2.0 hydrology 2.1 introduction

City of Lincoln Drainage Criteria Manual Update

Project No.020-1827 May 2021

9

2.0 HYDROLOGY

2.1 INTRODUCTION Hydrologic analysis of stream systems is done to estimate the peak rate of runoff, volume of

runoff, and time distribution of water flow during storms to design for stormwater drainage. Errors

in analysis may result in an undersized structure, which will cause drainage problems (flooding,

inundated properties, safety issues), or in an oversized structure that increases costs.

Site characteristics that have an impact on hydrology include:

• Drainage basin – Size and shape, slope, ground cover and land use, geology, soil types,

surface infiltration, ponding and storage, watershed development potential

• Stream channel – Geometry and configuration, natural controls, artificial controls, channel

modifications, aggradation, degradation, debris, Manning’s n equation (used to determine

velocity), slope

• Floodplain – Slope, vegetation, alignment, storage, location of structures, obstructions of

flow

• Meteorological – Time rate and amounts of precipitation, historical flood events

2.2 HYDROLOGIC ANALYSIS METHODS While there are many ways to conduct a hydrological analysis, it is recommended that projects

being developed within the City use the following:

Rational Method:

• Applies to drainage areas up to 150 acres

• Used for estimating peak flows and the design of subdivision storm drainage systems

(e.g., inlets, storm drainpipes)

• Shall not be used for design of storage facilities, road culverts, or bridges

• Curve Number (CN) Method

o Applies to drainage areas up to 2,000 acres

o Used for estimating peak flows and hydrographs

o Used for the design of all drainage structures including bridges, culverts, and

storage facilities

o HEC-HMS is the City’s preferred CN methodology program for stormwater master

planning and recommends its use for determining flows for hydraulic structure

design (e.g., bridges, culverts)



10

These methods are recommended for use in the City’s jurisdiction because they are familiar to

City officials and local consulting engineers and have available computer programs. The methods

provide consistent results because of tested equations and well-sourced monographs.

Other methods may be used if the Director of Lincoln Transportation and Utilities approves of their

use beforehand. Source documentation and/or calibration may be required.

2.3 HYDROLOGIC DESIGN CRITERIA It is not practical to design drainage structures such as culverts to accommodate the maximum

runoff a watershed can produce. For this reason, local design frequencies are established for the

storm drainage structures and systems. The specifications for the drainage structures and

systems are detailed below.

Culverts/bridges designed to transport stormwater runoff under roadways shall convey at a

minimum the 50-year flood event with 1-foot of freeboard from the centerline of the roadway.

Arterial roadways will convey the 100-year flood event without overtopping the roadway. Flow rate

shall be based on the ultimate upstream land-use build-out. Stormwater detention may be taken

into consideration when sizing culverts for residential or commercial roads, but stormwater

detention may not be used when sizing culverts or bridges for arterial roads. The 100-year flood

event shall be routed through all culverts and bridges to be sure adjacent structures are not

flooded or that damage does not occur to the roadway or adjacent property.

Storm drains shall accommodate the 5-year flood event for residential areas; the 10-year flood

event for commercial, downtown, and industrial areas; and the 10-year flood event for residential

areas downstream from commercial, downtown, or industrial areas.

Designs must not increase flooding on adjacent property; cause exit velocities so high as to cause

scour or too low to cause deposition; or encroach onto the street or highway in a manner that

impedes traffic, merging vehicles, or pedestrian movements.

Designs may involve temporary street or road inundation for flood events greater than the design

event but not for floods equal to or less than the design event.

Inlets shall accommodate the 5-year flood event for residential developments and the 10-year

flood event for downtown, industrial, commercial, and arterial roads.

Detention and retention storage facilities shall be designed to accommodate 2-year, 10-year, and

100-year flood events so that post development peak discharge rates do not exceed

predevelopment discharge rates. In addition, stormwater runoff shall not increase flooding or

erosion hazards for adjacent, upstream, or downstream property or cause safety hazards

associated with the facility. To accomplish this, an emergency spillway may need to be provided,



11

yet designs must not increase flood peaks at the downstream property line. If combined with a

stormwater quality facility, the combined facility must meet criteria for both facilities.

