-basin characteristics to design-flood discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years that are independent of hydrologic regionalization; (2) define corroborative equations for Iowa that relate channel-geometry characteristics to the same design-flood recurrence intervals; and (3) define application and reliability of drainage-basin and channel-geometry flood-estimation methods.

Both the drainage-basin and channel- geometry flood-estimation methods described in

2 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA

this report are applicable to unregulated rural streams located within the State. The drainage-basin flood-estimation method is limited to streams with drainage areas less than 1,060 mi2 . The channel-geometry flood- estimation method is applicable to stabilized stream channels in Iowa.

Acknowledgments

The U.S. Army Corps of Engineers, Omaha and Rock Island Districts, contributed to the funding of the sediment-sample analyses. James J. Majure, formerly with the U.S. Geological Survey, Iowa City, Iowa, and now with the Iowa State University, GIS Support and Research Facility, Ames, Iowa, developed the computer software used to quantify the drainage-basin characteristics and the software used to integrate the overall GIS procedure.

FLOOD-FREQUENCY ANALYSES OF STREAMFLOW-GAGING STATIONS IN IOWA

Flood-frequency curves were developed for 188 streamflow-gaging stations operated in Iowa by the U.S. Geological Survey. They were developed according to procedures outlined in Bulletin 17B of the Interagency Advisory Committee on Water Data (IACWD, 1982, p. 1-28). These flood-frequency curves include data collected through the 1990 water year for both active and discontinued continuous-record and crest-stage gaging stations having at least 10 years of gaged annual-peak discharges. A water year is the 12-month period from October 1 through September 30 and is designated by the calendar year in which it ends. The locations of the 164 gaging stations studied using the drainage-basin flood-estimation method are shown in figure 1, and the locations of the 157 gaging stations studied using the channel- geometry flood-estimation method are shown in figure 2. Map numbers for the gaging stations shown in figures 1 and 2 are referenced to gaging-station numbers and names in tables 8 and 9 (at end of this report). The observed annual-peak discharge record at each site includes water years during which the gaging station was operated, which is termed the systematic period of record. The observed annual-peak discharge record also may include historic-peak discharges that occurred during

water years outside the systematic period of record.

A flood-frequency curve relates observed annual-peak discharges to annual exceedance probability or recurrence interval. Annual exceedance probability is expressed as the chance that a given flood magnitude will be exceeded in any 1 year. Recurrence interval, which is the reciprocal of the annual exceedance probability, is the average number of years between exceedances of a given flood magnitude. For example, a flood with a magnitude that is expected to be exceeded once on the average during any 100-year period (recurrence interval) has a 1-percent chance (annual exceedance probability = 0.01) of being exceeded during any 1 year. This flood, commonly termed the 100-year flood, is generally used as a standard against which flood peaks are measured. Although the recurrence interval represents the long-term average period between floods of a specific magnitude, rare floods could occur at shorter intervals or even within the same year.

Flood-frequency curves were developed by fitting the logarithms (base 10) of the observed annual-peak discharges to a Pearson Type-Ill distribution using U.S. Geological Survey WATSTORE flood-frequency analysis programs (Kirby, 1981, p. C1-C57). Extremely small discharge, values (low outliers) were censored, adjustments were made for extremely large discharge values (high outliers), and the coefficient of skew was weighted for each gaging station with skew values obtained from a generalized skew-coefficient map (IACWD, 1982). Whenever possible, historically adjusted flood-frequency curves were developed to extend the flood record for gaging stations with historic peak-flood information.

The recommended equation (IACWD, 1982, p. 9) for fitting a Pearson Type-Ill distribution to the logarithms of observed annual-peak discharges of a gaging station is

log(QT , ffJ = x + ks, (1)

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FLOOD-FREQUENCY ANALYSES OF STREAMFLOW-GAGING STATIONS IN IOWA 3

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k is the standardized Pearson Type-Ill deviate for a selected T-year recurrence interval and weighted skew coefficient; and

s is the standard deviation of the logarithms (base 10) of the observed annual-peak dis charges.

Results of the Pearson Type-Ill flood- frequency analyses are presented in table 8 (listed as method B17B, at end of this report) for the 188 streamflow-gaging stations analyzed using either the drainage-basin or channel- geometry flood-estimation techniques. Included in table 8 is information about the type of gage operated, the effective record length of the gage, whether a systematic or historical analysis was performed, the observed annual-peak discharge record (listed as flood period), and the maximum known flood-peak discharge and its recurrence interval. An example flood-frequency curve is shown in figure 3.

DEVELOPMENT OFMULTIPLE-REGRESSIONEQUATIONS

Multiple linear-regression techniques were used to independently relate selected drainage- basin and channel-geometry characteristics to design-flood discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years. A general overview of the ordinary least-squares and weighted least-squares multiple linear- regression techniques used to develop the equations is presented in the following two sections. Specific information on the multiple- regression analyses for either flood-estimation method is presented in later sections entitled "Drainage-Basin Characteristic Equations" and "Channel-Geometry Characteristic Equations."

Ordinary Least-Squares Regression

Ordinary least-squares (OLS) multiple linear-regression techniques were used to

develop the initial multiple-regression equations, or models, for both the drainage- basin and channel-geometry flood-estimation methods. In OLS regression, a design-flood discharge (termed the response variable) is estimated on the basis of one or more significant drainage-basin or channel-geometry character istics (termed the explanatory variables) in which each observation is given an equal weight. The response variable is assumed to be a linear function of one or more of the explanatory variables. Logarithmic transforma tions (base 10) were performed for both the response and explanatory variables used in all of the OLS regression analyses. Data transformations were used to obtain a more constant variance of the residuals about the regression line and to linearize the relation between the response variable and explanatory variables. The general form of the OLS regres sion equations developed in these analyses is

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62log 10 (X2) + ... + 6p log

(2)

where Q? is the response variable, the estimated design-flood discharge, in cubic feet per second, for a selected T-year recurrence interval;

C is a constant;

6j is the regression coefficient for the ith explanatory variable (i =

is the value of the ith explanatory variable, a drainage-basin or channel-geometry characteristic (i = 1, ... ,p); and

is the total number of explanatory variables in the equation.

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OLS regression analyses were performed using the Statit1 statistical procedures ALLREG and SREGRES (Statware, Inc., 1990, p. 6-10 - 6-27). Initial selections of significant explanatory variables for the OLS regression models were performed by using the ALLREG procedure. The ALLREG procedure uses an all-possible subsets regression to identify the best-possible combinations of explanatory variables on the basis of the Mallows' Cp statistic (Mallows, 1973). The SREGRES procedure, based ' on stepwise-regression algorithms, then was used to perform an OLS regression analysis on each best-possible combination of explanatory variables. The final selection of explanatory variables was based on the following criteria as described by Koltun and Roberts (1990, p. 11):

1. The selection of explanatory variables, as well as the signs and magnitudes of their respective regression coefficients, need to be hydrologically valid in the context of flood runoff. This criterion takes precedence over all other criteria.

2. All explanatory variables should be statistically significant at the 95-percent confidence level.

3. The selection of explanatory variables, within the constraints of criteria 1 and 2, should minimize the prediction error sum of squares [the PRESS statistic, an index of the prediction error associated with the regression equation (Alien, 1971; Montgomery and Peck, 1982)], maximize the coefficient of determination (R2 , a measure of the proportion of the variation in the response variable accounted for by the regression equation), and minimize the standard error of estimate. Correlation between explanatory variables and the variance inflation factor (VIF) (Marquardt, 1970; Montgomery and Peck, 1982) was used to assess multicollinearity in the regression models.

Weighted Least-Squares Regression

A weighted least-squares (WLS) regression technique described by Tasker (1980 p. 1107- 1109) was used to develop the final

1 Use of brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

multiple-regression equations for both the drainage-basin and channel-geometry flood- estimation methods. WLS regression analyses improved R2 values and standard errors of estimate obtained in the majority of the OLS regression analyses. Tasker (1980, p. 1107) reports that OLS regression assumes that the time-sampling variance in the response- variable estimates (design-flood discharges) are the same for each gaging station used in the analysis (assumption of homoscedasticity). In hydrologic regression, this assumption usually is violated because the reliability of response- variable estimates depends primarily on the length of the observed annual-peak discharge records. The error associated with response-variable estimates is inversely proportional to record length (Choquette, 1988, p. 16-17). WLS regression adjusts for the variation in the reliability of the response- variable estimates by using a weighting function to account for differences in the lengths of observed annual-peak discharge records at gaging stations. The weighting function (Tasker, 1980, p. 1107) is based on theory and on analysis of residuals from the initial OLS regression equation. The weighting function assumes that the response variable (the design-flood discharge) is determined by fitting the logarithms (base 10) of the observed annual-peak discharges to a Pearson Type-Ill distribution.

The variance of the response variable, QT, is estimated to determine the appropriate weight factors for the WLS regression analysis. To be effective, the weight factors should be inversely proportional to the variance of QT- The variance of Qf, for a selected T-year recurrence interval, can be partitioned into two components-the variance due to OLS regression-model error, c0 , and the variance due to time-sampling error, tj, which is related to the standard deviation and weighted skew coefficient of the logarithms of the observed annual-peak discharges and to the number of years of effective record length (Tasker, 1980, p, 1107-1109; Choquette, 1988, p. 16-17). The weight function has the form

(4)

where W{ is the weight factor for the ith gaging station used in the


analysis.

The model error (c0) is estimated by

cn = maximumERLU

(5)

where SE is the standard error of the estimate from the OLS equation;

GI is a constant; and

ERL is the mean effective record length, in years, for all gaging stations used in each respective regression-model data set.

The time-sampling error (tj) is estimated by

(6)

where ERL^-) is the effective record length, in years, for the ith gaging station used in each respective regression-model data set.

The constant, c^, is related to the recurrence interval of the response variable and to the weighted skew coefficient (g) of the observed annual-peak discharges. It is determined by

c, = maximum-2

If(7)

where s is the mean standard deviation of the logarithms (base 10) of the observed annual-peak dis charges;

k is the mean standardized Pearson Type-Ill deviate for selected T-year recurrence interval and mean weighted skew coefficient g (IACWD, 1982, p. 3-1 - 3-27); and

g is the mean weighted skew coefficient of the logarithms (base 10) of the observed annual-peak discharges (IACWD, 1982, p. 12-15).

The values s and g are statewide estimates determined by the averages of the 188

streamflow-gaging stations analyzed using either the drainage-basin or channel-geometry flood-estimation techniques. These estimation methods are based on the assumption that s and g are approximately constant for all the gaging stations in the State.

The effective record length (ERL) of a gaging station is based on an empirical analysis made by Gary D. Tasker (U.S. Geological Survey, written commun., March 1992) of results reported in Tasker and Thomas (1978) and Stedinger and Cohn (1986). It is determined by

ERL =LS+(HST-LS)a, (8)

where LS

HST

is the systematic record length of a gaging station, in years, the number of water years during which the gaging station was operated;

is the historic record length of a gaging station, in years, as used in a Pearson Type-Ill historical flood-frequency analysis; if a systematic flood-frequency analysis was performed, HST = LS; if (HST-LS) > 200, set

(9)

In the last equation, ph = 1.0 (np I HST), and np is the number of historic and extremely large discharge (high-outlier) peaks.

The ERL used in the weighted least-squares regression analyses for each gaging station is listed in table 8 (at end of this report).

ESTIMATING DESIGN-FLOOD DISCHARGES USING DRAINAGE-BASIN CHARACTERISTICS

The drainage-basin flood-estimation method uses selected drainage-basin characteristics to estimate the magnitude and frequency of floods for stream sites in Iowa. Multiple-regression equations were developed by relating significant drainage-basin characteristics to Pearson Type-Ill, design-flood discharges for 164

ESTIMATING DESIGN-FLOOD DISCHARGES USING DRAINAGE-BASIN CHARACTERISTICS 9

streamflow-gaging stations in Iowa (fig. 1). Drainage-basin characteristics were quantified using a GIS procedure to process topographic maps and digital cartographic data. An overview of the GIS procedure is provided in the following section.

Geographic-Information-System Procedure

The GIS procedure developed by the U.S. Geological Survey (USGS) quantifies for each drainage basin the 26 basin characteristics listed in Appendix A (at end of this report). These characteristics were selected for the GIS procedure on the basis of their hypothesized applicability in flood-estimation analysis and their general acceptability as measurements of drainage-basin morphology and climate. Techniques for making manual measurements of selected drainage-basin characteristics from topographic maps are outlined in Appendix B (at end of this report). The GIS procedure uses ARC/INFO computer software and other software developed specifically to integrate with ARC/INFO (Majure and Soenksen, 1991; Eash, 1993).

The GIS procedure entails four main steps: (1) creation of four GIS digital maps (ARC/INFO coverages) from three cartographic data sources, (2) assignment of attribute information to three of the four GIS digital maps, (3) quantification of 24 morphologic basin characteristics from the four GIS digital maps, and (4) quantification of two climatic basin characteristics from two precipitation data sources.

The first step creates four GIS digital maps representing selected aspects of a drainage basin. Examples of these maps are shown in figure 4. The drainage-divide digital map (fig. 4A) is created by delineating the surface-water drainage-divide boundary for a streamflow- gaging station on 1:250,000-scale U.S. Defense Mapping Agency (DMA) topographic maps. This drainage-divide delineation is manually digitized into a polygon digital map using GIS software. If noncontributing drainage areas are identified within the drainage-divide boundary, then each noncontributing drainage area also is delineated and digitized.

The drainage-network digital map (fig. 4B) is created by extracting the drainage network for the basin from l:100,000-scale USGS digital line graph (DLG) data. The extraction process uses GIS software to select and append together the DLG data contained within the drainage-divide polygon.

The elevation-contour digital map (fig. 4C) is created from l:250,000-scale DMA digital elevation model (DEM) data that are referenced to sea level (in meters). GIS software is used to convert the DEM data to a lattice file of point elevations for an area slightly larger than the drainage-divide polygon. This lattice file of point elevations is contoured with a 12-meter (39.372-ft) or smaller contour interval using ARC/INFO software. The contour interval is chosen to provide at least five contours for each drainage basin. GIS software selects the contours contained within the drainage-divide polygon to create the elevation-contour digital map. Elevation contours then are converted to units of feet.

The basin-length digital map (fig. 4D) is created by delineating and digitizing the basin length from l:250,000-scale DMA topographic maps. The basin length characteristic is delineated by first identifying the main channel for the drainage basin on l:100,000-scale topographic maps. The main channel is identified by starting at the basin outlet and proceeding upstream, repetitively selecting the channel that drains the greater area at each stream junction. The most upstream channel is extended to the drainage-divide boundary defined for the drainage-divide digital map. This main channel identified on l:100,000-scale topographic maps is used to define the main channel on l:250,000-scale topographic maps. The basin length is centered along the main-channel, flood-plain valley from basin outlet to basin divide and digitized with as straight a line as possible from the l:250,000-scale maps. When comparing the basin length shown in figure 4D to those stream segments corresponding to the main channel in figure 4.B, it can be seen that the basin length does not include all the sinuosity of the stream segments.

The second step assigns attributes to specific polygon, line-segment, and point


A. Drainage-divide digital map digitized from Waterloo topographic map.

Base from U.S. Defense Mapping Agency, 1:250,000, 1976Universal Transverse Mercator projection, Zone 15

B. Drainage-network digital map extracted from Marshalltown-West digital line graph data, with stream-order numbers.

Base from U.S. Geological Survey digital data, 1:100,000, 1984 Universal Transverse Mercator projection, Zone 15

C. Elevation-contour digital map created from Waterloo-East digital elevation model, sea-level data, with contour intervals at 39.372 feet.


984.300

D. Basin-length digital map digitized from Waterloo topographic map.


0 2.5 5 MILES

2.5 5 KILOMETERS

EXPLANATIONA STREAMFLOW-GAGING STATION

Figure 4. Four geographic-information-system maps that constitute a digital representation of selectedaspects of a drainage basin.


features in the first three of the four GIS digital maps shown in figure 4. As a prerequisite, the digital maps are edited to ensure that drainage-divide boundaries, stream segments, and the basin-length line segments are connected properly. If noncontributing drainage areas are identified, they are assigned attributes with separate polygon designations so that the basin-characteristic programs can distinguish between contributing and noncontributing areas. Each line segment in the drainage-network digital map is assigned a Strahler stream-order number (Strahler, 1952) and a code indicating whether the line segment represents part of the main channel or a secondary channel. Specific GIS programs have been developed to assign the proper stream- order number to each line segment and to code those line segments representing the main channel. Figure 45 shows the Strahler stream-order numbers for streams in the Black Hawk Creek at Grundy Center (station number 05463090; map number 73, fig. 1) drainage basin. A description on how to order streams using Strahler's method is included in Appendix B (at end of this report).

The line segments in the elevation-contour digital map were assigned elevations from the processing of the DEM data. Line segments overlain by noncontributing drainage-area polygons are assigned attributes designating noncontributing contour segments. Two point attributes are added to the elevation-contour digital map to represent the maximum and minimum elevations of the drainage basin. The maximum basin elevation is defined from the highest DEM-generated contour elevation within the contributing drainage area. The minimum basin elevation is defined at the basin outlet as an interpolated value between the first elevation contour crossing the main channel upstream of the basin outlet and the first elevation contour crossing the main channel downstream of the basin outlet.

The third step uses the four GIS digital maps shown in figure 4 and a set of programs developed by the USGS (Majure and Soenksen, 1991) to quantify the 24 morphologic basin characteristics listed in Appendix A (at end of this report). These basin characteristics include selected measurements of area, length, shape, and topographic relief that define selected

aspects of basin morphology, and several channel characteristics. The programs access the information automatically maintained by the GIS for each of the four digital maps, such as the length of line segments and the area of polygons, as well as the previously described attribute information assigned to the polygon, line-segment, and point features of three of the four GIS digital maps. The GIS programs then use this information to automatically quantify the 24 morphologic basin characteristics.

The fourth step uses a software program developed to quantify the remaining two basin characteristics listed in Appendix A (at end of this report). These two climatic characteristics are quantified using GIS digital maps representing the distributions of mean annual precipitation and 2-year, 24-hour precipitation intensity for the area contributing to all surface-water drainage in Iowa. This area includes a portion of southern Minnesota. The mean annual precipitation digital map was digitized from a contour map for Iowa, supplied by the Iowa Department of Agriculture and Land Stewardship, State Climatology Office (Des Moines), and from a contour map for Minnesota (Baker and Kuehnast, 1978). The 2-year, 24-hour precipitation intensity digital map was digitized from a contour map for Iowa (Waite, 1988, p. 31) and interpolated contours for southern Minnesota that were digitized from a United States contour map (Hershfield, 1961, p. 95). The digital map representing the distribution of 2-year, 24-hour precipitation intensity for Iowa and southern Minnesota is shown in figure 5. The weighted average for each climatic characteristic is computed for a drainage basin by calculating the mean of the area-weighted precipitation values that are within the drainage-divide polygon.

Of the 26 drainage-basin characteristics listed in Appendix A, 12 are referred to as primary drainage-basin characteristics because they constitute specific GIS procedure or manual topographic-map measurements. They are listed under headings containing the word "measurement." The remaining characteristics are calculated from the primary drainage-basin characteristics; they are listed in Appendix A under headings containing the word "computation." Each drainage-basin character istic listed in Appendix A is footnoted with a


2.55

44

93°

Base from U.S. Geological Survey digital data, Missouri \ 1:2.000,000,1979 IVIIOOWUH

Hjniveisal Transverse Mercator projection.Zone 15

3.35

20 40 60 KILOMETERS ,'

EXPLANATION

AREA OF EQUAL 2-YEAR, 24-HOUR PRECIPITATION INTENSITY-Number is precipitation intensity, in inches

Figure 5. Distribution of 2-year, 24-hour precipitation intensity for Iowa and southern Minnesota.

reference and the cartographic data source used for both GIS procedure and manual measurements.

Verification of Drainage-Basin Characteristics

To verify that the drainage-basin characteristics quantified using the GIS procedure are valid, manual topographic-map measurements of selected drainage-basin characteristics were made for 12 of the

streamflow-gaging stations used in the drainage-basin flood-estimation method. These comparison measurements were made for those primary drainage-basin characteristics identified as being significantly related to flood runoff in the multiple-regression equations presented in the following section entitled "Drainage-Basin Characteristic Equations." Comparison measurements were made from topographic maps of the same scales as were used in the GIS procedure. The results of the comparisons are shown in table 1.