Stormwater quality facilities shall be designed to detain the 70th and 80th percentile rainfall events

measured at every discharge point from the project area (See Table 2-1 City of Lincoln Rainfall

Amounts by Recurrence Interval and Percentile). More detail on stormwater quality is provided in

Chapter 8.

2.4 RATIONAL METHOD The Rational method estimates the peak rate of runoff at any location in a watershed as a function

of the drainage area, runoff coefficient, and mean rainfall intensity for a duration equal to the time

of concentration. Here is the formula:

Equation 2.1 The Rational Method Equation (Q = CIA)

Q = peak rate of runoff, in cubic feet per second (cfs)

C = runoff coefficient representing a ration of runoff to rainfall for future land use

conditions

I = average rainfall intensity for a duration equal to the time of concentration (Tc) for a

selected return period, in inches per hour (in/hr)

A = drainage area contributing runoff to the design location, in acres

This method is sensitive to the parameters used for estimating peak rate of runoff (Q). Parameters

Tc, I, and C are discussed in the next section.

2.4.1 Time of Concentration (Tc)

Time of concentration is the time required for water to flow from the remotest point,

hydraulically, of the drainage area to the point under investigation (design point), which must

be determined for each design point within the drainage basin.

The duration of rainfall is set equal to the time of concentration used to estimate rainfall

intensity. There are several methods available for estimating time of concentration. Section

2.5.4 describes the recommended method for estimating the Tc.

For inlet design, the minimum time of concentration shall not be less than eight minutes. The

time of concentration for a storm drain system equals the inlet time plus the time of flow in a

closed conduit or open channel to the design point where inlet time is the time required for

runoff to flow over the surface to the nearest inlet. Inlet time is a function of the length of

overland flow, the slope, and the surface cover. The flow time for pipe or open channel,

sometimes referred to as travel time (shown as Tt in the equations), is calculated from the

hydraulic properties of the conduit or channel and added to the inlet time to obtain the total



12

time of concentration. Travel time is obtained by dividing velocity of the flow into the pipe or

channel length. Manning’s n equation can be used to determine velocity (see Chapter 5 for

more discussion on Manning’s n equation).

Be aware that there are a few common issues with time of concentration calculations. In some

cases, runoff from a highly impervious portion of the drainage area may result in a greater

flow and therefore produce a distorted time of concentration than if the area were considered

as a whole. Adjustments should be made to the drainage area by disregarding those areas

where flow time is too slow to add to Q. It may require estimates of different times of

concentration to determine the design flow critical to a specific application.

Occasionally, the overland flow path is not necessarily perpendicular to the contours shown

on available maps because of grading and swales that intercept the natural contour and

conduct the water to the streets, which reduces time of concentration. Therefore, sheet flow

paths greater than 100 feet for urban areas and 150 feet for rural areas shall not be used:

2.4.2 Rainfall Intensity (I)

Rainfall intensity is defined as the average rainfall rate (in/hr) for a duration equal to the time

of concentration for a selected return period (i.e., design frequency). Once a return period has

been selected for the design and a time of concentration is determined, use of an intensity-

duration-frequency (IDF) curve determines I (See Table 2-1 and Table 2-2).

2.4.3 Runoff Coefficient (C)

The runoff coefficient (C) represents the integrated effects of many drainage basin

parameters, mainly soil type, land use, and average land slope. C requires engineering

judgement, because it is an imprecise value, especially as related to land use.

Soil properties influence the relationship between runoff and rainfall because of differing

infiltration rates. The United States Soil Conservation Service (or SCS, now the Natural

Resources Conservation Service, or NRCS) has divided soils into four hydrologic soil groups,

used in both the Rational method and the SCS method:

Group A Soils having a low runoff potential because of high infiltration rates. These soils

consist primarily of deep, well-drained sands and gravels.

Group B Soils having a moderately low runoff potential because of moderate infiltration

rates. These soils consist primarily of moderately deep to deep, moderately

well to well-drained soils with moderately fine to moderately coarse textures.

Group C Soils having a moderately high runoff potential because of slow infiltration

rates. These soils consist primarily of soils in which a layer exists near the



13

surface that impedes the downward movement of water or soils with

moderately fine to fine textures.

Group D Soils having a high runoff potential because of very slow infiltration rates.

These soils consist primarily of clays with high swelling potential, soils with

permanently high water tables, soils with a claypan or clay layer at or near the

surface, and shallow soils over nearly impervious parent material.

A list of soils for the City and the soils’ hydrologic classifications are in the Lancaster County

Soil Survey (USDA 1980).

For the purposes of this drainage criteria manual, common soil classifications for City of

Lincoln area soil classifications are groups C and D. If a soil classification listed in the County

Soil Survey has a split classification (e.g., B/D), use the lower classification (e.g., D). If a soil

classification is listed in the County Soil Survey as urban or X, use soil classification D.

Soil groups are used in many cases as a significant parameter for determining hydrologic

classifications (i.e., runoff coefficient and curve number). The value of C shall be based on

fully built-out land use conditions. The minimum C shall be 0.4, unless it can be clearly

demonstrated that a value of less than 0.4 is adequate. It is common to develop a composite

C based on the percentage of diverse types of land surfaces in the drainage area.

Table 2-3 provides runoff coefficients for the Rational method based on selected land uses.

Table 2-4 provides runoff coefficients for the Rational method based on hydrologic soil groups

and slope ranges.

Infrequent, higher intensity storms require modifying C because infiltration and other losses

have a proportionally smaller effect on runoff. Adjusting the Rational method formula requires

multiplying the right side of the formula by a frequency factor (Cf), making the formula read as

follows:

Equation 2.2 Q = Cf CIA

Cf values are:

1.1 for a 25-year recurrence interval

1.2 for a 50-year recurrence interval

1.25 for a 100-year recurrence interval.

NOTE: The product of Cf times C shall not exceed 1.0.

2.4.4 Rational Method Limitations

Be aware that the Rational method has limitations. The Rational method requires a good

topographic map to define the drainage area. A field inspection should be completed to



14

determine whether the natural drainage divides have been altered. When determining C for

the drainage area, future possible land use changes for the facility should be considered as

these changes could result in an inadequate drainage area.

The effects of permanent upstream detention facilities should be considered as restrictions to

the natural flow, such as highway crossing and dams in the drainage area, may affect the

design flow. The unintentional detention of flow, such as behind road crossings, should be

excluded as future improvements may negate such detention.

The Rational method is best suited to small, highly impervious areas and least suitable for

large drainage areas or drainage areas in natural or undeveloped conditions. The tables in

this manual are not intended to replace reasonable and prudent engineering judgment.

2.5 CURVE NUMBER METHOD Techniques developed by the SCS for calculating rates of runoff require the same basic data as

the Rational method: drainage area, a runoff factor, time of concentration, and rainfall.

The SCS method, referred to in this manual as the CN method, also considers the following:

• Time distribution of the rainfall

• Initial rainfall losses to interception and depression storage

• Infiltration rate that decreases during a storm

The CN method allows for calculating direct runoff for any storm, either real or fabricated, by

subtracting infiltration and other losses from the rainfall to obtain the precipitation excess (runoff

volume).

Parameters necessary for the CN method include direct runoff (Q), drainage area, rainfall, time

of concentration, and a runoff factor (i.e., CN).

2.5.1 Direct Runoff (Q)

The rainfall runoff equation for direct runoff is a method of estimating direct runoff from a 24-

hour, or one-day, storm rainfall. It is derived from experimental plots for numerous soils and

vegetative cover conditions, including land-treatment measures (i.e., contouring and

terracing). It was mainly developed for small watersheds from available daily rainfall and

watershed data. Below is the equation:

Equation 2.3 Q = (P – 0.2S)2 / (P +0.8S)

Q = accumulated direct runoff, in inches

P = accumulated rainfall (potential maximum runoff), in inches

S = potential maximum retention, in inches



15

Note: S considers initial abstraction, including surface storage, interception, and infiltration prior to runoff, which, altogether, are assumed to be 20 percent of a basin’s potential losses.