Table 1. Comparisons of manual measurements and geographic-information-system-procedure measurements of selected drainage-basin characteristics at selected streamftow-gaging stations

[TDA, total drainage area, in square miles; BP, basin perimeter, in miles; BR, basin relief, in feet;FOS, number of first-order streams; TTF, 2-year, 24-hour precipitation intensity, in inches; MAN,

manual measurement; GIS, geographic-information-system procedure; % DIFF, percentage differencebetween MAN and GIS]

Station number

05411600

05414450

05414600

05462750

05463090

05470500

05481000

05489490

06483430

06609500

Measure ment technique

MANGIS%DIFF

MANGIS%DIFF

MANGIS%DIFF

MANGIS%DIFF

MANGIS%DIFF

MANGIS% DIFF

MANGIS%DIFF

MANGIS%DIFF

MANGIS%DIFF

MANGIS% DIFF

Selected drainage-basin characteristics

TDA1

111178+0.6

21.622.3+3.2

1.541.53

-0.6

11.611.9+2.6

56.957.0+0.2

204208+2.0

844852+0.9

22.922.2-3.1

29.930.0+0.3

871869

-0.2

BP

73.373.9+0.8

21.921.3-2.7

5.325.97

+12.2

15.015.5+3.3

33.533.1-1.2

69.867.7-3.0

139139

0

24.826.2+5.6

28.828.9+0.3

206210

+1.9

BR

297274

-7.7

444394-11.3

280291+3.9

160129-19.4

181160-11.6

318292

-8.2

303300

-1.0

280263

-6.1

198182

-8.1

582550

-5.5

FOS

8484

0

10100

110

660

2828

0

6051

-15.0

152155+2.0

10100

12120

477475

-0.4

TTF

3.053.050

3.053.050

3.053.050

3.053.050

3.153.150

3.153.150

3.053.050

3.253.250

2.852.850

3.053.050


Table 1. Comparisons of manual measurements and geographic-information-system-procedure measurements of selected drainage-basin characteristics at selected stream flow-gaging

stations Continued

Stationnumber

06807780

06903400

Measure menttechnique

MANGIS%DIFF

MANGIS%DIFF

Selected drainage-basin characteristics

TDA1

42.742.8+0.2

182184+1.1

BP

47.448.8+3.0

79.079.6+0.8

BR

268280+4.5

224256+14.3

FOS

1819+5.6

8080

0

TTF

3.053.050

3.253.250

WILCOXON SIGNED-RANKS TEST STATISTIC2 -1.726 p-VALUE STATISTIC 0.0844

-1.334 0.1823

-1.843 -0.365 NO TEST3 0.0653 0.7150

1 Manual TDA measurements are streamflow-gaging-station drainage areas published by the U.S. Greological Survey in annual streamflow reports. Noncontributing drainage areas (NCDA) are not listed because none were identified for these drainage basins.

2 Using a 95-percent level of significance, the T-value statistic = 2.2010 (Iman and Conover, 1983, p. 438).

3 All values for % DIFF = 0.

Comparison measurements for total drainage area (TDA) indicate that the GIS procedure was within about 1 percent of the drainage areas published by the USGS in annual streamflow reports for 8 of the 12 selected gaging stations. This comparison indicates that delineations of drainage areas used in the GIS procedure, made from l:250,000-scale topographic maps, were generally valid. The Wilcoxon signed-ranks test was applied to four of the five drainage-basin characteristics listed in table 1 using STATIT procedure SGNRNK (Statware, Inc., 1990, p. 3-25 - 3-26). Results (table 1) indicate that GIS procedure measurements of total drainage area, basin perimeter (BP), basin relief (BR), and number of first-order streams (FOS) were not significantly different from manual topographic-map measurements at the 95-percent level of significance. The greater

variation in measurement comparisons of basin relief are believed to be due to limitations in the l:250,000-scale DEM data. Results of the comparison tests (table 1) indicate that GIS procedure measurements are generally valid for the primary drainage-basin characteristics used in the regression equations presented in the following section.

Basin slope (BS) is another drainage-basin characteristic that was quantified using DEM data. It is hypothesized that basin slope may have a significant effect on surface-water runoff. Basin slope was indicated as being a significant characteristic in a few of the initial multiple-regression analyses. Comparison measurements indicated that the GIS procedure greatly underestimated basin slope. Measure ment differences for basin slope were between minus 9 and 66 percent, with an average


underestimation of 40 percent for the 10 drainage basins tested (Eash, 1993, p. 180-181). For this reason, the basin-slope characteristic was deleted from the drainage-basin characteristics data set during the initial multiple-regression analyses. Basin-slope comparisons appear to indicate that the l:250,000-scale DEM data used to create the elevation-contour digital maps are not capable of reproducing all the sinuosity of the elevation contours depicted on,the l:250,000-scale DMA topographic maps. The elevation contours generated using the GIS procedure are much more generalized than the topographic-map contours; thus, the total length of the elevation contours are undermeasured when using the "contour-band" method of calculating basin slope (BS) (Appendix A). A comparison of the elevation contours shown in figure 4C for the Black Hawk Creek at Grundy Center (station number 05463090; map number 73, fig. 1) drainage basin to those depicted on the DMA l:250,000-scale Waterloo topographic map showed a significant difference in the sinuosity of the elevation contours depicted.

Drainage-Basin Characteristic Equations

The 26 drainage-basin characteristics listed in Appendix A were quantified for 164 streamflow-gaging stations (fig. 1) and investigated as potential explanatory variables in the development of multiple-regression equations for the estimation of design-flood discharges. Because of the previously described problems concerning measurement verification of basin slope and because of the difficulty associated with manual measurements of total stream length, six basin characteristics were deleted from the regression data set. The excluded characteristics were basin slope IBS), total stream length (TSL\ stream density (SD\ constant of channel maintenance (CCAf), ruggedness number (RN), and slope ratio (SR).

Several other drainage-basin characteristics also were deleted from the data set because of multicollinearity. Multicollinearity is the condition where at least one explanatory variable is closely related to (that is, not independent of) one or more other explanatory variables. Regression models that include variables with multicollinearity may be

unreliable because coefficients in the models may be unstable. Output from the ALLREG analysis and a correlation matrix of Pearson product-moment correlation coefficients were used as guides in identifying the variables with multicollinearity. The hydrologic validity of variables with multicollinearity in the context of flood runoff was the principal criterion used in determining which drainage-basin character istics were deleted from the data set. Upon completion of the ALLREG analyses, any remaining multicollinearity problems were identified with the SREGRES procedure by checking each explanatory variable for variance inflation factors greater than 10.

Statewide flood-estimation equations were developed from analyses of the drainage-basin characteristics using the ordinary least-squares and weighted least-squares multiple-regression techniques previously described. The best equations developed in terms of PRESS statistics, coefficients of determination, and standard errors of estimate are listed in table 2. The characteristics identified as most significant in the drainage-basin equations are contributing drainage area (CDA), relative relief (RR), drainage frequency (Z)F), and 2-year, 24-hour precipitation intensity (TTF). Table 9 (at end of this report) lists these significant drainage-basin characteristics, as quantified by the GIS procedure, for 164 streamflow-gaging stations in Iowa.

Three of the four characteristics listed in the drainage-basin equations (table 2) are calculated from primary drainage-basin characteristics. The drainage-basin equations are comprised of six primary drainage-basin characteristics. Contributing drainage area (CDA) is a measure of the total area that contributes to surface-water runoff at the basin outlet. The primary drainage-basin characteristics used to calculate contributing drainage area are total drainage area (TDA) and noncontributing drainage area (NCDA). Relative relief (RR) is a ratio of two primary drainage-basin characteristics, basin relief (BR) and basin perimeter (BP). Drainage frequency (DF) is a measure of the average number of first-order streams per unit area and is an indication of the spacing of the drainage network. The primary drainage-basin characteristics used to calculate drainage


Table 2. Statewide drainage-basin characteristic equations for estimating design-flood discharges in Iowa

[Q, peak discharge, in cubic feet per second, for a given recurrence interval, in years; CDA,contributing drainage area, in square miles; RR, relative relief, in feet per mile; DF, drainage

frequency, in number of first-order streams per square mile; TTF, 2-year, 24-hour precipitationintensity, in inches]

Estimation equation

Average AverageStandard standard error equivalent

error of estimate of prediction years ofLogic Percent (percent) record

Number of streamflow-gaging stations = 164

Q2 =53.1 CDA0-799 flflO.6430^0.381 (TTf, .2.5)1-36 0 m 41 Q

Q5 = 98.8 CDA0-755 #fl0-652 DF°-380 (TTF -2.5)0-985 .156 37.2

Q10 = 136 CDA0-733 RR0-654 DF0-384 (TTF - 2.5)°-801

Q2s = 188 CDA0- 709 RR0-655 DF0-393 (TTF - 2.5)a61°

Qso =231CI>A0-694 ^?0-656 DF0-401 (TTF-2.5)0-491

Q100 = 277 CDA0-681 RRQM6 DF0-409 (TTF - 2.5)°-389

.160 38.2

.172 41.3

.185 44.5

.198 48.0

38.6

39.8

43.2

46.5

50.2

3.9

5.4

6.5

7.8

9.5

11.5

Note: Basin characteristics are map-scale dependent. See Appendix A and Appendix B for basin-characteristic descriptions, computations, and scales of maps to use for manual measurements.

frequency are the number of first-order streams (FOS) and contributing drainage area (CDA). The value of FOS is determined by using Strahler's method of ordering streams (Strahler, 1952). A description of Strahler's stream-ordering method is included in Appendix B. The 2-year, 24-hour precipitation intensity (TTF) is a primary drainage-basin- characteristic measurement of the maximum 24-hour precipitation expected to be exceeded on the average once every 2 years.

Additional information pertaining to the characteristics used in the drainage-basin equations (table 2) is included in Appendix A. Techniques on how to make manual measurements from topographic maps for the primary drainage-basin characteristics used in the equations are outlined in Appendix B. Several of the primary drainage-basin

characteristics are map-scale dependent. Use of maps of scales other than the scales used to develop the equations may produce results that do not conform to the range of estimation accuracies listed for the equations in table 2. The scale of map to use for manual measurements of each primary drainage-basin characteristic is outlined in Appendix A and Appendix B.

Examination of residuals, the difference between the Pearson Type-Ill and multiple- regression estimates of peak discharge for the drainage-basin equations, indicated no evidence of geographic bias. The drainage-basin equations thus were determined to be independent of hydrologic regionalization within the State.


The drainage-basin flood-estimation method developed in this study is similar to the regional flood-estimation method developed by Lara (1987) because both methods estimate flood discharges on the basis of morphologic relations. While the standard errors of estimate appear to be higher for the drainage-basin equations than for Lara's equations (Lara, 1987, p. 28), a direct comparison cannot be made because of the different methodologies used to develop the equations. Lara's method is based on the physiography of broad geographic landform regions defined for the State, whereas the drainage-basin method presented in this report is based on specific measurements of basin morphology. The drainage-basin equations are independent of hydrologic regionalization. The application of regional equations often requires that subjective judgments be made concerning basin anomalies and the weighting of regional discharge estimates. This subjectivity may introduce additional unmeasured error to the estimation accuracy of the regional discharge estimates. The drainage-basin regression equations presented in this report provide a flood-estimation method that minimizes the subjectivity in its application to the ability of the user to measure the characteristics.

Example of Equation Use- Example 1

Example 1. An application of the drainage- basin flood-estimation method can be illustrated by using the equation (listed in table 2) to estimate the 100-year peak discharge for the discontinued Black Hawk Creek at Grundy Center crest-stage gaging station (station number 05463090; map number 73, fig. 1), located in Grundy County, at a bridge crossing on State Highway 14, at the north edge of Grundy Center, in the NW1/4, sec. 7, T. 87 N., R. 16 W. Differences between manually computed values (table 1) and values computed using the GIS procedure (tables 1 and 9) are due to differences in applying the techniques.

Step 1. The characteristics used in the drainage-basin equation (table 2) are contributing drainage area (CDA), relative relief (RR), drainage frequency (DF), and 2-year, 24-hour precipitation intensity (TTF). The primary drainage-basin characteristics used in this equation are total drainage area (TDA),

noncontributing drainage area (NCDA), basin relief (BR), basin perimeter (BP), number of first-order streams (FOS), and 2-year, 24-hour precipitation intensity (TTF). These primary drainage-basin characteristic measurements and the scale of maps to use for each manual measurement are described in Appendix A and Appendix B.

Step 2. The topographic maps used to delineate the drainage-divide boundary for this gaging station are the DMA l:250,000-scale Waterloo topographic map and the USGS l:100,000-scale Grundy County map. Figure 4A shows the drainage-divide boundary that was delineated for this gaging station on the l:250,000-scale map. Contributing drainage area (CDA) is calculated from the primary drainage-basin characteristics total drainage area (TDA) and noncontributing drainage area (NCDA). The total drainage area published for this gaging station in the annual streamflow reports of the U.S. Geological Survey is 56.9 mi2 (table 9). Inspection of the l:100,000-scale map does not show any noncontributing drainage areas within the drainage-divide boundary of this basin. The contributing drainage area (CDA) is calculated as

CDA = TDA-NCDA,

= 56.9-0,

= 56.9 mi 2 .

(10)

Step 3. Relative relief (RR) is calculated from the primary drainage-basin characteristics basin relief (BR) and basin perimeter (BP). The difference between the highest elevation contour and the lowest interpolated elevation in the basin measured from the l:250,000-scale topographic map gives a basin relief of 181 ft (table I). Figure 4C shows an approximate representation of the topography for this drainage basin. The drainage-divide boundary delineated on the l:250,000-scale topographic map (fig. 4A) is used to measure the basin perimeter, which is 33.5 mi (table 1). Relative relief (RR) is calculated as


(ID

33.5'

= 5.40 ft/mi.

Step 4. Drainage frequency (DF) is calculated from the primary drainage-basin characteristics number of first-order streams (FOS) and contributing drainage area (CDA). A total of 28 first-order streams are counted within the drainage-divide delineation for this gaging station on the l:100,000-scale topographic map (table 1). These first-order streams are shown in figure 4B. Drainage frequency (DF) is calculated as

DF = FOS CDA'

28 56.9'

(12)

.2= 0.492 first-order streams/mi

Step 5. The 2-year, 24-hour precipitation intensity (TTF) for the drainage basin is determined from figure 5. Because the drainage-divide boundary for this gaging station is completely within the polygon labeled as 3.15 in., the 2-year, 24-hour precipitation

intensity is given a value of 3.15 in. (table 1).

Step 6. The 100-year flood estimate using the drainage-basin equation (table 2) is calculated as

Q100 = 277 (CDA)0- 681 (flfl)0-656 (DF)0 -409 (TTF - 2.5)°-389,

= 277 (56.9)0'681 (5.40)0-656 (0.492)0'409 (3.15- 2.5)0'389,

= 8,310 ft3/s.

Discharge estimates listed in this report are rounded to three significant figures. The difference between the above estimate of 8,310 ft3/s and the estimate of 7,740 ft3/s listed in table 8 (method GISDB) is due to measurement differences between manual measurement and GIS procedure techniques (table 1).

ESTIMATING DESIGN-FLOOD DISCHARGES USING CHANNEL-GEOMETRY CHARACTERISTICS

The channel-geometry flood-estimation method uses selected channel-geometry characteristics to estimate the magnitude and frequency of floods for stream sites in Iowa. The channel-geometry method is based on measure ments of channel morphology, which are assumed to be a function of streamflow discharges and sediment-load transport. Multiple-regression equations were developed by relating significant channel-geometry characteristics to Pearson Type-Ill, design-flood discharges for 157 streamflow-gaging stations in Iowa (fig. 2).

Channel-Geometry Data Collection

The channel-geometry parameters that were measured for each of the gaging stations are as follows:

ACW - average width of the active channel, in feet;

ACD - average depth of the active channel, in feet;

BFW - average width of the bankfull channel, in feet;

BFD - average depth of the bankfull channel, in feet;

- silt-clay content of channel-bed material, in percent;

bk - silt-clay content of left channel-bank material, in percent;

- silt-clay content of right channel-bank material, in percent;

£>50 - diameter size of channel-bed particles for which the total weight of all particles with diameters greater than D^Q is equal to the total weight of all particles with diameters less than or equal to D$Q, in millimeters; and

GRA - local gradient of channel, in feet per mile.

ESTIMATING DESIGN-FLOOD DISCHARGES USING CHANNEL-GEOMETRY CHARACTERISTICS 19

NOT TO SCALE

EXPLANATION

ACTIVE-CHANNEL REFERENCE LEVEL

BANKFULL REFERENCE LEVEL

LOW-FLOW WATER LEVEL

BANKFULL WIDTH

ACTIVE-CHANNEL WIDTH

Figure 6. Block diagram of a typical stream channel.

The active-channel and bankfull reference levels for a typical stream channel are illustrated in figure 6. Photographs of active-channel and bankfull reference levels at six gaging stations in Iowa are shown in figure 7.

A standard particle-size analysis (dry sieve, visual accumulation tube, and wet sieve) was performed for each of the composite sediment samples collected from the channel bed and the left and right channel banks (Guy, 1969). The local gradient (GRA) was measured from l:24,000-scale topographic maps and was calculated as the slope of the channel between

the nearest contour lines crossing the channel upstream and downstream of the gaging station.

Of the 157 gaging stations selected for study using the channel-geometry flood-estimation method, 46 were on stream channels that were or were suspected of being channelized. Bankfull width (BFW) and bankfull depth (BFD) measurements could not be made for these sites because channelization affects the long-term, stabilizing conditions of stream channels. Active-channel width (ACW) and active-channel depth (ACD) measurements were made at these 46 sites because channel conditions indicated that the active-channel portions of these


A. Willow Creek near Mason City (station number 05460100; map number 69, fig. 2)

B. Black Hawk Creek at Grundy Center (station number 05463090;

map number 73, fig. 2)

C. Keigley Branch near Story City (station number 05469990; map number 85, fig. 2)

E. Middle Raccoon River near Bayard (station number 05483450; map number 115, fig. 2)

D. Big Cedar Creek near Varina (station number 05482170;


F. West Branch Floyd River near Struble (station number 06600300;


Figure 7. Active-channel (B-B1) and bankful (C-C1 ) reference levels at six streamf low-gaging stations inIowa.


channels had stabilized. Commonly, the active-channel portion of the channel will adjust back to natural or stable conditions within approximately 5 to 10 years after channelization occurs (Waite Osterkamp, U.S. Geological Survey, oral commun., October 1992). Two data sets thus were compiled for the channel-geometry multiple-regression analyses: a 157-station data set that did not include bankfull measurements and a Ill-station data set (a subset of the 157-station data set) that included both the active-channel and bankfull measurements.

Channel-Geometry Characteristic Equations

Analysis of Channel-Geometry Data on a Statewide Basis

Multiple-regression analyses initially were performed on both data sets. Statewide equations were developed for each data set using the ordinary least-squares (OLS) and weighted least-squares (WLS) multiple- regression techniques previously described. The best equations developed in terms of PRESS statistics, coefficients of determination, and standard errors of estimate for each data set are listed in table 3. The channel-geometry characteristics identified as most significant for the Ill-station data set were bankfull width (BFW) and bankfull depth (BFD). The channel-geometry characteristic identified as most significant in the 157-station data set was active-channel width (ACW). Table 9 (at end of this report) lists the average values for BFW, BFD, and ACW for the streamflow-gaging stations analyzed in the 111- and 157-station data sets. Appendix C (at end of this report) outlines the procedure for conducting channel- geometry measurements of these characteristics.

Comparison of the average standard errors of prediction listed in table 3 indicate that the data set that included bankfull measurements provided better estimation accuracy for the design-flood discharges investigated in this study than did the active-channel measure ments in the other data set. The size and shape of the channel cross section is assumed to be a function of streamflow discharge and sediment- load transport. The bankfull channel is a longer

term geomorphic feature predominately sculptured by larger magnitude discharges, whereas the active channel is a shorter term geomorphic feature that is sculptured by continuous fluctuations in discharge. Because the design-flood discharge equations developed in this study estimate larger magnitude discharges, a multiple regression relation with better estimation accuracy was defined using bankfull characteristics.

In an attempt to further improve the estimation accuracy of the equations, each gaging station was classified into one of six channel types for which separate multiple- regression analyses were performed. Gaging stations were classified according to channel- type classifications described by Osterkamp and Hedman (1982, p. 8). This classification is based on the results of the sediment-sample analyses of percent silt-clay content (SC^) and diameter size (Z?5o) of the channel-bed particles, and the percent silt-clay content of the left (SQb^and right bank GSCrbk) material. The channel- geometry flood-estimation equations developed using this procedure were inconclusive because the estimation accuracy of some channel-type equations improved while the estimation accuracy of other equations decreased. An analysis of covariance procedure described by W.O. Thomas, Jr., (U.S. Geological Survey, written commun., 1982), wherein each channel- type classification was identified as a qualitative variable, was used to test whether there was a statistical difference due to channel-type classifications. Based on the results of this analysis, there was no significant difference between the channel-type equations and the equations developed without channel-type classification. Because of the results of these two channel-type analyses, statewide channel-geometry equations classi fied according to sediment-sample analyses were determined to not significantly improve the estimates of design-flood discharges for streams in Iowa.

Analysis of Channel-Geometry Data by Selected Regions

Examination of residuals for both sets of statewide channel-geometry equations listed in table 3 indicated evidence of geographic bias with respect to the Des Moines Lobe landform


Table 3. Statewide channel-geometry characteristic equations for estimating design-flood dischargesin Iowa

[Q, peak discharge, in cubic feet per second, for a given recurrence interval, in years; BFW, bankfull width, in feet; BFD, bankfull depth, in feet; ACW, active-channel width, in feet]

Standard error of estimate

Estimation equation Log1C) Percent

Average standard error

of prediction (percent)

Average equivalent years of

record

Bankfull equations


Q2 = 4.56 BFW0- 982 BFDL02

Q5 - 14.7 BFW0-915 BFD0-899

Q 10 =26.7 BFW0-874 BFD0-846

Q25 =49.5 BFW0- 828 BFD0- 797

Q50 = 73.2 BFW0- 796 BFD0- 769

Qwo = 104 BFW0- 766 BFD0- 747

0.169

.173

.186

.206

.221

.236

40.4

41.5

44.9

50.2

54.4

58.7

41.0

42.2

45.8

51.4

55.8

60.4

4.2

4.6

5.1

5.8

7.0

8.5

Active-channel equations


Q2 =38.5 ACW1-06Q5 =98.2 ACW0-980

Q10 =157 ACW0-937

Q25 =256 ACW0-891

Q50 =349 ACW0- 861

Q100 = 458ACW°-833

0.267

.247

.246

.251

.258

.267

67.8

61.9

61.5

63.0

65.1

67.7

68.3

62.3

61.9

63.6

65.8

68.4

1.6

2.1

2.8

3.6

4.8

6.3

Note: Bankfull equations may provide improved accuracies over active-channel equations for channels unaffected by channelization. For channels affected by channelization, the active-channel equations only are applicable when the active channels have stabilized (approximately 5 to 10 years after channelization). See Appendix C for a discussion of stabilized channels.


region (fig. 2). Consequently, both data sets were split into regional data sets, and additional multiple-regression analyses were performed for two regions in Iowa.

The State was divided into two hydrologic regions using information on areal trends of the residuals for the statewide regression equations, the Des Moines Lobe landform region, and topography as guides. The delineation of channel-geometry Regions I and II is shown in figure 2. The topography of the Des Moines Lobe landform region (Region II) is characteristic of a young, postglacial landscape that is unique with respect to the topography of the rest of the State (Region I) (Prior, 1991, p. 30-47). The region generally comprises low-relief terrain, accentuated by natural lakes, potholes, and marshes, where surface-water drainage typically is poorly defined and sluggish. The shaded area between hydrologic Regions I and II (fig. 2) represents a transitional zone where the channel morphology of one region gradually merges into the other. This regionalization process served to compensate for the geographic bias observed in the statewide residual plots, which was not accounted for otherwise in the 111- and 157-station channel- geometry regression equations listed in table 3.

Using the OLS and WLS multiple- regression techniques previously described, two sets of flood-estimation equations were developed for each channel-geometry region. Of the Ill-station data set, 78 stations were in Region I and 33 stations were in Region II. Of the 157-station data set, 120 stations were in Region I and 37 stations were in Region II. Gaging stations located within the regional transition zone (fig. 2) were compiled into either Region I or Region II data sets on the basis of residuals from the statewide regression equations and on the regional locations of their stream channels. The best equations developed in terms of PRESS statistics, coefficients of determination, and standard errors of estimate for the Region I data sets are listed in table 4 and the best equations developed for the Region II data sets are listed in table 5.

The channel-geometry characteristic that was identified as most significant in the Region I 78-station bankfull equations was bankfull width (BFW). The characteristic identified as

most significant in the Region I 120-station active-channel equations was active-channel width (ACW). The channel-geometry character istics that were identified as most significant in the Region II 33-station bankfull equations were bankfull width (BFW) and bankfull depth (BFD), and the most significant characteristic in the Region II 37-station active-channel equations was active-channel width (ACW), Appendix C (at end of this report) outlines the procedure for conducting channel-geometry measurements of these characteristics.

Comparison of Regional and Statewide Channel-Geometry Equations

Comparison of the Region I and II equations with the statewide equations shows an improvement in the average standard errors of prediction for all of the regional equations except the 25-, 50- and 100-year recurrence intervals of the Region II active-channel equations. The regional equations listed in tables 4 and 5 may provide improved accuracies for estimating design-flood discharges based on channel-geometry measurements. The statewide equations listed in table 3 also can be used to estimate design-flood discharges, although their accuracies may be less than for the regional equations. Comparison of the bankfull equations with the active-channel equations listed in tables 3-5 shows an improvement in the average standard errors of prediction for all of the bankfull equations. The bankfull equations may provide improved estimation accuracies in comparison to active- channel equations for estimating design-flood discharges for channels unaffected by channelization.

Bankfull depth (BFD) was identified as a significant channel-geometry characteristic in the statewide bankfull equations (table 3). It is also a significant channel-geometry character istic in the estimation of design-flood discharges for stream sites located within the Des Moines Lobe landform region (fig. 2, Region II). While bankfull depth was not identified as significant in estimating flood discharges in Region I, it appears to be a significant morphologic feature distinguishing stream channels in Regions I and II.


Table 4. Region I channel-geometry characteristic equations for estimating design-flood discharges inIowa outside of the Des Moines Lobe landform region^

[Q, peak discharge, in cubic feet per second, for a given recurrence interval, in years; BFW, bankfullwidth, in feet; ACW, active-channel width, in feet]

Standard error of estimate

Estimation equation Logjo Percent




record

Bankfull equations


Q2

Q5QioQ25QsoQlOC

= 4.55 BFWIA5= 15.6 BFW1 -32

= 29.2 BFW1 -25

= 55.7 BFW1 - 18

= 84.2B/5WL13

) = 122B/5WL09

0.160

.140

.146

.162

.176

.192

38.1

33.1

34.5

38.5

42.3

46.4

38.9

33.8

35.4

39.8

43.9

48.3

4.8

7.4

8.8

9.8

12.6

16.1



Q2 =45. Q5 =118 ACW0-982

Q10 =190 ACW0-937

Q25 =312 ACW0- 889

Q50 =427 ACW0- 858

= 566ACW°-828

0.213

.180

.175

.179

.188

.198

52.1

43.2

41.9

43.1

45.3

48.2

53.0

44.2

43.0

44.5

46.9

50.0

2.4

4.0

5.4

7.0

8.9

11.0

lrrhe Des Moines Lobe landform region is delineated as Region II in figure 2.Note: Bankfull equations may provide improved accuracies over active-channel

equations for channels unaffected by channelization. For channels affected by channelization, the active-channel equations only are applicable when the active channels have stabilized (approximately 5 to 10 years after channelization). See Appendix C for a discussion of stabilized channels.


Table 5. Region II channel-geometry characteristic equations for estimating design-flood dischargesin Iowa within the Des Moines Lobe landform region1

[Q, peak discharge, in cubic feet per second, for a given recurrence interval, in years; BFW, bankfull width, in feet; BFD, bankfull depth, in feet; ACW, active-channel width, in feet]

Estimation equation

Standard error of estimateLog10 Percent




record

Bankfull equations


Q2 = 2.77 BFW0-844 BFDIAS

Q5 = 7.42 BFW0- 783 BFDL43

Q10 = 12.1 BFW0- 748 BFDL41

Q25 = 19.7 BFW0- 715 BFDL38

Q50 =26.7 BFW0-694 BFD1-37

Q IQO = 34.9 BFW0-675 BFD1'36

0.123

.131

.143

.162

.176

.190

28.8

30.8

33.9

38.6

42.3

45.9

30.3

33.6

37.7

43.4

47.8

52.1

6.5

6.1

6.3

6.6

7.9

9.3



Q2 =7.80ACWL3°

Q5 = 19.1 AC W1 -23

Q10 =29.6ACWL19

Q25 =45.6ACWL16

Q50 =59.5ACWL14

Q 100 = 75.0ACW1- 12

0.236

.235

.240

.248

.255

.262

58.5

58.4

59.7

62.0

64.2

66.4

59.7

60.1

61.8

64.8

67.4

70.0

1.9

2.1

2.6

3.3

4.4

. 5.7

lrrhe Des Moines Lobe landform region is delineated as Region II in figure 2.

Note: Bankfull equations may provide improved accuracies over active-channel equations for channels unaffected by channelization. For channels affected by channelization, the active-channel equations only are applicable when the active channels have stabilized (approximately 5 to 10 years after channelization). See Appendix C for a discussion of stabilized channels.


The differences in peak-discharge estimation between regional and statewide active-channel width (ACW) equations are shown in figures 8B and 9B for the 2- and 100-year recurrence intervals, respectively. Figures 85 and 9B illustrate the higher estimated peak discharges obtained from the Region I equations relative to those obtained from the Region II equations for a specified active-channel width. The slopes of the Region I regression lines are parallel to those of the statewide regression lines at a higher estimated discharge. The Region II regression lines have steeper slopes relative to the Region I and statewide regression lines but at a lower estimated discharge. Figures 8A and 9A illustrate the relation of the Region I, bankfull regression equations for 2- and 100-year recurrence-interval discharges, respectively. Tests performed using STATIT procedure REGGRP (Statware, Inc., 1990, p. 6-32 - 6-36) indicated that there were statistically significant differences in the slopes and intercepts of the Region I and Region II regression lines for both the bankfull and active-channel equations.

The paired-t test was used to test whether design-flood discharge estimates obtained by both the bankfull and active-channel regression equations for the same gaging station were significantly different at the 95-percent level of significance. The paired-t test was applied using STATIT procedure HYPOTH (Statware, Inc., 1990, p. 3-21 - 3-23). For table 3, discharge estimates for 111 stations were not significantly different for all design-flood recurrence intervals. For table 4, discharge estimates for 78 stations were significantly different for the 2-year recurrence interval, but estimates were not significantly different for the 5-year to 100-year recurrence intervals. For table 5, discharge estimates for 33 stations were not significantly different for all design-flood recurrence intervals.

The application of the channel-geometry regression equations listed in tables 4 and 5 for a stream site are determined by two factors, and the application of the channel-geometry equations listed in table 3 are determined only by the second factor. First, the stream site is located in figure 2 to determine whether Region I or Region II equations apply. The user may be

faced with a dilemma if design-flood discharges are to be estimated for a stream site located within the shaded transitional zone or for a stream that crosses regional boundaries. The discharges could be estimated using both the Region I and II equations and hydrologic judgment used to select the most reasonable design-flood estimate, or a weighted average based on the proportion of drainage area within each region could be applied. The most reasonable alternative to resolving this dilemma may be to use the statewide equations listed in table 3 because they preclude regional subjectivity and the majority of statewide design-flood estimates calculate between Region I and Region II estimates.

Second, the stream site is inspected to determine whether the stream was channelized. If evidence of channelization is not found, then the bankfull equations are applicable (the first set of equations listed in tables 3, 4, and 5); if evidence of channelization is found, then the active-channel equations may be applicable for stabilized channels (the second set of equations listed in tables 3, 4, and 5). Appendix C (at end of this report) outlines a procedure for identifying channelized streams and describes the stabilization conditions for which channel- geometry measurements of channelized streams are applicable.

Examples of Equation Use-Examples 2-4

Example 2.--Use a regional, channel- geometry equation to estimate the 100-year peak discharge for the discontinued Black Hawk Creek at Grundy Center crest-stage gaging station (station number 05463090; map number 73, fig. 2), located in Grundy County, at a bridge crossing on State Highway 14, at the north edge of Grundy Center, in the NW1/4 sec. 7, T. 87 N., R. 16 W.

Step 1. The appropriate regional equation is determined on the basis of which hydrologic region the stream site is located in and whether the stream has been channelized. This gaging station is located in Region I, and an inspection of the USGS l:100,000-scale Grundy County map and a visit to the site show no evidence of channelization. Therefore the 100-year bankfull equation for Region I, listed in the first set of


100,000

10,000

1,000o

O

COccUJ Q.

tLUJ

CD

OzUJ

IoCOo

100

10

O = 2-year recurrence-interval discharge

BFW= Bankfull width

Region I O =455 8FVY1

.45

Region I streamflow-gaging stations, n - 78

10 100

BANKFULL WIDTH, IN FEET

1,000

< 100,000ccUJ

o

ccD O UJ CC CC

UJ

10,000

1 ,000

100

10

B.O = 2-year recurrence-interval discharge

ACW= Active-channel width

Statewide'Q =38.£

2

Region II . Q =7.QQACW

2


+ Region II streamflow-gaging stations, n - 37

10 100

ACTIVE-CHANNEL WIDTH, IN FEET

1,000

Figure 8. Relation between 2-year recurrence-interval discharge and channel width for (A) bankfull and (B)active-channel width regression equations.


1,000,000

100,000

o

ouu(OIT LU 0.I- LU LU ULO CDD O

LU O IT

1 O

10,000

1,000

100

A.

Q = 100-year recurrence-interval discharge100 ' *

BFW= Bankfull width

Region I-Q =1228FW

100


10 100

BANKFULL WIDTH. IN FEET

1,000

1,000,000ccLU

LUoLU

en 100,000D O LU ITCC< LU

6o 10,000

1,000

100

B.

Q = 100-year recurrence-interval discharge 100 7 *

ACW= Active channel width

RegionQ =566

100

StatewideQ =458

100

n

streamflow-gaging 120

Region II streamflow-gaging stations, n,37

10 100

ACTIVE-CHANNEL WIDTH, IN FEET

1,000

Figure 9. Relation between 100-year recurrence-interval discharge and channel width for (A) bankfull and(B) active-channel width regression equations.


equations in table 4, is determined to be the most applicable. The only channel-geometry characteristic used for the Region I bankfull equation is the bankfull width (BFW). Appendix C describes the procedure for conducting this channel-geometry measurement.

Step 2. Three bankfull widths measuring 52, 50, and 52 ft, measured along a straight channel reach about 0.75-1.0 mi downstream of the gaging station, were used to calculate an average bankfull width (BFW) of 51 ft. Figure IB shows the bankfull reference level at one of these channel measurement sections.

Step 3. The 100-year flood estimate for the Region I bankfull equation (table 4) is calculated as

Q100 = 122 (BFW)1-09, = 122 (51)1-09,

= 8,860 ft3/s.

Example 3.-Use a regional channel- geometry equation to estimate the 50-year peak discharge for the Big Cedar Creek near Varina continuous-record gaging station (station number 05482170; map number 108, fig. 2), located in Pocahontas County, at a bridge crossing on County Highway N33, 5.5 mi northeast of Varina, in the NE1/4 sec. 24, T. 91 N., R. 34 W.

Step 1. This gaging station is located in Region II, and an inspection of the USGS l:100,000-scale Pocahontas County map and a visit to the site show evidence of channelization. Therefore, the 50-year active-channel equation for Region II, listed in the second set of equations in table 5, is determined to be the most applicable. Features that are character istic of channelized streams are illustrated in figure ID, which shows the straightened and leveed channel reach downstream of the gage. The only channel-geometry characteristic used for the Region II active-channel equation is the active-channel width (ACW). Appendix C describes the procedure for conducting this channel-geometry measurement.

Step 2. Three active-channel widths measuring 25.6, 25.3, and 24.2 ft, measured along a straight channel reach about 0.25-0.5 mi downstream of the gaging station, were used to calculate an average active-channel width

(ACW) of 25.0 ft. Figure ID shows the approximate active-channel reference level for the channel reach measured to calculate an average active-channel width.

Step 3. The 50-year flood estimate for the Region II active-channel equation (table 5) is calculated as

Q50 = 59.5= 59.5 (25.0) 1' 14 , = 2,330 ft3/s.

Example 4.-Use a statewide channel- geometry equation in table 3 to estimate the 100-year peak discharge for the gaging station used in example 2.

Step .1. Because a statewide equation is to be used and no evidence of channelization is evident, as determined in example 2, the 100-year bankfull equation listed in the first set of equations in table 3 is applicable. Bankfull width (BFW) and bankfull depth (BFD) are the channel-geometry characteristics used for this equation. Appendix C describes the procedure for conducting these channel-geometry measurements.

Step 2. The average bankfull width (BFW) calculation of 51 ft for this stream channel is outlined in example 2.

Step 3. The average bankfull depth (BFD) for this stream channel was calculated to be 6.0 ft. The bankfull depth measurements used to determine this average are listed in the "Bankfull-Depth (BFD) Measurements" section of Appendix C, and they are illustrated in figure 10.

Step 4. The 100-year flood estimate for the statewide bankfull equation (table 3) is calculated as

Q100 = 104 (BFW)0-766 (BFD)0-747, = 104(51)°-766 (6.0)0-747,

= 8,060 ft3/s.

Examples 2 and 4 illustrate the use of bankfull measurements in computing 100-year flood estimates for this gaging station using regional and statewide multiple-regression equations. The regional estimate was determined to be 8,860 ft3/s, and the statewide estimate was determined to be 8,060 ft3/s.


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APPLICATION AND RELIABILITY OF FLOOD-ESTIMATION METHODS

The regression equations developed in this study for both the drainage-basin and channel-geometry flood-estimation methods apply only to streams in Iowa where peak streamflow is not affected substantially by stream regulation, diversion, or other human activities. The drainage-basin method does not apply to basins in urban areas unless the effects of urbanization on surface-water runoff are negligible. The channel-geometry method does not apply to channels that have been altered substantially from their stabilized conditions by human activities, as outlined in Appendix C.

Limitations and Accuracy of Equations

The applicability and accuracy of the drainage-basin and channel-geometry flood- estimation methods depend on whether the drainage-basin or channel-geometry character istics measured for a stream site are within the range of the characteristic values used to develop the regression equations. The acceptable range for each of the drainage-basin characteristics used to develop the statewide equations (table 2) are tabulated as maximum and minimum values in table 6. Likewise, the acceptable range for each of the channel- geometry characteristics used to develop the statewide and regional equations (tables 3-5) also are tabulated as maximum and minimum values in table 6. The applicability of the drainage-basin and channel-geometry equations is unknown when the characteristic values associated with a stream site are outside of the acceptable ranges.

The standard errors of estimate and average standard errors of prediction listed in tables 2-5 are indexes of the expected accuracy of the regression-equation estimates in that they provide measures of the difference between the regression estimate and the Pearson Type-Ill estimate for a design-flood recurrence interval. If all assumptions for applying regression techniques are met, the difference between the regression estimate and the Pearson Type-Ill estimate for a design-flood recurrence interval

will be within one standard error approximately two-thirds of the time.

The standard error of estimate is a measure of the distribution of the observed annual-peak discharges about the regression surface (Jacques and Lorenz, 1988, p. 17). The average standard error of prediction includes the error of the regression equation as well as the scatter about the equation (Hardison, 1971, p. C228). Although the standard error of estimate of the regression gives an approximation of the standard error of peak discharges, the average standard error of prediction provides more precision in the expected accuracy with which estimates of peak discharges can be made. The average standard error of prediction is estimated by taking the square root of the PRESS statistic mean. Because the standard errors of estimate and average standard errors of prediction are expressed as logarithms (base 10), they are converted to percentages by methods described by Hardison (1971, p. C230).

The average standard errors of prediction for the regression models ranged as follows: statewide drainage-basin equations, 38.6 to 50.2 percent (table 2); statewide channel-geometry bankfull equations, 41.0 to 60.4 percent (table 3); statewide channel-geometry active-channel equations, 61.9 to 68.4 percent (table 3); Region I channel-geometry bankfull equations, 33.8 to 48.3 percent (table 4); Region I channel- geometry active-channel equations, 43.0 to 53.0 percent (table 4); Region II channel-geometry bankfull equations, 30.3 to 52.1 percent (table 5); and Region II channel-geometry active- channel equations, 59.7 to 70.0 percent (table 5).