S is related to the soil and cover conditions of the watershed through the CN or runoff factor CN. CN

has a range of 0 to 100, and S is related to CN by the following equation:

Equation 2.4 S = (1000 / CN) – 10

2.5.2 Drainage Area

Drainage area is determined from topographic maps and field surveys. Knowledge of the

drainage area may be required to divide large drainage areas into subdrainage areas and

account for major land use changes, to obtain analysis results at different points within the

drainage area, or to locate stormwater drainage facilities and assess the effects on flood flows.

2.5.3 Rainfall

Rainfall is based on a 24-hour storm event with various time distributions, depending on the

watershed (drainage area) location. NOAA Atlas 14 data will be used as the total rainfall

amount (See Table 2-1) for the specified return period to use with the typical time distribution

for the Lincoln area. The MSE3 (Midwest/Southeast) storm distribution (See Figure 2-2) is

the time distribution from the NOAA Atlas 14 data and shall be used as the “typical” time

distribution used for Lincoln, Nebraska. The tabular data needed for input into the software is

shown in Table 2-5.

2.5.4 Time of Concentration

Travel time (Tt) is the time it takes water to travel from one location to another in a watershed.

Travel time is a component of time of concentration, which is the time needed for runoff to

travel from the hydraulically most distant point of the watershed to a point of interest within

the watershed. Time of concentration is computed by summing all the travel times for

consecutive components of the drainage conveyance system. Procedures and equations for

calculating travel time and time of concentration are discussed in the following sections.

Water moves through a watershed as sheet flow, shallow concentrated flow, open channel

flow, or some combination of these. The type of flow that occurs is a function of the

conveyance system and is best determined by a field inspection.

As shown in the equation below, travel time is the ratio of flow length to flow velocity:

Equation 2.5 Tt = L/(3600V)

Tt = travel time, in hours (hr)

L = flow length, in feet (ft)

V = average velocity, in feet per second (ft/s)

3600 = factor to convert velocity from seconds to hours



16

The time of concentration is the sum of travel time values for the various consecutive flow

segments:

Equation 2.6 Tc = Tt1 + Tt2 + ... Ttm

Tc = time of concentration, hr

m = number of flow segments

Note: Minimum Tc is 8 minutes

Sheet Flow

Sheet flow is flow over planar surfaces. It usually occurs in the headwater of watersheds. With

sheet flow, the friction value (Manning’s n) is an effective roughness coefficient that includes

the effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges,

and rocks; and erosion and transportation of sediment. These n values are for very shallow

flow depths of about 0.1 foot or so. Table 2-6 gives Manning’s n values for sheet flow for

various surface conditions.

Sheet flow conditions are unlikely to occur at lengths in excess of 150 feet in rural areas and

100 feet in urban areas. In urban residential developments, sheet flow conditions may occur

in rear yards and other open areas but generally change to a shallow concentrated flow when

that flow occurs between buildings. For sheet flow, use Manning’s kinematic solution to

compute travel time, which is shown in the equation below:

Equation 2.7 Tt = [0.42 (nL)0.8 / (P2) 0.5s0.4]

Tt = travel time, in minutes (min)

n = Manning’s roughness coefficient (Table 2-6)

L = flow length, feet

P2 = 2-year, 24-hr rainfall, in. (3.0 inches in Lincoln)

s = slope of HGL (land slope), in feet per feet (ft/ft)

This simplified form of the Manning’s kinematic solution is based on a shallow, steady, uniform

flow, a constant intensity of rainfall excess (rain available for runoff), a rainfall duration of 24

hours, and a minor effect of infiltration on travel time.

Shallow Concentrated Flow

Beyond the maximum lengths of 150 feet in rural areas and 100 feet in urban areas, sheet

flow usually becomes a shallow concentrated flow. Also, once flow gets to a curb line, do not

use the equations used for sheet flow. The average velocity for this flow can be determined

from the following equations in which average velocity is a function of watercourse slope and

the type of channel the flow travels through.