The average equivalent years of record represents an estimate of the number of years of actual streamflow record required at a stream site to achieve an accuracy equivalent to each respective drainage-basin or channel-geometry design-flood discharge estimate. The average equivalent years of record as described by Hardison (1971, p.C231-C233) is a function of the standard deviation and skew of the observed annual-peak discharges at the gaging stations analyzed for each respective regression equation, the accuracy of the regression equation, and the recurrence interval of the design flood. The average equivalent years of record for a design flood with a recurrence


Table 6. Statistical summary for selected statewide drainage-basin and channel-geometry characteristics, and for selected regional channel-geometry characteristics at streamflow-gaging

stations in Iowa

[CDA, contributing drainage area, in square miles; RR, relative relief, in feet per mile; DF, drainage frequency, in number of first-order streams per square mile; TTF, 2-year, 24-hour precipitation inten sity, in inches; BFW, bankfull width, in feet; BFD, bankfull depth, in feet; ACW, active-channel width,

in feet]

Statistic

Statistic

Statewide drainage-basin characteristics

CDA RR DF TTF

MaximumMinimumMeanMedianNo. of sites

1,060.338

20980.7

164

48.71.576.484.45

164

2.95.043.520.510

164

3.262.823.113.14

164

Statewide channel-geometry characteristics

BFW BFD ACW


5239.6

11082.7

111

17.11.77.06.7

111

5104.2

77.049.8

157

Regional channel-geometry characteristics

Region I

Statistic


BFW

5239.6

10671.078

ACW

5104.2

73.746.1

120

BFW

36119.3

12010633

Region IE

BFD

12.52.06.66.6

33

ACW

3396.9

87.473.337

interval of T-years is calculated as (Hardison, 1971, p. C231)

SE.(13)

where E is the average equivalent years of record, in years;

r is a factor that is a function of the

mean weighted skew coefficient of the logarithms (base 10) of the observed annual-peak discharges at the gaging stations used in each respective regression-model data set and the recurrence interval relating the standard error of a T-year peak discharge to the index of variability (s) and the number of observed annual-

APPUCATION AND RELIABILITY OF FLOOD-ESTIMATION METHODS 33

SEr

peak discharges;

is an index of variability equal to the mean standard deviation of the logarithms (base 10) of the observed annual-peak discharges at the gaging stations used in each respective regression-model data set; and

is the average standard error of prediction, in log units (base 10), estimated using the Press statistic.

Several of the primary drainage-basin characteristics used in the regression equations listed in table 2 are map-scale dependent. Use of maps of scales other than the scales used to develop the equations may produce results that do not conform to the range of estimation accuracies listed for the equations in table 2. The scale of map to use for manual measurements of each primary drainage-basin characteristic is outlined in Appendix A and Appendix B.

An additional constraint in the application and reliability of the channel-geometry characteristic equations is the requirement to obtain onsite measurements of bankfull or active-channel width, and possibly bankfull depth. Training and experience are required to properly identify the bankfull and active- channel features in order to make these measurements. The variability in making these measurements can be large, even among experienced individuals. As reported by Wahl (1976), based on a test conducted in northern Wyoming, the standard error in estimated discharge due to variation in width measure ments alone was about 30 percent (0.13 log unit). Variation in bankfull-depth measure ments probably would increase this standard error in estimated discharge. Wahl (1976) also noted an average bias with respect to the mean channel width of about 14 percent (0.06 log unit). A truer total standard error, in log units, for a channel-geometry discharge estimate is calculated by Wahl (1984, p. 63) as the square root of the sums of the squares of the errors of the regression equation and of the variation and average bias in width measurements. Using the standard error of estimate for the Region I,

100-year flood bankfull equation (table 4) and assuming the standard errors for measuring channel width reported by Wahl (1976), the

true standard error = [(0.192)2 + (0.13)2 + (0.06)2] ° 5 , = 0.240.

This yields an average standard error of 59.6 percent compared to 46.4 percent for the regression equation alone. Wahl (1984, p. 64) notes that the variability of the measurements collected in the Wyoming test probably is greater than normally would be encountered in applying channel-geometry measurements in a particular hydrologic area. Sites in the Wyoming test were chosen for their diversity, and they ranged from ephemeral streams in a nearly desert environment to perennial streams in a high mountain environment.

Despite the limitations associated with the channel-geometry method, the equations presented in this report are considered to be useful as a corroborative flood-estimation method with respect to the drainage-basin method. The channel-geometry equations are applicable to all unregulated, stabilized stream channels in the State, whereas the drainage- basin equations are applicable only to stream sites with drainage areas less than 1,060 mi2 . Although the error of measurement may be larger for channel-geometry characteristics than for drainage-basin characteristics, the variability of channel-geometry measurements made in Iowa are assumed to be not as great as reported by Wahl (1984) for the Wyoming test. An additional advantage in utilizing the channel-geometry method is that design-flood discharge estimates obtained from each flood-estimation method can be used to calculate a weighted average as described in the following section.

Weighting Design-Flood Discharge Estimates

Design-flood discharges determined using both the drainage-basin and channel-geometry flood-estimation methods are presumed to be independent from each other. Each flood- estimation method thus can be used to verify results from the other; when design-flood discharge estimates are independent, the independent estimates can be used to obtain a weighted average (IACWD, 1982, p. 8-1).


Calculation of Estimates Example of Weighting-Example 5

Design-flood discharge estimates calculated using both the drainage-basin and channel- geometry flood-estimation methods can be weighted inversely proportional to their variances to obtain a weighted average that has a smaller variance than either of their individual estimates. According to the Interagency Advisory Committee on Water Data (IACWD, 1982), the weighted average is calculated as

QT (dbcg),(14)

where QT(dbcg)is *-ne weighted average design-flood discharge, in cubic feet per second, for a selected T-year recurrence interval;

is the drainage-basin regression- equation design-flood discharge, in cubic feet per second;

is the standard error of estimate, in log units (base 10), of the channel-geometry regression equation (tables 3-5);

is the channel-geometry regression-equation design-flood discharge, in cubic feet per second; and

SE.(eg)

^T(cg)

SE,(db) is the standard error of estimate, in log units (base 10), of the drainage-basin regression equation (table 2).

The standard error of estimate log units (base 10), of the weighted average design-flood discharge estimate QT(dbcg) can calculated as

SE (dbcg)

0.5

(15)

Example 5.-Use the 100-year drainage- basin and channel-geometry regression estimates (table 8) to obtain a weighted average, 100-year peak-discharge estimate for the discontinued Black Hawk Creek at Grundy Center crest-stage gaging station (station number 05463090; map number 73, figs. 1 and 2).

The 100-year flood estimate calculated for this gaging station using the drainage-basin equation is 7,740 fl3/s (listed as method GISDB in table 8), and the standard error of estimate, in log units (base 10), for this equation is 0.198 (table 2). The 100-year flood estimate calculated for this gaging station using the Region I, bankfull channel-geometry equation is 8,860 fl3/s (listed as method BFRI in table 8), and the standard error of estimate, in log units, for this equation is 0.192 (listed in the first set of equations in table 4). The weighted average, 100-year flood estimate is calculated using equation 14 as

Q 100(db) (db)) 2

r 100(dbcg)

_ 7, 740 (0.192) 2 + 8.860 (0.198) 2

(0.198) 2 + (0.192) 2

= 8,320 ft3/s.

The standard error of estimate for this weighted average, 100-year peak-discharge estimate is calculated using equation 15 as

SE (dbcg)

(SE 0.5

(Q.198) 2 (Q.192) 2 "| 0- 5

.(0.198) 2 +(0.192) 2J = 0.138 log units or 32.6 percent.

Weighting Design-Flood Discharge Estimates for Gaged Sites

Weighted design-flood discharges are estimated for a gaged site based on either the Pearson Type-Ill estimate and regression- equation estimates from both the drainage-

APPLICATION AND RELIABILITY OF FLOOD-ESTIMATION METHODS 35

basin and channel-geometry flood-estimation methods or on the Pearson Type-Ill estimate and only one of the regression-equation estimates. The design-flood discharge estimate is a weighted average of these values in which the Pearson Type-Ill estimate for the gaged site is weighted by the effective record length (ERL) at the gaged site, and the regression-equation estimates are weighted by the average equivalent years of record associated with their respective regression equations.

Calculation of Estimates

The- weighted design-flood discharge estimate for a gaged site as outlined by the Interagency Advisory Committee on Water Data (IACWD, 1982, p. 8-1 - 8-2) is calculated as

Et(eg)

regression-equation design-flood discharge for a gaging station, in cubic feet per second, (listed as either method BFRI, ACRI, ACRII, or BFRII in table 8); and

is the average equivalent years of record for the channel-geometry regression equation used to determine Q-r(gcg) (table 4 or 5).

If both the drainage-basin regression- equation estimate QT(gdb) anc^ the channel- geometry regression-equation estimate QT(gcg) are not available for a gaged site, then equation 16 used to calculate the weighted design-flood discharge estimate QT(w) *s simplified to the

Q

where

ERL

is weighted design-flood discharge for a gaging station, in cubic feet per second, for a selected T-year interval;

recurrence

^T(gcg)

is the Pearson Type-Ill design-flood discharge for a gaging station, in cubic feet per second, as determined by the analysis of the observed annual-peak discharge record (listed as method B17B in table 8);

is the effective record length for a gaging station, in years, representing the Q-r(g) analysis (table 8);

*s the drainage-basin regression- equation design-flood discharge for a gaging station, in cubic feet per second, (listed as method GISDB in table 8);

is the average equivalent years of record for the drainage-basin regression equation used to determine QT(gdb) (table 2);

*s the channel-geometry

weighting of two estimates based on Qx(g) and ERL and either Qrtgdb) and E(db) or QT(gcg) an^ j£(cg). An example of weighting a gaged site with only one regression-equation estimate is illustrated in "Example 7."

By including both the drainage-basin and channel-geometry regression-equation esti mates, or only one of these estimates, with the computed Pearson Type-Ill estimate for a gaged site, design-flood histories for a relatively long period of time are incorporated into the weighted estimate for the gaged site and tend to decrease the time-sampling error (Choquette, 1988, p. 41). Climatic conditions during a short gaged period of record often are not indicative of the longer term climatic variability associated with a particular gaging station. Such time-sampling error may be particularly large when the observed gaged period of record represents an unusually wet or dry climatic cycle compared to the longer term average climatic conditions. Time-sampling error thus is minimized for a gaging station by weighting the design-flood discharge estimate

Examples of Weighting-Examples 6-7

Example 6. -Calculate a weighted 100-year peak-discharge estimate for the discontinued Black Hawk Creek at Grundy Center crest-stage gaging station (station number


05463090; map number 73, figs. 1 and 2). An inspection of table 8 lists regression-equation estimates for both the drainage-basin and channel-geometry flood-estimation methods. The 100-year Pearson Type-Ill estimate is 8,320 ft3/s, and the effective record length is 24

years (table 8). The 100-year drainage-basin regression estimate is 7,740 ft3/s (table 8), and the average equivalent years of record for this regression equation is 11.5 (table 2). The 100-year Region I, bankfull channel-geometry regression estimate is 8,860 ft3/s (table 8), and the average equivalent years of record for this regression equation is 16.1 (listed in the first set of equations in table 4). The weighted 100-year flood estimate for this gaging station is calculated using equation 16 as

Estimating Design-Flood Discharges for an Ungaged Site on a Gaged Stream

Design-flood discharges for an ungaged site on a gaged stream can be estimated if the total drainage area of the ungaged site is between 50 and 150 percent of the total drainage area of the gaged site by an adjustment procedure described by Choquette (1988, p. 42-45) and Koltun and Roberts (1990, p. 6-8). This procedure uses flood-frequency information from the Pearson Type-Ill and regression-equation estimates at the gaged site to adjust the regression-equation estimate at the ungaged site.

<«100(gdb)>

100(wg)

(8, 320) (24) + (7, 740) (11.5) + (8, 860) (16.1)

24+ 11.5 +16.1

= 8,360 ftS/s.

Example 7.-Calculate a weighted 50-year peak-discharge estimate for the discontinued Fox River at Bloomfield gaging station (station number 05494300; map number 133, fig. 1), located in Davis County, at a bridge crossing on a county highway, about 0.5 mi north of Bloomfield, in the SE1/4 sec. 13, T. 69 N., R. 14 W. Table 8 lists a regression-equation estimate for only the drainage-basin flood-estimation method. The 50-year Pearson Type-Ill estimate is 10,600 fl3/s, and the effective record length is 21 years (table 8). The flood-frequency curve developed from the Pearson Type-Ill analysis for this gaging station is shown in figure 3. The 50-year drainage-basin regression estimate is 7,600 fl3/s (table 8), and the average equivalent years of record for this regression equation is 9.5 (table 2). The weighted 50-year flood estimate for this gaging station is calculated using a simplified version of equation 16 as

Calculation of Estimates

The regression-equation estimate for the ungaged site is determined as one of the following: (1) the weighted average QT(dbcg) calculated from both the drainage-basin and channel-geometry regression-equation esti mates using equation 14 or (2) the regression- equation estimate of QT(db) or ^T(cg) calculated from either one of these flood-estimation methods. The calculation for this adjustment procedure is

where QT(au)

Q50(wg) ru)

(10,600) (21) + (7,600) (9.5)

21 + 9.5

< 9,670 ft3/s.

is the adjusted design-flood discharge for the ungaged site, in cubic feet per second, for a selected T-year recurrence interval;

is the regression design-flood discharge for the ungaged site, in cubic feet per second, determined as one of the following: (1) the weighted average of both the drainage-basin and channel-

APPUCATION AND RELIABILITY OF FLOOD-ESTIMATION METHODS 37

AF

AF =Q

geometry regression-equation estimates Qx(dbcg) (equation 14); (2) only the drainage-basin regression-equation estimate QT(db)J or (3) only the channel- geometry regression-equation estimate QT(cg>

is the adjustment factor for the gaged site and is calculated as

(18)T(rg)

where QiCwg) is the weighted design-flood discharge for the gaged site, in cubic feet per second, as defined by equation 16;

*s the regression design-flood discharge for the gaged site, in cubic feet per second, determined as one of the following: (1) the weighted average of both the drainage-basin and channel- geometry regression-equation estimates QT(dbcg)» as defined by equation 14; (2) only the drainage-basin regression- equation estimate QT(db)J or (3) only the channel-geometry regression-equation estimate

ATDA is the absolute value of the difference between the total drainage area of the gaged site (TDAg) and the total drainage area of the ungaged site; and

TDAg is the total drainage area of the gaged site, in square miles, listed as the published drainage area in table 9.

This procedure (1) adjusts the regression-equation estimate for the ungaged site QT(FU) by the rati° AF when the total drainage area of the ungaged site equals the total drainage area of the gaged site TDAg and (2) prorates the adjustment to 1.0 as the difference in total drainage area between the gaged site and the ungaged site approaches

either 0.5 or 1.5 of the total drainage area of the gaged site. In other words, when the total drainage area of the ungaged site is 50 percent larger or 50 percent smaller than the total drainage area of the gaged site, no adjustment is applied to the regression-equation estimate for the ungaged site

Example of Estimation Method-Example 8

Example 8.~Determine the 50-year peak- discharge estimate for an ungaged site on Otter Creek, located on the Osceola and Lyon County line, at a bridge crossing on County Highway L26, 4.75 mi southwest of Ashton, in the SW1/4 sec. 31, T. 98 N. f R. 42 W. Because a crest-stage gaging station is located on this stream, Otter Creek near Ashton (station number 06483460; map number 139, fig. 1), the 50-year recurrence interval regression-equation estimate calculated for the ungaged site can be adjusted by the weighted 50-year flood-discharge estimate calculated for the gaged site. Estimating the adjusted 50-year peak discharge for the ungaged site Qso(au) (equation 17) involves four steps.

Step 1. A regression-equation estimate Qso(ru) (equation 17) is calculated for the ungaged site. Both drainage-basin and channel- geometry flood-estimation methods could be used to calculate a weighted average estimate QsO(dbcg) (equation 14) for the regression estimate (Qso(ru)) or only one of these flood-estimation methods could be used to calculate the regression-equation estimate (Qs(Xru))- For this example, only the statewide drainage-basin estimate (Qso(dbP (table 2) will be used for the 50-year recurrence interval regression-equation estimate (Qso(ru)) at the ungaged site because channel-geometry measurements were not collected for calculating a channel-geometry estimate (Q50(Cg))-

(A). The characteristics used in the drainage-basin equation (table 2) are contributing drainage area (CDA\ relative relief (RR), drainage frequency (DF)y and 2-year, 24-hour precipitation intensity (TTF). The primary drainage-basin characteristics used in this equation are total drainage area (TDA), noncontributing drainage area (NCDA), basin relief (BR), basin perimeter (BP), number of first-order streams (FOS), and 2-year, 24-hour


precipitation intensity (7TF). These primary drainage-basin characteristic measurements and the scale of maps to use for each manual measurement are described in Appendix A and Appendix B.

(B). The topographic maps used to delineate the drainage-divide boundary for this ungaged site are the DMA 1:250, 000-scale Fairmont topographic map and the USGS l:100,000-scale Osceola County map. Contributing drainage area (CDA) is calculated from the primary drainage-basin characteristics total drainage area (TDA) and noncontributing drainage area (NCDA). The drainage-divide boundary for this basin is delineated on the l:250,000-scale map, and the total drainage area (TDA) for the ungaged site is listed in Larimer (1957, p. 313) as 120 mi2 . The total drainage area published for the gaged site, Otter Creek near Ashton (station number 06483460; map number 139, fig. 1), is 88.0 mi2 (table 9). Because the total drainage area of the ungaged site is 136.4 percent of the total drainage area of the gaged site and within the 50- and 150-percent limits for application, the adjustment procedure is determined to be applicable to the ungaged site. Inspection of the l:100,000-scale map does not show any noncontributing drainage areas within the drainage-divide boundary of this basin. The contributing drainage area (CDA) for the ungaged site is calculated using equation 10 as

= TDA-NCDA,

= 120 - 0,

= 120 mi2.

(C). Relative relief (RR) for the ungaged site is calculated from the primary drainage-basin characteristics basin relief (BR) and basin perimeter CBP). The difference between the highest elevation contour and the lowest interpolated elevation in the basin measured from the l:250,000-scale topographic map gives a basin relief of 286 ft. The drainage-divide boundary delineated on the l:250,000-scale topographic map is used to measure the basin perimeter, which is 57.8 mi. Relative relief (RR) is calculated using equation 11 as

RR- BR RR ~BP'

286

= 4.95 ff mi.

(D). Drainage frequency (DF) for the ungaged site is calculated from the primary drainage-basin characteristics number of first-order streams (FOS) and contributing drainage area (CDA). A total of 57 first-order streams are counted within the drainage-divide delineation for the ungaged site on the l:100,000-scale topographic map. Drainage frequency (DF) is calculated using equation 12 as

DF = FOS CDA'

_ 57 120'

= 0.475 first-order streams/mi2.

(F). The 2-year, 24-hour precipitation intensity (7TF) for the ungaged drainage basin is determined from figure 5. Because the drainage-divide boundary of this ungaged site overlies two of the 2-year, 24-hour precipitation intensity polygons shown in figure 5, a weighted average for the basin is computed using equation 19 as outlined in Appendix B. According to figure 5, approximately 60 percent of the total drainage area (TDA) for the ungaged site is located within the polygon labeled as 2.85 in., and approximately 40 percent of the

total drainage area is located within the polygon labeled as 2.95 in. The weighted average for the 2-year, 24-hour precipitation intensity (7TF) is calculated using equation 19 (Appendix B) as

TTF= (A^TTF^ + CAaMITFa), = (0.60) (2.85)+ (0.40) (2.95), = 2.89 in.

(G). The 50-year flood estimate for the ungaged site using the drainage-basin equation (table 2) is calculated as

Qso = 231 (CDA)0-694 (flfl)0'656 (DF)0-401 (TTF- 2.5)°-491 ,

= 231 (120)0' 694 (4.95)0' 656 (0.475)0-401 (2.89 - 2.5)°-491 ,

= 8,550 ft3/s.

Because Q50 = QsO(db)> and Qso(ru) (equation 17 ) = Qso(db) in this example, then Q5Q(rVL) = 8,550 ft3/s.

APPUCATION AND REUABILITY OF FLOOD-ESTIMATION METHODS 39

Step 2. The weighted 50-year peak discharge for the gaged site Qso(Wg) (equation 16) is estimated next. Because table 8 lists both the drainage-basin and channel-geometry regression-equation estimates for this gaged site, Otter Creek near Ashton (station number 06483460, map number 139, fig. 1), the weighted estimate will be based on the Pearson Type-Ill estimate and both of these regression-equation estimates.