17

Equation 2.8 Unpaved (V = 16.1345(s)0.5)

Equation 2.9 Paved (V = 20.3282(s)0.5)

V = average velocity, feet/second

s = slope of HGL (watercourse slope), ft/ft

These two equations are based on the solution of Manning’s equation with different

assumptions for n (Manning’s roughness coefficient) and r (hydraulic radius, feet). For

unpaved areas, n is 0.05 and r is 0.4 foot; for paved areas, n is 0.025 and r is 0.2 foot.

After determining the average velocity, use the equation for travel time from Equation 2.5 to

estimate travel time for the shallow concentrated flow segment.

Pipe and Open Channels

Open channels are assumed to begin where surveyed cross-section information has been

obtained, where channels are visible on aerial photographs, or where blue lines (indicating

streams) appear on United States Geological Survey (USGS) quadrangle sheets. Either

Manning’s equation or water surface profile information can be used to estimate the average

flow velocity. Average flow velocity is usually determined for bank-full elevation.

Manning’s equation is as follows:

Equation 2.10 V = (1.486 r2/3 s1/2)/n )

V = average velocity, ft/s

r = hydraulic radius, ft (this is equal to a/pw)

a = cross-sectional flow area, ft2

pw = wetted perimeter, ft

s = slope of the HGL, ft/ft

n = Manning’s roughness coefficient

After average velocity is computed, travel time for the channel segment can be estimated

using the equation for travel time from Equation 2.5.

Reservoir or Lake

Sometimes it is necessary to compute a time of concentration for a watershed that has a

relatively large body of water in the flow path. This travel time is normally very small and can

be assumed as zero.

One must not overlook the fact that this does not account for the travel time involved with the

passage of the inflow through spillway storage and the reservoir or lake outlet. This time is

generally much longer and is added to the travel time across the lake. The travel time through



18

lake storage and its outlet can be determined by the storage routing procedures given in

Chapter 6.

Limitations to the Use of Tc

Manning’s kinematic solution should not be used for sheet flow longer than 150 feet for rural

areas and 100 feet for urban areas. In watersheds with storm drains, carefully identify the

appropriate hydraulic flow path to estimate time of concentration. Storm drains generally

handle only a small portion of a large event. The rest of the peak flow travels by streets, lawns,

and so on, to the outlet. Consult Chapter 3 to determine average velocity in pipes. A culvert

or bridge can act as a reservoir outlet if there is significant storage behind it. Detailed storage

routing procedures should be used to determine the outflow through the culvert.

2.5.5 Runoff Factor

The runoff factor is defined as the amount by which rainfall exceeds the capability of the land

to absorb or otherwise retain the rainfall based on land use, land treatment, soil types, and

land slope.

The runoff factor will be affected by land uses – watershed cover, both agricultural and

nonagricultural uses – by the types of vegetation found on the land, or by the presence of any

water surfaces, roads, or roofs. The runoff factor will also be affected by how the land has

been treated – by mechanical practices such as contouring or terracing and by management

practices such as rotating crops.

The CN method uses soil conditions and land use to assign a runoff factor (CN) when the soil

is not frozen. The higher the CN, the higher the runoff potential. Runoff from snowfall or frozen

ground cannot be accounted for using the CN method.

The CN method accounts for surface runoff only and does not take interflow or groundwater

into consideration.

The soil groups identified in Section 2.4.3 for the Rational method are also used for the CN

method. They are based on infiltration rates and can be found in the Lancaster County Soil

Survey. When considering hydrologic soil groups, the effects of urbanization, such as heavy

equipment compaction of soil, should be considered. The CN method describes average

conditions, and since a watershed or subwatershed is described by one CN value, the

watershed or sub-watershed should be delineated (to the extent feasible) so that it is

hydrologically homogeneous.

The CN can vary with antecedent soil moisture conditions, which is influenced by the amount

of rainfall occurring in a selected period preceding a given storm. The more rain, the more

direct runoff. A five-day period is used as the minimum for estimating antecedent moisture

conditions.