The 50-year Pearson Type-Ill estimate is 11,100 ft3/s, and the effective record length is 39 years (table 8). The 50-year drainage-basin regression estimate is 6,710 ft3/s (listed as method GISDB in table 8), and the average equivalent years of record for this regression equation is 9.5 (table 2). The 50-year Region I, active-channel channel-geometry regression estimate is 9,260 ft3/s (listed as method ACRI in table 8), and the average equivalent years of record for this regression equation is 8.9 (listed in the second set of equations in table 4). The weighted 50-year flood estimate for the gaged site is calculated using equation 16 as

Q«?50(g)> «?50(gdb)

50 (wg)

9,260 ft3/s (listed as method ACRI in table 8), and the standard error of estimate, in log units, for this equation is 0.188 (listed in the second set of equations in table 4). The weighted average, 50-year flood estimate for the gaged site is calculated using equation 14 as

QQ '50(eg)

50(dbcg)

6. 710 (0.188) 2 + 9, 260 (0.185) 2

(0.185) 2 +(0. 188) 2= 7,960 ft3/s.

Because Q5(xdbcgl= ^50(rg) m = 7,960 ft3/s.

example, then

Step 4. The final step adjusts the 50-year recurrence interval regression-equation est imate of 8,550 ft3/s (Q5Q(ruP calculated for the ungaged site by the 50-year recurrence interval information determined for the gaged site. The adjusted 50-year flood estimate for the ungaged site Qsjxau) is calculated using equations 17 and 18 as

(11.100) (39) + (6. 710) (9.5) + (9. 260) (8.9) 39 + 9.5 + 8.9

=10,100ft3/s.

Step 3. The regression-equation estimate for the gaged site Qsofrg) (equation 18) is determined next. Because table 8 lists both the drainage-basin and channel-geometry regression estimates for this gaged site, Otter Creek near Ashton, the weighted average of these regression estimates Qsocdbcg) (equation 14) is calculated to determine the regression estimate

The 50-year flood estimate calculated for this gaging station using the drainage-basin equation is 6,710 ft3/s (listed as method GISDB in table 8), and the standard error of estimate, in log units (base 10), for this equation is 0.185 (table 2). The 50-year flood estimate calculated for this gaging station using the Region I, active-channel channel-geometry equation is

Q50(au) f50(ru)(2&TDA (-TDA- }(AF-

AlDA is the absolute value of the difference between the total drainage area of the gaged site (88.0 mi2) and the total drainage area of the ungaged site (120 mi2),

ATDA = 32.0 mi2; TDAg = 88.0 mi2;

QAF = 50(wg) ,

AF = iai00 7,960 '

AF = 1.27;

Q50 ( au) =8,55o[l.27-(

= 9,180 ft3/s.

(2) (32.0) 88.0

(1.27-1)],

This adjustment procedure has increased the 50-year recurrence interval regression- equation estimate for the ungaged site Qsod-u) by


107.4 percent based on the 50-year recurrence interval information determined for the gaged site upstream of this ungaged site.

SUMMARY AND CONCLUSIONS

Drainage-basin and channel-geometry equations are presented in this report for estimating design-flood discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years at stream sites on rural, unregulated streams in Iowa. The equations were developed using ordinary least-squares and weighted least-squares multiple-regression techniques. Statewide equations were developed for the drainage-basin flood-estimation method and statewide and regional equations were developed for the channel-geometry flood- estimation method. The drainage-basin equations are applicable to stream sites with drainage areas less than 1,060 mi2 , and the channel-geometry equations are applicable to stabilized stream channels in Iowa.

Flood-frequency curves were developed for 188 continuous-record and crest-stage gaging stations on unregulated rural streams in Iowa. Pearson Type-Ill estimates of design-flood discharges are reported for these gaging stations.

Regression analyses of Pearson Type-Ill design-flood discharges and selected drainage- basin characteristics, quantified using a geographic-information-system (GIS) proce dure, were used to develop the statewide drainage-basin flood-estimation equations. The significant characteristics identified for the drainage-basin equations included contributing drainage area; relative relief; drainage frequency; and 2-year, 24-hour precipitation intensity. The regression coefficients for these equations indicated an increase in design-flood discharges with increasing magnitude in the values of each drainage-basin characteristic. The average standard errors of prediction for the drainage-basin equations ranged from 38.6 to 50.2 percent.

Techniques on how to make manual measurements from topographic maps for the primary drainage-basin characteristics used in the regression equations are presented along with examples. Several of the primary

drainage-basin characteristics used in the regression equations are map-scale dependent. Use of maps of scales other than the scales used to develop the equations may produce results that do not conform to the range of estimation accuracies listed for the equations.

Regression analyses of Pearson Type-Ill design-flood discharges and selected channel-geometry characteristics were used to develop both statewide and regional channel- geometry equations. On the basis of a geographic bias identified from the statewide regression residuals, two channel-geometry hydrologic regions were defined for Iowa relative to the Des Moines Lobe landform region. The significant channel-geometry characteristics identified for the statewide and regional regression equations included bankfull width and bankfull depth for natural channels unaffected by channelization, and active- channel width for stabilized channels affected by channelization. The regression coefficients for the statewide and regional channel- geometry equations indicated an increase in design-flood discharges with increasing magnitude in the values of each channel- geometry characteristic. The average standard errors of prediction for the statewide regression equations ranged from 41.0 to 68.4 percent and for the regional regression equations from 30.3 to 70.0 percent. The regional channel-geometry regression equations provided an improved estimation accuracy compared to the statewide regression equations, with the exception of the Region II active-channel regression equations developed for design floods having recurrence intervals of 25, 50, and 100 years. Guidelines for measuring the channel-geometry character istics used in the statewide and regional regression equations are presented along with examples.

Procedures for applying the drainage-basin and channel-geometry regression equations vary and depend on whether the design-flood discharge estimate is for a site on an ungaged stream, an ungaged site on a gaged stream, or a gaged site. When both a drainage-basin and a channel-geometry regression-equation estimate are available for a stream site, a procedure is presented for determining a weighted average of the two flood estimates. The procedure for estimating a design-flood discharge for an

SUMMARY AND CONCLUSIONS 41

ungaged site on a gaged stream is based on information from the Pearson Type-Ill estimate for the gaged site, and on information from either both flood-estimation methods, or from only one of the methods. At a gaged site, a weighted design-flood discharge is estimated from the Pearson Type-Ill estimate, and from either both flood-estimation methods, or from only one of the methods. Examples are provided for each of these procedures.

The drainage-basin and channel-geometry flood-estimation methods presented in this report each measure characteristics that are presumed to be independent of each other. The drainage-basin flood-estimation method is based on measurements of morphologic and climatic characteristics that are related to how water flows off the land. The drainage-basin method measures the varying flood potential at stream sites as defined by differences in basin size, topographic relief, stream development, and precipitation. The channel-geometry flood-estimation method, in contrast, is based on measurements of channel morphology that are assumed to be a function of streamflow discharges and sediment-load transport. The channel-geometry method measures the variability of floods that have actually occurred as defined by differences in channel width and depth.

The drainage-basin flood-estimation method developed in this study is similar to the regional flood-estimation method developed in a previous study because both methods estimate flood discharges on the basis of morphologic relations. While the standard errors of estimate for the drainage-basin equations in this study appear to be higher, a direct comparison cannot be made because of the different methodologies used to develop the equations.

The statewide drainage-basin and statewide channel-geometry regression equations presented in this report provide flood- estimation methods that minimize the subjectivity in their application to the ability of the user to measure the characteristics. Although the user of the regional channel- geometry equations may still encounter a dilemma when a stream site is located within the transitional zone or when a stream crosses regional boundaries, application of the

statewide channel-geometry equations may be utilized to preclude the regional subjectivity associated with estimating a design-flood discharge in this situation. Despite the greater variability in the error of measurement associated with the channel-geometry characteristics, the channel-geometry equations presented in this report are considered to be useful as a corroborative flood-estimation method with respect to the drainage-basin method.

The estimation accuracy of the drainage- basin regression equations possibly could be improved if drainage-basin characteristics were quantified from larger scale data. The drainage-basin characteristics quantified by the GIS procedure were limited to the 1:250,000- and l:100,000-scale digital cartographic data currently available for Iowa.

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Melton, M.A., 1957, An analysis of the relations among elements of climate, surface properties, and geomorphology: Office of Naval Research, Geography Branch, Columbia Univ. Dept. of Geology, New York, Technical Report 11, Project NR 389-042, 102 p.

Montgomery, D.C., and Peck, E.A., 1982, Introduction to linear regression analysis: New York, Wiley and Sons, 504 p.

Office of Water Data Coordination, 1978, Physical basin characteristics for hydrologic analysis, in National handbook of recom mended methods for water-data acquisition: U.S. Geological Survey, Reston, Va., chap. 7, 38 p.

REFERENCES 43

Osterkamp, W.R., 1979, Variation of alluvial- channel width with discharge and character of sedim'ent: U.S. Geological Survey Water-Resources Investigations 79-15, lip.

Osterkamp, W.R., and Hedman, E.R., 1977, Variation of width and discharge for natural high-gradient stream channels: Water Resources Research, v. 13, no. 2, p. 256-258.

__1982, Perennial-streamflow characteristics related to channel geometry and sediment in the Missouri River Basin: U.S. Geological Survey Professional Paper 1242, 37 p.

Parrett, Charles, Hull, J.A., and Omang, R.J., 1987, Revised techniques for estimating peak discharges from channel width in Montana: U.S. Geological Survey Water- Resources Investigations Report 87-4121, 34 p.

Pickup, G., and Rieger, W.A., 1979, A conceptual model of the relationship between channel characteristics and discharge: Earth Surface Processes, v. 4, p. 37-42.

Prior, J.C., 1991, Landforms of Iowa: Iowa City, Iowa Department of Natural Resources, University of Iowa Press, 153 p.

Riggs, H.C., 1974, Flash flood potential from channel measurements: International Association of Scientific Hydrology Publication 112, Proceedings of Paris Symposium, p. 52-56.

Robbins, C.H., 1986, Techniques for simulating flood hydrographs and estimating flood volumes for ungaged basins in central Tennessee: U.S. Geological Survey Water-Resources Investigations Report 86-4192, 32 p.

Schwob, H.H., 1953, Iowa floods-Magnitude and frequency: Iowa Highway Research Board Bulletin 1, 171 p.

__1966, Magnitude and frequency of Iowa floods: Iowa Highway Research Board Bulletin 28, parts I and II, 423 p.

Statware, Inc., 1990, Statit statistics reference manual, release 2.3X: Corvallis, Oregon, Statware, Inc., chap. 6, 50 p.

Stedinger, J.R., and Cohn, T.A., 1986, Flood frequency analysis with historical and paleoflood information: Water Resources Research, v. 22, no. 5, p. 785-793.

Strahler, A.N., 1952, Dynamic basis of geomorphology: Bulletin of the Geological Society of America, v. 63, p. 923-938.

__1958, Dimensional analysis applied to fluvially eroded landforms: Bulletin of the Geological Society of America, v. 69, p. 279-300.

__1964, Quantitative geomorphology ofdrainage basins and channel networks, inChow, V.T., ed., Handbook of appliedhydrology: New York, McGraw-Hill,

p. 4-39-4-76.

Tasker, G.D., 1980, Hydrologic regression with weighted least squares: Water Resources Research, v. 16, no. 6, p. 1107-1113.

Tasker, G.D., and Thomas, W.O., 1978, Flood frequency analysis with pre-record information: Journal of Hydraulics Division, American Society of Civil Engineers, v. 104, no. 12, p. 249-259.

Wahl, K.L., 1976, Accuracy of channel measurements and the implications in estimating streamflow characteristics, in Modern Developments in Hydrometry: Padua, Italy, World Meteorological Organization, v. 2, p. 311-319.

__1984, Evolution of the use of channel cross-section properties for estimating streamflow characteristics, in Meyer, E.L., ed., Selected papers in the hydrologic sciences 1984: U.S. Geological Survey Water-Supply Paper 2262, p. 53-66.

Waite, Paul, 1988, Iowa precipitation frequencies: Iowa Department of Agriculture and Land Stewardship, State Climatology Office, Climatology of Iowa Series 2, revised, 42 p.

Webber, E.E., and Roberts, J.W., 1981, Floodflow characteristics related to channel geometry in Ohio: U.S. Geological Survey Open-File Report 81-1105, 28 p.


APPENDIX A

Selected Drainage-Basin Characteristics Quantified Using a Geographic Information-System Procedure

[ , A primary drainage-basin characteristic used in the regression equations (table 2); superscripts 3~n , footnotes at end of the appendix reference the literary and data source for

each drainage-basin characteristic and list topographic-map scales to use for manual measurements of primary drainage-basin characteristics used in the regression equations]

Basin-Area Measurements

TDA* - Total drainage area3, in square milesb , includes noncontributing areas.

NCDA* - Noncontributing drainage area3, in square milesb, total area that does not contribute to surface-water runoff at the basin outlet.

Basin-Length Measurements

BL - Basin length0, in milesb , measured along the main-channel, flood-plain valley from basin outlet to basin divide.

BP* - Basin perimeter3, in milesb , measured along entire drainage-basin divide.

Basin-Relief Measurements

BS - Average basin slope3, in feet per mileb'd, measured by the "contour-band" method, within the contributing drainage area (CDA),

( total length of all selected elevation contours ) ( contour interval )BS .

CDA

BR* - Basin relief6 , in feetd'f, measured as the sea-level elevation difference between the highest contour elevation and the lowest interpolated elevation at basin outlet within the CDA.

Basin Computations

CDA - Contributing drainage area3, in square miles, defined as the total area that contributes to surface-water runoff at the basin outlet,

CDA = TDA M7DA.

BW - Effective basin width3, in miles,

BL '

SF - Shape factor3, dimensionless, ratio of basin length to effective basin width,

BL

ER - Elongation ratio3, dimensionless, ratio of (1) the diameter of a circle of area equal to that of

APPENDIX A 45

the basin to (2) the length of the basin,

RB -Rotundity of basina, dimensionless,

RB = = 0.785 SF. 4CDA

CR - Compactness ratioa, dimensionless, is the ratio of the perimeter of the basin to the circumference of a circle of equal area,

CR.2(*CZ)A)°-5

RR - Relative relief6, in feet per mile,

RR- BR RR ~BP'

Channel- (Stream-) Length Measurements

MCL - Main-channel lengtha, in milesg, measured along the main channel from the basin outlet to the basin divide.

TSL - Total stream length6, in milesg, computed by summing the length of all stream segments within the CDA.

Channel-Relief Measurement

MCS - Main-channel slopea, in feet per mile, an index of the slope of the main channel computed from the difference in streambed elevationd at points 10 percent and 85 percent of the distance5 along the main channel from the basin outlet to the basin divide,

0.75 MCL

Channel (Stream) Computations

MCSR - Main-channel sinuosity ratioa, dimensionless,

MCLMCSR =BL

SD - Stream density3, in miles per square mile, within the CDA,

TSLSD =CDA

CCM - Constant of channel maintenancea, in square miles per mile, within the CDA,


CDA 1

MCSP - Main-channel slope proportion11 , dimensionless,

MCLMCSP =(Af CS) °'5

RN - Ruggedness number1, in feet per mile,

SR - Slope ratio of main-channel slope to basin slope6 , dimensionless, within the CDA,

MCSSR =BS '

First-Order Streams Measurement

FOS* - Number of first-order streams within the CDA*'g'k, using Strahler's method of ordering streams.

Drainage-Frequency Computation

DF - Drainage frequency6 , in number of first-order streams per square mile, within the CDA,

CDA

Climatic Measurements

AP - Mean annual precipitation0, in inches1, computed as a weighted average within the TDA.

TTF* - 2-year, 24-hour precipitation intensity0, in inchesm'n, defined as the maximum 24-hour precipitation expected to be exceeded on the average once every 2 years, computed as a weighted average within the TDA.

aModified from Office of Water Data Coordination (1978, p. 7-9 - 7-16).

bMeasured from l:250,000-scale U.S. Defense Mapping Agency topographic maps.

cModified from National Water Data Storage and Retrieval System (Dempster, 1983, p. A-24-A-26).

dMeasured from l:250,000-scale U.S. Defense Mapping Agency digital elevation model sea-level data.

6Modified from Strahler (1958, p. 282-283).

Use l:250,000-scale U.S. Defense Mapping Agency topographic maps for manual measurements.

Measured from l:100,000-scale U.S. Geological Survey digital line graph data.

APPENDIX A 47

hModified from Robbins (1986, p. 12).

Modified from Melton (1957).

"Modified from Strahler (1952).

kUse l:100,000-scale U.S. Geological Survey topographic maps (County Map Series) for manual measurements.

Determined from Iowa Department of Agriculture and Land Stewardship, State Climatology Office (Des Moines), and from Baker and Kuehnast (1978); mean annual precipitation maps.

mDetermined from Waite (1988, p. 31) and Hershfield (1961, p. 95); 2-year, 24-hour precipitation intensity maps.

nUse figure 5 for manual measurements.


APPENDIX B

Techniques for Manual, Topographic-Map Measurements of Primary Drainage-Basin Characteristics Used in the Regression Equations

The drainage-basin flood-estimation method is applicable to unregulated rural stream sites in Iowa with drainage areas less than 1,060 mi2 . Specific information concerning techniques for making manual measurements is outlined for the six primary drainage-basin characteristics that are used to calculate the four basin characteristics listed in the regression equations in table 2. Comparisons between manual measurements made from different scales of topographic maps are shown in table 7 for four of these six-primary drainage-basin characteristics. Table 7 demonstrates that several of these primary drainage-basin characteristics are map-scale dependent. Map-scale dependency refers to a condition whereby a drainage-basin characteristic value is affected substantially by the scale of topographic map used in the measurement. The comparisons in table 7 list the percentage differences between manual measurements made at the same scale used for geographic- information-system (GIS) measurements (the base scale) and manual measurements made at different scales. Use of maps of scales other than the scales used to develop the equations may produce results that do not conform to the range of estimation accuracies listed for the equations in table 2. The scale of map to use for manual measurements of each primary drainage-basin characteristic is outlined in this section and in the footnotes at the end of Appendix A.

Total Drainage Area (TDA)

The stream site is located and the drainage-divide boundary upstream of the site is delineated on l:250,000-scale U.S. Defense Mapping Agency (DMA) topographic maps. The drainage-divide boundary is delineated along the topographic divide that directs surface-water runoff from precipitation to the basin outlet located at the stream site. The drainage-divide boundary is an irregular line that traces the perimeter of the drainage area and is perpendicular to each elevation contour that it crosses (Office of Water Data Coordination, 1978, p. 7-9 - 7-10). In some cases it may be difficult to delineate the drainage-divide boundary on l:250,000-scale topographic maps, particularly for small drainage basins or for drainage basins located in areas of low relief. In such cases it may be necessary to use larger scale topographic maps, such as l:100,000-scale or l:24,000-scale maps, to facilitate the delineation. Figure 4A shows the drainage-divide boundary for the Black Hawk Creek at Grundy Center streamflow-gaging station (station number 05463090; map number 73, fig. 1).

Because GIS measurements of total drainage area were quantified from l:250,000-scale topographic maps, the appropriate scale for manual measurements of total drainage area is 1:250,000. Total drainage areas for many Iowa stream sites are listed in "Drainage Areas of Iowa Streams" (Larimer, 1957). The total drainage areas listed in this publication can be used to calculate contributing drainage area (CDA) once any necessary adjustments for noncontributing drainage areas (NCDA) are accounted for. Manual measurements of total drainage area for stream sites typically are planimetered or digitized from topographic maps if drainage areas are not listed in Larimer's (1957) publication.

Noncontributing Drainage Area (NCDA)

Noncontributing drainage areas usually are identified as either an area of internal drainage or as an area draining into a disappearing stream. Internal drainage areas drain into depressions, which are represented by hachured contour lines on topographic maps. Internal drainage areas may include potholes or marshes, which are common within the Des Moines Lobe landform region in north-central Iowa (Region II, fig. 2). Disappearing streams do not connect with the drainage network that reaches the basin outlet. In the karst topography of northeast Iowa, sinkholes are a common cause of disappearing streams.

APPENDIX B 49

Tab

le 7

. C

ompa

riso

ns o

f man

ual

mea

sure

men

ts m

ade

from

dif

fere

nt s

cale

s o

f top

ogra

phic

map

s o

f pri

mar

y dr

aina

ge-b

asin

cha

ract

eris

tics

ESTIMATING

DESIGN-FLOOD

DISCHARGES

FOR

STREAMS

IN

IOWA

used

in

the

regr

essi

on e

quat

ions

[TD

A, t

otal

dra

inag

e ar

ea,

in s

quar

e m

iles

; B

P, b

asin

per

imet

er, i

n m

iles

; BR

, bas

in r

elie

f, in

fee

t; F

OS,

num

ber

of fi

rst-

orde

r st

ream

s; 2

50K

, m

anua

l m

easu

rem

ents

mad

e fr

om l

:250

,000

-sca

le U

.S. D

efen

se M

appi

ng A

genc

y to

pogr

aphi

c m

aps;

*, b

ase

scal

e us

ed f

or g

eogr

aphi

c-

info

rmat

ion-

syst

em m

easu

rem

ents

; 10

0K,

man

ual

mea

sure

men

ts m

ade

from

l:1

00,0

00-s

cale

U.S

. G

eolo

gica

l S

urve

y C

ount

y M

ap S

erie

s to

pogr

aphi

c m

aps;

24K

, m

anua

l m

easu

rem

ents

mad

e fr

om l

:24,

000-

scal

e U

.S.

Geo

logi

cal

Sur

vey

topo

grap

hic

map

s; %

DIF

F, p

erce

ntag

e di

ffer

ence

bet

wee

n ba

se-s

cale

and

com

pari

son-

scal

e m

anua

l m

easu

rem

ents

]

Sta

tion

num

bers

of t

hree

str

eam

flow

-gag

ing

stat

ions

rep

rese

ntin

g

0541

4450

(m

ap n

umbe

r 11

, fig

. 1)

Bas

in

char

ac

teri

stic

s 25

0K*

100K

%

DIF

F

2TD

A

22.7

21

.8

-4.0

B

P

21.9

22

.1

+0.9

B

R

444

490

+10.

4

250K

10

0K*

%D

IFF

F

OS

1

10

-90.

0

24K

%

DIF

F

250K

*

22.3

-1

.8

189

22.4

+2

.3

79.0

50

2 +1

3.1

224

24K

%

DIF

F

250K

41

+3

10.0

7

smal

l, in

term

edia

te,

and

(map

100K

184 84.0

23

4

100K

* 80

0690

3400

nu

mbe

r 18

3, f

ig.

% D

IFF

-2.6

+6

.3

+4.5

% D

IFF

-9

1.2

24K

185 85.5

23

1 24K

27

2

larg

e dr

aina

ge b

asin

s, r

espe

ctiv

ely

0660

9500

1)

(m

ap n

umbe

r 15

7,

% D

IFF

25

0K*

-2.1

90

6 +8

.2

206

+3.1

58

2

% D

IFF

25

0K

+240

.0

32

100K

870

228

520

100K

* 47

7

fig.

1)

%D

IFF

-4.0

+1

0.7

-10.

7

%D

IFF

-9

3.3

Deg

ress

ion

equa

tion

s ar

e li

sted

in

tab

le

2.

Com

pari

son

mea

sure

men

ts

for

2-ye

ar,

24-h

our

prec

ipit

atio

n in

tens

itie

s (T

TF

) w

ere

not

appl

icab

le.

A p

lani

met

er w

as u

sed

for

man

ual

mea

sure

men

ts o

f T

DA

. N

onco

ntri

buti

ng d

rain

age

area

s (N

CD

A)

are

not

list

ed b

ecau

se n

o si

gnif

ican

t N

CD

A w

ere

iden

tifi

ed f

or t

hese

dra

inag

e ba

sins

.

Noncontributing drainage areas are delineated on l:250,000-scale topographic maps. When questionable noncontributing drainage areas are encountered, hydrologic judgment is required to determine whether to delineate these areas as noncontributing. Larger scale topographic maps facilitate the delineation of questionable noncontributing areas.

Basin Perimeter CBP)

The basin perimeter is measured along the drainage-divide boundary delineated on l:250,000-scale topographic maps. Because GIS measurements of basin perimeter were quantified from l:250,000-scale topographic maps, the appropriate scale for manual measurements is 1:250,000.

Basin Relief (BR)

Basin relief is the difference between the maximum elevation contour and the minimum interpolated elevation within the contributing drainage area delineated on l:250,000-scale topographic maps. The minimum basin elevation is defined at the basin outlet as an interpolated elevation between the first elevation contour crossing the main channel upstream of the basin outlet and the first elevation contour crossing the main channel downstream of the basin outlet. Because GIS measurements of basin relief were quantified from l:250,000-scale digital elevation model (DEM) data, the appropriate scale for manual measurements is 1:250,000. Figure 4C shows the elevation contours created from DEM data for the Black Hawk Creek at Grundy Center drainage basin.

Number of First-Order Streams (FOS)

The number of first-order streams is a count of all the stream segments defined as being a first-order drainage using Strahler's method of ordering streams (Strahler, 1952). First-order streams are defined for contributing drainage areas on l:100,000-scale topographic maps. Figure 4B shows the stream ordering for the Black Hawk Creek at Grundy Center drainage basin. As shown in figure 4fi, a stream segment with no tributaries is defined as a first-order stream. Where two first-order streams join, they form a second-order stream; where two second-order streams join, they form a third-order stream; and so forth. Because GIS measurements of the number of first-order streams were quantified from l:100,000-scale digital line graph data, the appropriate scale for manual measurements is 1:100,000. Comparison measurements listed in table 7 indicate that the number of first-order streams is clearly map-scale dependent and use of map scales other than 1:100,000 may produce results that do not conform to the range of estimation accuracies listed for the equations in table 2.

2-Year, 24-Hour Precipitation Intensity (TTF)

The map shown in figure 5 is used to calculate 2-year, 24-hour precipitation intensities for drainage basins in Iowa and for basins that extend into southern Minnesota. This map shows polygon areas that represent averages for maximum 24-hour precipitation intensities, in inches, that are expected to be exceeded on the average once every 2 years. These polygons were created from the precipitation contours depicted on 2-year, 24-hour precipitation intensity maps for Iowa (Waite, 1988, p. 31) and the United States (Hershfield, 1961, p. 95). The polygon areas for southern Minnesota were interpolated from the precipitation contours depicted on the United States map. The polygons shown in figure 5 represent the average value, in inches, of rainfall between the precipitation contours and are not intended to represent interpolated values between the contours. Figure 5 was used to compute a weighted average of the 2-year, 24-hour precipitation intensity for each drainage basin processed by the GIS procedure. A manual measurement of 2-year, 24-hour precipitation intensity can be made by delineating the approximate location of the drainage-divide boundary for a stream site in figure 5. The approximate percentage of the total drainage area for the

APPENDIX B 51

stream site that falls within each precipitation polygon shown in figure 5 is calculated, and a weighted average for the basin is computed as

7TF = (Aj) (TTFj) + (A2) (7TF2) + . . . + (Ap) (7TFp), ( 19)

where TTF is the weighted average for 2-year, 24-hour precipitation intensity, in inches;

Aj is the approximate percentage of the total drainage area of a basin within the ith 2-year, 24-hour precipitation intensity polygon shown in figure 5 (i = 1, . . . , p);

TTFi is the 2-year, 24-hour precipitation intensity, in inches, for the ith polygon shown in figure 5 (i = 1, . . . , p); and

p is the total number of 2 -year, 24-hour precipitation intensity polygons shown in figure 5 everlain by the drainage-divide boundary of a basin.

For example, if approximately 70 percent of the total drainage area for a stream site overlies the polygon labeled as 3.15 in. and approximately 30 percent of the total drainage area overlies the polygon labeled 3.05 in., then the weighted average for the basin is calculated as

TTF = (Aj) (TTFJ + (A2) (7TF2), = (0.70) (3.15) + (0.30) (3.05), = 3.12 in.


APPENDIX C

Procedure for Conducting Channel-Geometry Measurements

The channel-geometry flood-estimation method is applicable to stream sites in Iowa with unregulated and stabilized stream channels. The following discussion outlines the procedure for conducting channel-geometry measurements.

Selection of Channel-Geometry Measurement Reaches

An inspection of 1:100,000- or l:24,000-scale topographic maps is made to evaluate the channel reach both upstream and downstream of the stream site. Channel-geometry measurements are made along a straight channel reach, and an inspection of topographic maps is helpful in determining whether to start searching upstream or downstream of the site for a measurement reach. If the channel for some distance upstream and downstream of the stream site is very sinuous, unnaturally wide, or in an area that may be affected by development, topographic maps can be inspected to locate more suitable channel reaches at nearby bridges upstream or downstream of the stream site.

Channel-geometry measurements can be made at some distance away from the stream site, either upstream or downstream, as long as the drainage area upstream of the measurement reach does not change by more than about 5 percent from the drainage area of the stream site. The 5-percent change in drainage area is an approximate limitation to ensure that channel-geometry measurements are representative of the streamflow discharges that occur at the stream site.

Topographic maps are useful in identifying linear channels that are usually indicative of channelization. Channels that appear to be channelized are noted because application of the channel-geometry equations listed in tables 3-5 are dependent on whether a stream has been channelized. A visual inspection of the channel also is made upon visiting the stream site to check for evidence of channelization. Features that are characteristic of channelized streams are illustrated in figure ID, which shows the straightened and leveed channel reach downstream of the Big Creek near Varina gaging station (station number 05482170; map number 108, fig. 2). If evidence of channelization is not found, then the bankfull equations (the first set of equations listed in tables 3-5) are applicable; if evidence of channelization is found, then the active-channel equations (the second set of equations listed in tables 3-5) may be applicable.

The channel-geometry method may not be applicable to poorly drained or pooled streams that have extremely low, local gradients (less than approximately 0.1 ft/mi.). A local gradient is measured from l:24,000-scale topographic maps and is calculated as the slope of the channel between the nearest contour lines crossing the channel upstream and downstream of a stream site. This slope measurement is performed only for those stream sites that are suspected of having extremely low, local gradients and typically is not required for channel-geometry measurements.

Selection of Channel-Geometry Measurement Sections

Measurements of channel-geometry characteristics are made at channel cross sections that represent stable and self-formed channel-bank conditions. Self-formed channels are natural channels or channels that have been affected by channelization for which at least the active-channel portion of the channel has had time to adjust back to natural conditions. Commonly, the active-channel portion of the channel will adjust back to natural or self-formed conditions within approximately 5 to 10 years after channelization occurs.

Measurements are made far enough away from bridges or other structures crossing the stream channel to avoid any alterations to the channel caused by construction. More distance is allowed downstream of bridges to avoid the effects of the channel constriction and more distance is allowed upstream of culverts to avoid the effects of backwater. Ideally, measurements are made in a

APPENDIX C 53

As shown in figure 6, the active-channel reference level is identified at a lower channel-bank elevation. At least three active-channel width measurements are made that are within 10 percent of the average, and active-channel measurement sections are separated by at least twice the active-channel width. The tagline or tape is staked in a similar manner as previously described, and width measurements are read to at least two significant figures. As defined by Osterkamp and Hedman (1977, p. 256),

"The active channel is a short-term geomorphic feature subject to change by prevailing discharges. The upper limit is defined by a break in the relatively steep bank slope of the active channel to a more gently sloping surface beyond the channel edge. The break in slope normally coincides with the lower limit of permanent vegetation so that the two features, individually or in combination, define the active channel reference level. The section beneath the reference level is that portion of the stream entrenchment in which the channel is actively, if not totally, sculptured by the normal process of water and sediment discharge."

Figures 7A and IE show photographs at two stream sites where a tape and a tagline, respectively, were staked at the active-channel reference level used to measure active-channel widths.

SB ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

atio

ns i

n Io

wa

[B, b

oth

cont

inuo

us-r

ecor

d an

d hi

gh-f

low

, pa

rtia

l-re

cord

gag

e; C

, con

tinu

ous-

reco

rd g

age;

P, h

igh-

flow

, par

tial

-rec

ord

gage

; ft

3/s,

cub

ic f

eet

per

seco

nd;

Met

h.,

met

hod

used

to

com

pute

flo

od-p

eak

disc

harg

e es

tim

ates

; B

17B

, Bul

leti

n 17

B (

Inte

rage

ncy

Adv

isor

y C

omm

itte

e on

Wat

er D

ata,

198

2) P

ears

on ty

pe-

Ill

anal

ysis

; G

ISD

B, g

eogr

aphi

c in

form

atio

nsy

stem

qua

ntif

ied

drai

nage

-bas

in c

hara

cter

isti

cs (

tabl

e 2)

; BFR

I, c

hann

el-g

eom

etry

Reg

ion

I (b

ankf

ull,

tabl

e 4)

; AC

RI,

chan

nel-

geom

etry

Reg

ion

I (a

ctiv

e ch

anne

l, ta

ble

4); A

CR

II.

chan

nel-

geom

etry

Reg

ion

II (

acti

ve c

hann

el,

tabl

e 5)

; BF

RII

, cha

nnel

-geo

met

ry R

egio

n II

(ba

nkfu

ll, t

able

5);

ER

L, e

ffec

tive

reco

rd l

engt

h, i

ndic

ates

sys

tem

atic

rec

ord

leng

th u

sed

in B

17B

ana

lysi

s w

hen

no v

alue

is

list

ed f

or H

ST;

yrs,

yea

rs;

HST

, his

tori

call

y ad

just

ed r

ecor

d le

ngth

use

d in

B17

B a

naly

sis;

Dis

ch.,

disc

harg

e; R

ecur

, in

ter,

app

roxi

mat

ere

curr

ence

int

erva

l in

terp

olat

ed f

rom

B17

B a

naly

sis,

rou

nded

to

near

est

5 ye

ars

for

20-

to 5

0-ye

ar r

ecur

renc

e in

terv

als

and

to n

eare

st 1

0 ye

ars

abov

e th

e 50

-yea

r re

curr

ence

inte

rval

; *,

ra

tio

of m

axim

um f

lood

to 1

00-y

ear

B17

B e

stim

ate;

--,

hist

oric

ally

adj

uste

d re

cord

leng

th w

as n

ot u

sed

in B

17B

ana

lysi

s]

Map

no

. (f

igs.

1

and

2)

Flo

od-p

eak

disc

harg

e es

tim

ates

, in

ft3

/s,

Rec

ord

Max

imum

flo

odfo

r in

dica

ted

recu

rren

ce i

nter

val,

in

year

s

Sta

tion

nu

mbe

rS

tati

on

nam

e

Typ

e of

ga

ge

2E

RL

5

10

25

50

100

Met

h.

(yrs

)H

ST

(yrs

)Fl

ood

peri

odW

ater

ye

arD

isch

.(f

t3/s

)

Rec

ur,

inte

r.(y

rs)

0538

7500

U

pper

Iow

a R

iver

B

at

Dec

orah

0538

8000

U

pper

Iow

a R

iver

C

ne

ar D

ecor

ah

5,93

0 10

,200

13

,200

17

,200

20

,100

23

,000

B

17B

5,62

0 10

,500

14

,400

19

,600

23

,800

28

,100

GIS

DB

5,23

0 9,

530

12,7

00

17,2

00

20,4

00

24,4

00

BFR

I

8,07

0 11

,700

14

,100

17

,000

19

,000

21

,000

B

17B

6,08

0 11

,200

15

,300

20

,600

24

,900

29

,400

GIS

DB

43

77

1941

,195

2-89

19

41

28,5

00

1.2

35

77

1914

,191

9-27

, 19

41

28,5

00

1.4

1933

-52

0538

8250

U

pper

Iow

a R

iver

C

ne

ar D

orch

este

r

0538

8500

P

aint

Cre

ek a

t W

ater

vill

e

6,39

0 9,

160

11,1

00

13,8

00

15,9

00

18,1

00

B17

B8,

190

15,0

00

20,2

00

27,1

00

32,6

00

38,2

00 G

ISD

B6,

820

12,1

00

16,0

00

21,4

00

25,1

00

29,8

00

BFR

I

2,24

0 3,

560

4,51

0 5,

780

6,76

0 7,

770

B17

B1,

890

3,82

0 5,

420

7,65

0 9,

520

11,5

00 G

ISD

B

22

77

1941

, 19

76-9

0 19

41

30,4

00

1.7

21

23

1951

,195

3-73

19

51

9,10

0 1.

2

I I 1

0538

9000

Y

ello

w R

iver

at

Ion

0541

1530

N

orth

Bra

nch

Tur

key

Riv

er

near

Cre

sco

0541

1600

T

urke

y R

iver

at

Spi

llvi

lle

8,00

12

,500

15

,600

19

,500

22

,400

25

,300

B

17B

4,93

0 9,

250

12,7

00

17,2

00

20,9

00

24,7

00 G

ISD

B

316

1,17

0 2,

310

4,76

0 7,

600

11,6

00

B17

B65

0 1,

350

1,94

0 2,

800

3,53

0 4,

310

GIS

DB

533

1,19

0 1,

770

2,69

0 3,

450

4,38

0 B

FR

I

2,85

0 5,

460

7,40

0 9,

980

11,9

00

13,9

00

B17

B2,

580

4,85

0 6,

640

9,04

0 11

,000

13

,000

GIS

DB

2,74

0 5,

300

7,28

0 10

,200

12

,400

15

,000

B

FRI

17 24

1935-51

1941

21

,200

40

1966

-90

1990 11,500

100

33

44

1947

,195

6-73

, 1947 10

,000

25

19

78-9

0

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

atio

ns i

n /o

wa-

Con

tinu

ed

ESTIMATING

DEf

M O 55 1 O

O 1? S CO O I | CO a o >

Map

no

. (f

igs.

1

Sta

tion

an

d 2)

nu

mbe

r

8 05

4116

50

9 05

4117

00

10

0541

2500

11

0541

4450

12

0541

4500

13

0541

4600

14

0541

7000

15

0541

7530

16

0541

7590

Typ

e S

tati

on

of

nam

e ga

ge

Cra

ne C

reek

tri

buta

ry P

near

Sar

atog

a

Cra

ne C

reek

nea

r P

Lou

rdes

Tur

key

Riv

er a

t C

Gar

ber

Nor

th F

ork

Lit

tle

PM

aquo

keta

Riv

erne

ar R

icka

rdsv

ille

Lit

tle

Maq

uoke

ta

BR

iver

nea

rD

uran

go

Lit

tle

Maq

uoke

ta

PR

iver

tri

buta

ryat

Dub

uque

Maq

uoke

ta R

iver

C

near

Man

ches

ter

Plum

Cre

ek a

t P

Ear

lvil

le

Kit

ty C

reek

nea

r P

Lan

gwor

thy

Floo

d-pe

ak d

isch

arge

est

imat

es,

in f

t3/s

, fo

r in

dica

ted

recu

rren

ce i

nter

val,

in y

ears

2 632

348

2,04

01,

560

1,76

0

15,4

009,

170

1,24

01,

350

862

6,52

04,

400

4,10

0

231

342

212

4,74

04,

710

7,54

0

1,34

01,

330

989

780

797

1,27

0

5

1,18

078

0

4,58

03,

060

3,55

0

20,8

0015

,900

2,20

02,

820

1,85

0

10,6

008,

420

7,63

0

508

811

483

8,31

08,

640

13,3

00

2,52

02,

670

2,09

0

1,36

01,

690

2,63

0

10

1,58

01,

170

6,67

04,

280

4,98

0

24,0

0020

,600

2,99

04,

060

2,68

0

14,0

0011

,600

10,3

00 750

1,24

072

9

11,0

0011

,700

17,4

00

3,48

03,

790

3,02

0

1,81

02,

450

3,75

0

25

2,12

01,

750

9,61

05,

950

7,12

0

27,8

0027

,200

4,15

05,

820

3,97

0

18,9

0016

,000

14,1

00

1,12

01,

900

1,12

0

14,8

0015

,800

23,2

00

4,91

05,

360

4,44

0

2,43

03,

550

5,45

0

50 2,52

02,

260

12,0

007,

320

8,76

0

30,4

0031

,600

5,15

07,

300

5,01

0

23,2

0019

,600

16,9

00

1,44

02,

500

1,46

0

17,9

0019

,000

27,2

00

6,11

06,

680

5,58

0

2,94

04,

480

6,78

0

100

2,93

02,

820

14,4

008,

780

10,8

00

32,8

0037

,200

6,25

08,

880

6,28

0

27,9

0023

,400

20,3

00

1,79

03,

160

1,86

0

21,1

0022

,400

32,1

00

7,42

08,

080

6,97

0

3,47

05,

490

8,41

0

Rec

ord

Max

imum

flo

od

ER

L

HST

Met

h.

(yrs

) (y

rs)

B17

B

23G

ISD

B

B17

B

38G

ISD

BB

FRI

B17

B

75

101

BFR

I

B17

B

37G

ISD

BB

FRI

B17

B

63

114

GIS

DB

BFR

I

B17

B

39G

ISD

BA

CR

I

B17

B

53

59G

ISD

BB

FRI

B17

B

24G

ISD

BB

FRI

B17

B

24G

ISD

BB

FRI

Floo

d W

ater

pe

riod

ye

ar

1953

-75

1962

1953

-90

1962

1902

, 19

14-1

6,

1922

1919

-27,

193

0,19

33-9

0

1951

-90

1972

1925

, 19

35-8

3,

1972

1986

-90

1951

-65,

19

5719

67-9

0

1925

, 19

28-3

0,

1925

1933

-73,

1976

-83

1966

-90

1974

1966

-90

1969

Rec

ur.

Dis

ch.

inte

r.

(ft3

/s)

(yrs

)

1,83

0 16

11,9

00

50

32,3

00

90

7,18

0 1.

1*

40,0

00

1.4*

1,65

0 80

25,4

00

1.2*

6,20

0 50

3,70

0 1.

1*

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

atio

ns i

n Jo

u>a-

-Con

tinue

d

Map

no

. T

ype

(fig

s. 1

S

tati

on

Sta

tion

of

an

d 2)

nu

mbe

r na

me

gage

? O o O i £> | O 3

17

0541

7700

B

ear

Cre

ek n

ear

Mon

mou

th

18

0541

8450

N

orth

For

kM

aquo

keta

Riv

erat

Ful

ton

19

0541

8500

M

aquo

keta

Riv

erne

ar M

aquo

keta

20

0542

0560

W

apsi

pini

con

Riv

erne

ar E

lma

21

0542

0600

L

ittl

e W

apsi

pini

con

C C C C P£

Riv

er t

ributa

ry n

ear

> § CA i 1 1 | I o a o 3

Ric

evill

e

22

0542

0620

L

ittl

e W

apsi

pini

con

Riv

er n

ear

Acm

e

23

0542

0640

L

ittl

e W

apsi

pini

con

Riv

er a

t E

lma

24

0542

0650

L

ittl

e W

apsi

pini

con

Riv

er n

ear

New

Ham

pton

25

0542

0690

E

ast

For

k

P P P PW

apsi

pini

con

Riv

erne

ar N

ew H

ampt

on

Flo

od-p

eak

disc

harg

e es

tim

ates

, in

ft3

/s,

for

indi

cate

d re

curr

ence

int

erva

l, i

n ye

ars

2

1,71

01,

730

6,35

06,

730

6,82

0

15,0

0011

,700

2,19

01,

720

1,24

0

213

158

233

439

419

634

1,17

098

91,

070

2,05

01,

910

1,62

0

1,46

01,

100

923

5

2,96

03,

430

9,62

012

,100

12,1

00

23,8

0019

,900

5,06

03,

410

2,58

0

559

381

528

866

911

1,40

0

2,43

02,

000

2,25

0

3,82

03,

700

3,28

0

3,80

02,

260

1,96

0

10

3,87

04,

830

11,6

0016

,200

16,0

00

29,8

0025

,400

7,51

04,

790

3,68

0

877

590

794

' 1,

240

1,34

02,

060

3,44

02,

830

3,23

0

5,32

05,

140

4,62

0

5,98

03,

220

2,85

0

25

5,05

06,

780

13,9

0021

,500

21,4

00

37,5

0033

,200

11,1

006,

690

5,35

0

1,36

092

11,

210

1,81

01,

970

3,09

0

4,87

04,

000

4,74

0

7,60

07,

110

6,64

0

9,43

04,

590

4,20

0

50 5,94

08,

410

15,4

0025

,800

25,1

00

43,2

0038

,300

14,0

008,

260

6,67

0

1,77

01,

220

1,58

0

2,31

02,

510

3,95

0

6,00

04,

970

5,93

0

9,58

08,

730

8,20

0

12,4

005,

740

5,29

0

100

6,84

010

,100

16,8

0030

,200

29,8

00

49,0

0044

,700

17,1

009,

920

8,28

0

2,22

01,

560

2,00

0

2,89

03,

100

4,99

0

7,18

06,

010

7,40

0

11,8

0010

,400

10,1

00

15,8

006,

970

6,62

0

Rec

ord

Max

imum

flo

od Rec

ur.

ER

L

HS

T

Floo

d W

ater

D

isch

. in

ter.

M

eth.

(y

rs)

(yrs

) pe

riod

ye

ar

(ft3

/s)

(yrs

)

B17

B

19

--

1944

, 19

58-7

6 19

65

7,34

0 1.

1*G

ISD

B

B17

B

14

--

1974

, 19

77-9

0 19

81

10,7

00

7G

ISD

BB

FRI

B17

B

79

88

1903

, 19

14-9

0 19

44

48,0

00

90B

FRI

B17

B

32

-- 19

59-9

0 19

74

10,1

00

20G

ISD

BB

FR

I

B17

B

37

-

1953

-90

1990

1,

900

60G

ISD

BA

CR

I

B17

B

38

-

1953

-90

1962

2,

380

50G

ISD

BB

FRI

B17

B

38

- 19

53-9

0 19

62

5,74

0 45

GIS

DB

BFR

I

B17

B

26

28

1966

-90

1990

14

,900

1.

3*G

ISD

BB

FRI

B17

B

24

- 19

66-9

0 19

69

11,0

00

35G

ISD

BB

FRI

stations in /owa Continued

-gaging

3 1"

"to

8

C

Table 6. Flood-freqm

/s, Record Maximum flood

sars ........... ...., ...,.. .,...,,, ,,, . _,._, . _, , ,

Recur.

.S '"of cd

en coS c' S ° 1"*CO CO CO Ua, C

II

m g^^s*^3 co *^<

fa c§

CO

cu .CO O

tj co co co""

Q S

<u h -M ca

M CU

&^ Cu

-fcj CO

o oTH

okO

kO CM

OTH

to

CN

COt*- bo o ca

bo

O CO' 5 S -2 S a> c

"ca § *-> 3 03 C

co *O

tC cO

*OO O °°. O OO kOkO CMCO CO TH CM C-CO CMCO

oo -^ o o oo o c-OOO CO O CM OO CMkO O5 00 CO O*l QO * ^ C^ O5 î* (^1 ^Q î* T|* T ^ C** *"

CM CM 00

^3 QO QO QO T|* ^D <^> ^* î* COCO CO CO t- OJOJ kOlO OJOJ OJ OJ OJ OJOJ OJOJTHTH TH TH TH TH TH THTH

o o oo co" coo o o 2 OJOJ kooo<° 9 9 9 2 ko 06 oj CN o CN rj« oj o 52 cot- T*koaj>OCO kO kO :T: OJOJ OJ OJ OJOJOJ OJ OJ :2 THTH THTHTHTH TH TH TH ^^

! 0 ! ! ! t> i i !OJ kO

CNO OJ TH kO COOO OCO THCO CO rf CM kOTH THCO

CQ CQ OQ CO ^^ CO CO ^^ CO CQ ^^ CQ CO ^^ CO ^T3 CQ CQ ^TJ CQ CQ CQ CQ t^C**Q ^*Q£ ^**Q£ ^**Q£ ^**Q^ ^"*^ ^*Q£ ^**Q ^*QOS1^^ Ô ^"^ Ô f^^ '"^ Ô ~^3 '"^ Ô ^^ ^^ Ô f^J 1^^ **j 1^^ rfi *^4 '"^ ^Q 1^^ Q^ ^ >CQ h*4 CQ i»i CQ CQ ^^ CQ CQ I^H OQ CQ t«^ CQ CQ CQ OQ h*4 CQ CQ h*4 CQ ^** Tioo o o o o oo*ÔO OOO OOOO OOO OOO OO OOO OO COOOO CNO OOO t-OJrl< OOJOJ n<Tl*CM OO OJCOt- kOCO OÔJ THO kOt-kO 'sl'COrl' OOCOt- TfCMOJ kOkO THOJOJ OOt- COOJTHn«oo ajTi*io THTH cocooo t-ri«t- ooco ajn«co cMko <*

CM CO CO TH 00 to

oo ooo t-oo ooo ooo oo ooo oo oocooCMOJ OOO OJ O 00 kOt O CNCOC OO ^CMOO t '* oOJt~ (N^ COOTH COCOO t OJTH CN^CO rfOO OJ » ' kO 0000 lOt kOco of to o" o TH" T-T aT kO t-" co" co" co oj oo co ^ ko CM" "*" co"

(N 00 CO (N ^*

OO OOO Ot-CO OOO OOO OO OOO OO COCOO OJOO OOO OJkOO COCMi-t n«OOO OO CNCOrl< OJOJ THCMrl* ^00 COÔJ COOJOO OJCMC OCOiH ^«00 THCO^« OJOJ ^COOJ<NTH THkOkO COrfkO IOOJU3 kOOi tOCOrl« THCO Oi

OJ (N CN (N *

oo ooo to-^co ooo ooo oo ooo oo THOOCMt> OOO O1OJOJ OOCOn< COOIO OO COOOCN t>CM OCOCO kOCN OJlOkO CNkOrf THOJOJ kOOOtO OCO THCOO OOOJ COrl«THTHTH kOOJOJ rfCNOO OOTHOO OCO OOOJOO THCM CN

TH TH TH CN CO

OOJ OOO OOCOOJ OOO OOO OO OOO kOO OJ^*O COU3 OOO kOÔ CMt-t- Orfrf OO OOOJ OOCM THOJOO OJOO OJOOO THCOOO COOt- kOCM 1'* 00rl« Ot>O OJrH CN(NkO

rHrflO CMCMCMCMTHCMlOCOCNTHCM CM THTH TH TH TH CN

THTH OOO lOOOTH OOO OCOO OO COCOOJ t-O COt-CN t-OJ THkOTH C-Tl«CM OiHkO OOt-t- CNO THCOOO OrH THCOOO CO CO '^ kO CO *H TH TH CO CO TH kO TH 00 CD 00 00 OJ tO ^ **^ TH 00

co" oo" oo" T-T TH" T-T TH" TH" aT co" *-iTH

p 1 **J P^ Qj **J ***) ^^ qj P^

j, c u S«£$" a> SSca ® m 2 C 3Q.U - ? <0 $ Co«co -22 * 12 D5^»^» i23° jwCts^ 'C£ * * cs^ca >- W H-I

11 .it |! Is- 1 |? 11 18 1h?l 11 1? J| <*! 1? 51 1 2|S « COHH ca ^ c Sii coca >*» +* > * > z?11 J-^ |S IS J| IS la |l llfi S> £ a, 02^0 ^woo o o o oo ooCOO O O OJ OC- OOojo TH CM oo o^r koc-OTH TH TH TH CMCN 0000CNCM (N CM CM <NOJ rf-rj*

lOlO kO >O to tOlO kOkOoo o o o oo oo

COC 00 OJ O THCN CO^« O1CM Oi CM CO COOO OOCO


Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amfl

ow-g

agin

g st

atio

ns i

n /o

u;a~

Con

tinu

ed

Map

no

. (f

igs.

1

and

2)

35 36 37 38 39 40 41 42

Typ

e S

tati

on

Sta

tion

of

nu

mbe

r na

me

gage

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i ft3

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Rec

ord

Max

imum

flo

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year

s

ER

L

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0 M

eth.

(y

rs)

(yrs

)

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00

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54

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0 B

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42

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0 G

ISD

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BFR

II

38,2

00

B17

B

65

8729

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B

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20,5

00

B17

B

55

9517

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GIS

DB

19,6

00

BFR

II

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00

B17

B

85

8857

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2,53

0 B

17B

25

3,16

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ISD

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II

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00 G

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0 B

17B

31

3,63

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B2,

760

AC

RII

Floo

d W

ater

pe

riod

ye

ar

1938

, 19

40-9

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38

1940

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1947

1905

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19

6519

14-2

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47-9

0

1918

, 19

32,

1918

1941

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1903

, 19

05-2

9,

1954

1931

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33-9

0

1966

-90

1979

1960

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1986

1960

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1962

Rec

ur.

Dis

ch.

inte

r.(f

t3/s

) (y

rs)

22,0

00

80

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00

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Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

atio

ns i

n Io

wa

Con

tinu

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Map

no

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tion

an

d 2)

nu

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117

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118

0548

4500

119

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120

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121

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122

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123

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124

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Typ

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2,27

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310

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Rec

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flo

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L

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M

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(y

rs)

(yrs

)

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BB

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B17

B

76B

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B17

B

19G

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BB

FRII

B17

B

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BA

CR

II

B17

B

51G

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CR

I

B17

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BA

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I

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BB

FRI

Rec

ur.

Floo

d W

ater

D

isch

. in

ter,

pe

riod

ye

ar

(ft3

/s)

(yrs

)

1940

-90

1958

35

,000

1.

1*

1915

-90

1947

41

,200

40

1972

-90

1986

12

,500

35

1972

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19

77

5,38

0 10

1981

-90

1940

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1947

32

,000

1.

3*

1940

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1947

34

,000

1.

6*

1930

, 19

40-9

0 19

90

38,1

00

100

1953

-81

1981

11

,800

1.

2*

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

am flo

w-g

agin

g st

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ns i

n /o

u>a-

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.(t

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tion

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d 2)

nu

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Rec

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Max

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35

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DB

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450

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0 12

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14

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397

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1,47

0 2,

370

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1,78

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2,09

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180

3,87

0 4,

700

5,29

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860

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390

2,56

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480

4,69

0 5,

670

6,68

0 G

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B1,

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3,18

0 4,

490

6,46

0 7,

990

9,85

0 B

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3,02

0 5,

110

6,77

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170

11,2

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480

4,21

0 5,

500

7,14

0 8,

420

9,72

0 G

ISD

B

37 24 23 26

1953

-88,

199

0 19

81

15,5

00

80

32

46

1962

-90

1982

37

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1.

9

21

36

1946

-62

1947

14

,000

25

1966

-90

1982

12

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60

49

139

1946

, 19

48-9

0 19

82

96,0

00

2.1

1966

-88

1970

4,

500

1.1

1965

-90

1977

4,

300

16

30

92

1905

, 19

23-2

8,

1905

33

,000

2.

5 19

30-3

1,

1959

-73

133

0549

4300

Fo

x R

iver

at

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B

2,71

0 4,

970

6,65

0 8,

890

10,6

00

12,4

00

B17

B

2,16

0 3,

730

4,91

0 6,

410

7,60

0 8,

810

GIS

DB

2119

53-7

3 19

60

8,60

0 25

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

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ns i

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134

135

136

137

138

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140

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2,18

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6,38

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080

6,06

0

i ft3

/s,

Rec

ord

Max

imum

flo

odin

yea

rs

ER

L

HST

100

Met

h.

(yrs

) (y

rs)

14,7

00

B17

B

14

1813

,000

GIS

DB

5,22

0 B

17B

23

2,64

0 G

ISD

B

33,5

00

B17

B

23

9330

,700

GIS

DB

1,78

0 B

17B

36

2,19

0 G

ISD

B66

1 B

FR

II

5,58

0 B

17B

35

4,30

0 G

ISD

B4,

880

AC

RI

15,8

00

B17

B

39

638,

310

GIS

DB

11,0

00

AC

RI

43,5

00

B17

B

49

9336

,700

B

FRI

8,96

0 B

17B

27

43

6,32

0 G

ISD

B

10,7

00

B17

B

42

568,

900

GIS

DB

7,32

0 A

CR

I

Floo

d W

ater

pe

riod

ye

ar

1920

, 19

41-5

1 19

46

1953

-75

1970

1960

-74

1969

1952

-88

1962

1952

-88

1953

1952

-72,

19

7919

74-8

8

1897

, 19

48-9

0 19

69

1926

, 19

34,

1953

1949

-69

1939

-69,

19

4419

81-9

0

Rec

ur.

Dis

ch.

inte

r.

(ft3

/s)

(yrs

)

16,5

00

1.1*

3,10

0 25

29,0

00

70

1,41

0 60

5,40

0 90

18,0

00

1.1*

40,4

00

80

10,9

00

1.2*

9,60

0 50

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

atio

ns i

n /o

u>a-

-Con

tinue

d

Floo

d-pe

ak d

isch

arge

est

imat

es,

in f

t3/s

,R

ecor

dM

axim

um f

lood

Map

no

. (f

igs.

1 an

d 2)

Sta

tion

nu

mbe

rS

tati

on

nam

e

Typ

eof

ga

ge

2

tor

indi

cate

d re

curr

ence

int

erva

l, in

yea

rs

ER

L

5 10

25

50

10

0 M

eth.

(y

rs)

HST

(yrs

)Fl

ood

peri

odW

ater

ye

arD

isch

.(f

t3/s

)

Rec

ur,

inte

r.

(yrs

)

143

0660

0100

Fl

oyd

Riv

er a

t A

lton

144

0660

0300

W

est B

ranc

h Fl

oyd

C

Riv

er n

ear

Str

uble

145

0660

0500

Fl

oyd

Riv

er a

t C

Ja

mes

146

0660

2020

W

est F

ork

ditc

h at

C

Hor

nick

147

0660

2400

M

onon

a-H

arri

son

C

ditc

h ne

ar T

urin

148

0660

5000

O

chey

edan

Riv

er

C

near

Spe

ncer

149

0660

5340

P

rair

ie C

reek

nea

r P

Sp

ence

r

150

0660

5750

W

illow

Cre

ek n

ear

P

Cor

nell

1,81

02,

530

1,99

0

2,70

02,

820

2,02

0

328

316

263

954

916

1,12

0

5,10

05,

080

3,96

0

2,16

0 4,

810

1,32

0 2,

940

2,43

0 4,

740

8,56

07,

180

5,52

0

6,95

04,

380

6,55

0

14,6

0010

,100

7,86

0

9,93

06,

490

9,23

0

20,4

0012

,500

9,63

0

27,5

00

B17

B

15,1

00 G

ISD

B

11,8

00

BFR

I

12,3

00

14,6

00

B17

B8,

310

10,3

00 G

ISD

B11

,200

13

,700

B

FRI

3,84

0 8,

450

12,7

00

19,6

00

25,9

00

33,1

00

B17

B4,

710

9,35

0 13

,200

18

,400

22

,700

27

,300

GIS

DB

5,35

0 9,

730

12,9

00

17,5

00

20,8

00

24,8

00

BFR

I

3,17

0 5,

880

7,88

0 10

,500

3,68

0 7,

180

10,0

00

13,9

002,

320

4,35

0 5,

940

8,18

0

12,6

00

14,7

00

B17

B17

,100

20

,500

GIS

DB

9,99

0 11

,900

A

CR

I

6,17

0 10

,900

14

,100

18

,100

21

,000

23

,800

B

17B

5,24

0 9,

920

13,7

00

18,6

00

22,6

00

26,8

00 G

ISD

B3,

580

6,47

0 8,

670

11,7

00

14,1

00

16,6

00

AC

RI

5,03

05,

580

3,68

0

817

702

542

1,84

01,

890

2,13

0

6,82

0 9,

290

7,84

0 10

,900

4,87

0 6,

410

1,25

0 1,

900

},04

0 1,

540

769

1,08

0

2,53

0 3,

510

2,71

0 3,

850

2,90

0 3,

910

11,3

0013

,400

7,69

0

2,44

01,

960

1,35

0

4,30

04,

810

4,77

0

13,3

00

B17

B16

,100

GIS

DB

8,98

0 B

FRII

3,01

0 B

17B

2,

420

GIS

DB

1,

640

BFR

II

5,13

0 B

17B

5,

820

GIS

DB

5,

660

BFR

II

41

115

1953

., 19

56-9

0 19

53

45,5

00

1.6

35

37

61

115

47 32 20

75

25 25

1956

-90

1962

8,

060

15

1935

-90

1953

71

,500

2.

2

1939

-69,

19

75-9

019

62

12,4

00

45

1959

-90

1971

19

,900

40

1953

, 19

69,

1953

26

,000

2.

0 19

78-9

0

1966

-90

1971

2,

200

40

1966

-90

1979

4,

200

45

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amflo

w-g

agin

g st

atio

ns i

n /o

n;a-

-Con

tinue

d

Map

no

.(f

igs.

1

Sta

tion

an

d 2)

nu

mbe

rS

tati

on

nam

e

Typ

eof

ga

ge

Flo

od-p

eak

disc

harg

e es

tim

ates

, in

ft3

/s,

for

indi

cate

d re

curr

ence

int

erva

l, i

n ye

ars

Rec

ord

1025

5010

0E

RL

H

ST

Fl

ood

Met

h.

(yrs

) (y

rs)

peri

od

Max

imum

flo

od Rec

ur.

Wat

er

Dis

ch.

inte

r,

year

(f

t3/s

) (y

rs)

151

0660

5850

L

ittl

e Si

oux

Riv

er

C

at L

inn

Gro

ve

152

0660

6600

L

ittl

e Si

oux

Riv

er

C

at C

orre

ctio

nvil

le

4,52

0 9,

470

13,8

00

20,3

00

25,9

00

32,1

00

B17

B

28

997,

240

12,6

00

16,5

00

21,1

00

25,1

00

29,1

00

BF

RII

6,47

0 11

,700

15

,800

21

,800

26

,700

32

,100

B

17B

69

99

6,89

0 12

,200

16

,100

21

,600

25

,300

30

,000

B

FR

I

1953

, 19

61-6

2 19

53

22,5

00

35

1965

, 19

73-9

0

1891

, 19

19-2

5,

1965

29

,800

80

19

29-3

2,

1937

-90

153

0660

6790

M

aple

Cre

ek n

ear

P

Alta

154

0660

7000

O

debo

lt C

reek

nea

r C

A

rthu

r

155

0660

7200

M

aple

Riv

er a

t C

M

aple

ton

156

0660

8500

S

oldi

er R

iver

at

C

Pis

gah

157

0660

9500

B

oyer

Riv

er a

t C

L

ogan

158

0661

0500

In

dian

Cre

ek a

t C

C

ounc

il B

luff

s

159

0661

0520

M

osqu

ito

Cre

ek

C

near

Ear

ling

134

702

828

1,74

055

3 1,

230

1,58

02,

530

1,83

0

3,63

03,

660

2,77

0

6,09

04,

620

3,55

0

9,56

0 B

17B

5,

660

GIS

DB

4,

500

BF

RI

990

2,01

0 2,

880

4,22

0 5,

380

6,67

0 B

17B

1,88

0 3,

800

5,41

0 7,

690

9,61

0 11

,700

GIS

DB

7,03

0 11

,900

15

,200

19

,400

22

,400

25

,400

B

17B

6,72

0 12

,100

16

,300

21

,800

26

,200

30

,700

GIS

DB

8,06

0 13

,600

17

,700

23

,000

27

,100

31

,000

A

CR

I

8,45

0 14

,300

18

,400

23

,600

27

,500

31

,200

B

17B

5,92

0 10

,900

14

,800

19

,900

24

,000

28

,400

GIS

DB

5,76

0 10

,000

13

,200

17

,400

20

,700

23

,900

A

CR

I

12,1

00

18,3

00

21,9

00

26,1

00

28,9

00

31,4

00

B17

B7,

750

13,5

00

17,9

00

23,5

00

27,9

00

32,3

00 G

ISD

B7,

990

13,5

00

17,5

00

22,8

00

26,9

00

30,8

00

AC

RI

561

1,52

0 2,

480

4,11

0 5,

640

7,44

0 B

17B

583

1,28

0 1,

880

2,76

0 3,

520

4,34

0 G

ISD

B

3,11

0 6,

170

8,59

0 12

,000

14

,700

17

,500

B

17B

946

1,93

0 2,

740

3,88

0 4,

820

5,83

0 G

ISD

B79

7 1,

630

2,33

0 3,

360

4,24

0 5,

180

AC

RI

24 18 49 51 61 15

1966

-89

1969

5,300

40

1951

, 19

58-7

5 1962

5,20

0 45

1942-90

1978 20

,800

35

1940-90

1950 22

,500

20

1881, 19

18-2

5,

1990 30

,800

90

1938

-90

25

35

1942, 1955-76

1942

9,200

1.2

1965-79

1972

12

,000

25

-acu3

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213

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Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

am fl

ow-g

agin

g st

atio

ns i

n lo

w a

Con

tinu

ed

3 0 s £> 1 1 52 2 o 9) i 1 0 | i 1 a o 25 CA S o

Map

no

. (f

igs.

1

and

2)

169

170

171

172

173

174

175

176

177

Sta

tion

nu

mbe

r

0680

9000

0680

9210

0680

9500

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0000

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1760

0681

1840

0681

1875

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7000

0681

8598

Typ

e S

tati

on

of

nam

e ga

ge

Dav

ids

Cre

ek n

ear

CH

amli

n

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t N

ishn

abot

na

CR

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nea

r A

tlan

tic

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t N

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abot

na

CR

iver

at

Red

Oak

Nis

hnab

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Riv

er

Cab

ove

Ham

burg

Tar

kio

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ear

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for

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2 892

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2,92

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500

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00

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00

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270

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0

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0

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00

3,11

04,

830

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00

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0034

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00

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3,17

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170

4,85

0

15,9

006,

410

6,50

0

3,78

03,

850

3,00

0

36,2

0036

,100

33,8

00

3,51

05,

770

7,88

0

i ft3

/s,

Rec

ord

Max

imum

flo

odn

year

s

ER

L

HST

10

0 M

eth.

(y

rs)

(yrs

)

10,3

00

B17

B

227,

200

GIS

DB

35,4

00

B17

B

32

4326

,400

GIS

DB

32,1

00

AC

RI

33,7

00

B17

B

69

8739

,800

GIS

DB

32,5

00

AC

RI

40,3

00

B17

B

69

139

37,0

00

AC

RI

3,85

0 B

17B

33

3,83

0 G

ISD

B6,

080

BFR

I

18,8

00

B17

B

377,

600

GIS

DB

7,84

0 A

CR

I

4,25

0 B

17B

25

4,60

0 G

ISD

B3,

710

AC

RI

40,4

00

B17

B

66

8741

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GIS

DB

38,4

00

AC

RI

3,91

0 B

17B

23

6,74

0 G

ISD

B9,

430

AC

RI

Floo

d W

ater

pe

riod

ye

ar

1952

-73

1958

1958

, 19

61-9

0 19

58

1917

-25,

19

7219

36-9

0

1917

, 19

22-2

3,

1947

1929

-90

1952

-90

1987

1952

, 19

54-5

6,

1967

1958

-90

1966

-90

1987

1903

, 19

18-2

5,

1947

1936

-90

1966

-88

1974

Rec

ur.

Dis

ch.

inte

r.(f

t3/s

) (y

rs)

22,7

00

2.2*

34,2

00

80

38,0

00

1.1*

55,5

00

1.4*

3,21

0 50

22,5

00

1.2*

3,08

0 20

31,1

00

25

3,12

0 25

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

amfl

ow-g

agin

g st

atio

ns i

n Io

wa C

onti

nued

Flo

od-p

eak

disc

harg

e es

tim

ates

, in

ft3

/s,

Rec

ord

Max

imum

flo

odm

ap

no.

(fig

s. 1

an

d 2)

Sta

tion

nu

mbe

rS

tati

on

nam

e

Typ

eof

ga

ge

2

lor

indi

cate

d re

curr

ence

int

erva

l, in

yea

rs

ER

L

5 10

25

50

10

0 M

eth.

(y

rs)

HS

T(y

rs)

Floo

d pe

riod

Wat

er

year

Dis

ch.

(ft3

/s)

Rec

ur,

inte

r,

(yrs

)

178

0681

8750

P

latt

e R

iver

nea

r D

iago

nal

179

0681

9190

E

ast

For

k O

ne

C

Hun

dred

and

Tw

o R

iver

nea

r B

edfo

rd

180

0689

7950

E

lk C

reek

nea

r C

D

ecat

ur C

ity

181

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8000

T

hom

pson

Riv

er a

t C

D

avis

City

182

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8400

W

eldo

n R

iver

nea

r C

L

eon

183

0690

3400

C

hari

ton

Riv

er

C

near

Cha

rito

n

184

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3500

H

oney

Cre

ek n

ear

C

Rus

sell

185

0690

3700

S

outh

For

k C

hari

ton

C

Riv

er n

ear

Pro

mis

e C

ity

186

0690

3900

C

hari

ton

Riv

er

C

near

Rat

hbun

5,00

03,

880

3,20

0

4,21

02,

880

2,72

0

5,10

03,

080

3,15

0

7,94

011

,400

7,79

0

5,87

03,

740

4,37

0

3,48

03,

580

1,91

0

609

847

5,80

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100

3,15

0

5,57

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040

6,76

0 7,

760

8,85

0 9,

560

10,2

00

B17

B6,

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8,28

0 10

,600

12

,400

14

,200

GIS

DB

5,83

0 7,

860

10,7

00

12,9

00

15,2

00

AC

RI

6,46

0 7,

960

9,82

0 11

,200

12

,500

B

17B

4,96

0 6,

520

8,51

0 10

,100

11

,700

GIS

DB

5,02

0 6,

810

9,31

0 11

,300

13

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A

CR

I

11,7

00

17,1

00

24,5

00

30,2

00

36,0

00

B17

B5,

460

7,28

0 9,

660

11,6

00

13,5

00 G

ISD

B6,

010

8,21

0 11

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13

,800

16

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B

FR

I

12,3

00

15,3

00

19,2

00

22,1

00

25,0

00

B17

B17

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22

,600

28

,200

32

,500

36

,800

GIS

DB

13,2

00

17,1

00

22,3

00

26,3

00

30,2

00

AC

RI

9,45

0 12

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17

,800

20

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B

17B

6,41

0 8,

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10,9

00

12,9

00

14,9

00 G

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B7,

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00

13,8

00

16,6

00

19,3

00

AC

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6,05

0 8,

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10,9

00

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00

15,6

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970

7,71

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11,6

00

13,2

00 G

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B3,

810

5,33

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590

9,32

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B

FR

I

1,35

0 2,

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3,05

01,

590

2,19

0 2,

990

3,98

0 5,

030

B17

B

3,65

0 4,

340

GIS

DB

9,32

0 11

,900

15

,400

18

,200

21

,100

B

17B

5,18

0 6,

710

8,62

0 10

,100

11

,600

GIS

DB

5,76

0 7,

760

10,5

00

12,8

00

15,0

00

AC

RI

12,0

00

17,6

00

25,8

00

32,7

00

40,3

00

B17

B11

,100

14

,000

17

,500

20

,200

22

,800

GIS

DB

24

25

1967

-90

1989

8,

630

20

24

--

1960-83

1974

9,98

0 25

24

--

1959, 1967-90

1990 18,000

11

68

106

1885

, 18

97,

1885 30

,000

1.

219

03, 1909,

1914

-15,

191

8-24

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26,1

942-

90

37

72

1959-90

1959 48

,600

2.

4*

30

44

1947, 1960,

1981 16,600

1.1

1966

-90

11 23 13

1952

-62

1959

4,100

50

1968-90

1981

28

,000

1.3

1957-691

1960 21

,800

17

Tab

le 8

. F

lood

-fre

quen

cy d

ata

for

stre

am fl

ow-g

agin

g st

atio

ns i

n Io

wa

Con

tinu

ed

Map

no

. (f

igs.

1

Sta

tion

an

d 2)

nu

mbe

r

187

0690

3990

188

0690

4000

Typ

e S

tati

on

of

nam

e ga

ge

Coo

per

Cre

ek a

t P

C

ente

rvil

le

Cha

rito

n R

iver

nea

r C

C

ente

rvil

le

Flo

od-p

eak

disc

harg

e es

tim

ates

, in

ft3

/s,

for

indi

cate

d re

curr

ence

int

erva

l, in

yea

rs

1 1 1 5 8

2

,570

,6

60

,170

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,9

50

5

3,16

0 2,

940

2,44

0

10,6

00

14,0

00

4 3 3 14

17

10 ,420

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30

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00

25

6,17

0 5,

210

5,09

0

21,0

00

21,8

00

50 7,55

0 6,

230

6,36

0

26,1

00

25,0

00

100

8,98

0 7,

280

7,90

0

31,6

00

28,2

00

Rec

ord

Max

imum

flo

od

ER

L

HST

M

eth.

(y

rs)

(yrs

)

B17

B

24

GIS

DB

B

FR

I

B17

B

25

31

GIS

DB

Rec

ur.

Floo

d W

ater

D

isch

. in

ter,

pe

riod

ye

ar

(ft3

/s)

(yrs

)

1966

-89

1982

7,

000

40

1938

-59

1946

21

,700

30

lStr

eam

flow

reg

ulat

ed d

urin

g pa

rt o

f gag

ed r

ecor

d. O

nly

unre

gula

ted

peak

dis

char

ges

at t

hese

sta

tion

s w

ere

used

in

floo

d-fr

eque

ncy

anal

ysis

.

£> o c> I 1 1 o Cfl 3 o £

Tab

le 9

. Se

lect

ed d

rain

age-

basi

n an

d ch

anne

l-ge

omet

ry c

hara

cter

isti

cs fo

r st

ream

flow

-gag

ing

stat

ions

in

Iow

a

ESTIMATED G

DESIGN-FLC s D D 8 1 o n CO 3 o 33 CO 1 CO 3 I

[mi

, squ

are

mile

s; D

rain

age-

basi

n ch

arac

teri

stic

mea

sure

men

ts,

valu

es q

uant

ifie

d us

ing

the

geog

raph

ic-i

nfor

mat

ion-

syst

em p

roce

dure

; CD

A,

cont

ribu

ting

dra

inag

e ar

ea,

in

squa

re m

iles;

RR

, re

lati

ve r

elie

f, i

n fe

et p

er m

ile; D

F, d

rain

age

freq

uenc

y,

in n

umbe

r of

firs

t -or

der

stre

ams

per

squa

re m

ile;

TTF

, 2-y

ear,

24-

hour

pre

cipi

tati

on i

nten

sity

, in

in

ches

; Cha

nnel

-geo

met

ry c

hara

cter

isti

c m

easu

rem

ents

, av

erag

e va

lues

mea

sure

d on

site

; BF

W, b

ankf

ull

wid

th, i

n fe

et; B

FD

, ban

kful

l de

pth,

in

feet

; AC

W, a

ctiv

e-ch

anne

l w

idth

, in

fee

t; --

, not

det

erm

ined

]

Map

nu

mbe

r (f

igs.

1

Sta

tion

an

d 2)

nu

mbe

r

1 2 3 4 5 6 7 8 9

0538

7500

0538

8000

0538

8250

0538

8500

0538

9000

0541

1530

0541

1600

0541

1650

0541

1700

Pub

lish

ed

drai

nage

S

tati

on

area

na

me

(mi2

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Upp

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Dec

orah

Upp

er I

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Upp

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ar D

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r

Pai

nt C

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at

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n

Nor

th B

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urke

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Tur

key

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ne C

reek

tri

buta

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ar S

arat

oga

Cra

ne C

reek

nea

r

511

568

770 42

.8

221 19

.5

177 4.

06

75.8

Lat

i-

Lon

gi

tude

tu

de

43°1

8'19

" 91

047'

48n

4301

8'20

" 91

045>

00W

43°2

5'16

" 91

030'

31"

43°1

2'37

" 91

018'

21"

43°0

6'35

" 91

°15'

55"

43°2

2'15

" 92

012'

49"

43°1

2'28

" 91

°56'

56"

43°2

2'00

" 92

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00"

43°1

4'57

" 92

°18'

32"

Dra

inag

e-ba

sin

Cha

nnel

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met

ry

char

acte

rist

ic m

easu

rem

ents

ch

arac

teri

stic

mea

surm

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CD

A

503

565

765 41

.9

207 19

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178 4.

13

74.7

RR 4.21

4.00

4.37

12.9 8.19

5.27

3.71

13.7 4.87

DF

0.50

3

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TT

F

2.99

3.00

3.00

3.05

3.05

3.05

3.05

3.05

3.05

BF

W

BF

D

AC

W

129

6.1

114

155

7.2

139

.. .. 26.7

3.

7 19

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82.7

5.

1 68

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61.0

4.

0 40

.9Lo

urde

s

10

05412500 Turkey Riv

er at

1,545

42°4

4'24

" 91

°15'

42"

-- --

-- --

190

13.7

156

Garber

Tab

le 9

. Se

lect

ed d

rain

age-

basi

n an

d ch

anne

l-ge

omet

ry c

hara

cter

isti

cs fo

r st

ream

flow

-gag

ing

stat

ions

in

7ou>

a--C

ontin

ued

DRAINAGE-BAS]

X | O i r n o 1 o 5 > n H w 2 on H n on

3 O on 1 | O P | 3 o GO >j O

2. GO 3 o

Map

nu

mbe

r (f

igs.

1

Sta

tion

an

d 2)

nu

mbe

r

11 12 13 14 15 16 17 18 19 20

0541

4450

0541

4500

0541

4600

0541

7000

0541

7530

0541

7590

0541

7700

0541

8450

0541

8500

0542

0560

Pub

lish

ed

Dra

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sin

Cha

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met

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drai

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men

ts

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Sta

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ea

Lat

i-

Lon

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me

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de

tude

C

DA

R

R

DF

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TF

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th F

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tle

21.6

42

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0.

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near

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kard

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le

Lit

tle

Maq

uoke

ta

130

42°3

3'18

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46"

136

9.69

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64

3.05

Riv

er n

ear

Dur

ango

Lit

tle

Maq

uoke

ta

1.54

42

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33"

90°4

1'38

" 1.

53

48.7

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55

3.05

Riv

er t

ribu

tary

at D

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ue

Maq

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ta R

iver

30

5 42

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6 4.

34

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3.

05ne

ar M

anch

este

r

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m C

reek

at

41.1

42

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13"

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4'53

" 40

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7.06

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16

3.05

Ear

lvil

le

Kit

ty C

reek

nea

r 14

.4

42°1

2'04

" 91

°12'

27"

14.8

12

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3.

05L

angw

orth

y

Bea

r C

reek

nea

r 61

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42°0

2'18

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59"

58.4

6.

39

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3.

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onm

outh

Nor

th F

ork

516

42°0

8'48

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33"

511

3.62

.6

66

3.05

Maq

uoke

ta R

iver

at F

ulto

n

Maq

uoke

ta R

iver

1,

553

42°0

5'05

W 90

°38'

04"

near

Maq

uoke

ta

Wap

sipi

nico

n R

iver

95

.2

43°1

4'34

" 92

°31'

48"

94.5

4.

87

.476

3.

02ne

ar E

lma

BF

W

BF

D

AC

W

37.2

3.

4 25

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109

9.4

46.8 4.2

166

6.4

125

40.9

3.

9 27

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48.6

4.

1 16

.8

..

155

10.2

10

3

225

13.8

17

3

47.9

5.

0 39

.0

Tab

le 9

. Se

lect

ed d

rain

age-

basi

n an

d ch

anne

l-ge

omet

ry c

hara

cter

istic

s fo

r st

ream

flow

-gag

ing

stat

ions

in

7ou>

a--C

ontin

ued

en

H 2 o o pq en o 2* r D o 1 1 pq en O 1 i en 5 o

Map

nu

mbe

r (f

igs.

1

and

2)

21 22 23 24 25 26 27 28 29 30

Sta

tion

nu

mbe

r

0542

0600

0542

0620

0542

0640

0542

0650

0542

0690

0542

0960

0542

1000

0542

1100

0542

1200

0542

1890

Pub

lish

ed

drai

nage

S

tati

on

area

na

me

(mi2

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Lit

tle

Wap

sipi

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tri

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Ric

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Lit

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Lit

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New

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1,

at I

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tri

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rop

Pin

e C

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nea

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Sil

ver

Cre

ek a

t

0.90

0

7.76

37.3

95.0

30.3 6.17

048 0.

334

28.3 9.03

Lat

i

tude

4302

1'31

"

43°1

9'37

"

43°1

4'30

"

43°0

3'58

"

43°0

5'11

"

42°2

9'52

"

42°2

7'49

"

42°2

9'17

"

42°2

8'11

"

41°5

4'54

"

Lon

gi

tude

92°2

9'08

"

92°2

9'07

"

92°2

7'04

" L

92°2

3'38

"

92°1

8'22

"

91°5

3'27

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" 27

8 4.

00

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3.

15

92°3

9'31

"

106

162

8 10

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83.3

138

Tab

le 9

. Se

lect

ed d

rain

age-

basi

n an

d ch

anne

l-ge

omet

ry c

hara

cter

istic

s fo

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flow

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ions

in

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DIS 0 B AGES

FOR

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94°1

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Tab

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. Se

lect

ed d

rain

age-

basi

n an

d ch

anne

l-ge

omet

ry c

hara

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102

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104

105

106

107

108

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2170

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Tab

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. Se

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n an

d ch

anne

l-ge

omet

ry c

hara

cter

istic

s fo

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flow

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tinue

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Tab

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. Se

lect

ed d

rain

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basi

n an

d ch

anne

l-ge

omet

ry c

hara

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cs fo

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flow

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O 1 O 5 £ o CA

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93°0

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13.5

Tab

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. Se

lect

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rain

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basi

n an

d ch

anne

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D W cn 53 21 a a I a W 09 1 1 O >

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42°5

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9600

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96°1

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9600

4'40

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Tab

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. Se

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rain

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n an

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anne

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