19

CNs can be found from the following exhibits depending upon their application:

• Table 2-7 is to be used as a reference for estimating CNs in urban areas

• Table 2-8 is to be used as a reference for estimating CNs for cultivated agricultural

areas

• Table 2-9 is to be used as a reference for estimating CNs for other types of agricultural

areas

• Table 2-10 is to be used to convert CNs (found from the other exhibits) to a dry or wet

corresponding CN based on antecedent soil moisture conditions (not typically used for

general applications)

• Table 2-11 is to be used to determine antecedent moisture condition (not typically

used for general applications)

Composite CNs can be calculated when a drainage area has more than one land use, if the

drainage area can be adequately represented by the composite CN. Composite CNs can be

calculated by using Table 2-12.



20

Table 2-1. City of Lincoln Rainfall Amounts by Recurrence Interval and Percentiles.

Water Quantity Calculations* Water Quality Calculations

Recurrence Rainfall (inches) Percentile (%) 24-hour Rainfall (inches)

2 years 3.04 70 0.62

5 years 3.79 75 0.70

10 years 4.48 80 0.83

25 years 5.52 85 0.97

50 years 6.40 90 1.25

100 years 7.33 95 1.65

500 years 9.79

* Source: NOAA Atlas 14 data (2013)

Figure 2-1. NOAA Atlas 14 Intensity-Duration-Frequency Curves for Lincoln, Nebraska.

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0 20 40 60 80 100 120 140

Inte

nsi

ty (

in/h

r)

Minutes

NOAA Atlas 14 IDF Curve for Lincoln, NE

5 year 10 year 100 year



21

Table 2-2. NOAA Atlas 14 Intensity-Duration-Frequency Table for Lincoln, Nebraska.

Intensity (inches/hour)

Minutes (Tc) 5-year event 10-year event 100-year event

5 6.98 8.20 12.48 8 5.58 6.55 9.96

10 5.11 6.00 9.12 15 4.16 4.88 7.40 30 3.04 3.58 5.42 60 1.98 2.33 3.58 120 1.22 1.44 2.23

Table 2-3. Recommended Rational Method Coefficient of Runoff Values for Various Selected Land Uses.

Description of Area Runoff Coefficients

Business: Downtown areas 0.70 – 0.95

Neighborhood areas 0.50 – 0.70

Residential: Single-family areas 0.30 – 0.50

Multi units, detached 0.40 – 0.60

Multi units, attached 0.60 – 0.75

Suburban 0.25 – 0.40

Residential (1-acre lots or larger) 0.30 – 0.45

Apartment dwelling areas 0.50 – 0.70

Industrial: Light areas 0.50 – 0.80

Heavy Areas 0.60 – 0.90

Parks, cemeteries 0.10 – 0.25

Playground areas 0.20 – 0.40

Railroad yard areas 0.20 – 0.40

Unimproved areas 0.04 – 0.38 (see Table 2-4)



22

Table 2-4. Recommended Rational Method Coefficient of Runoff for Pervious Surfaces (Unimproved Areas) by Selected Hydrologic Soil Groupings and Slope Ranges.

Slope Soil Group A Soil Group B Soil Group C Soil Group D

Flat 0.04 – 0.09 0.07 – 0.12 0.11 – 0.16 0.15 – 0.20 (0 – 1%)

Average 0.09 – 0.14 0.12 – 0.17 0.16 – 0.21 0.20 – 0.25

(2 – 6%)

Steep 0.13 – 0.18 0.18 – 0.24 0.23 – 0.31 0.28 – 0.38

(Over 6%) Note: Soil Groups are further described in Section 2.4.2 Rainfall Intensity (I).

Figure 2-2. Graphic Data for the MSE3 Distribution.

Note: The Type II rainfall distribution from Technical Paper 40 is shown for comparison. Use the MSE3

rainfall distribution from NOAA Atlas 14.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 5.0 10.0 15.0 20.0

Cu

mu

lati

vfe

Rai

nfa

ll, C

um

ula

tive

un

it in

ches

Time Hours

Design Storm Curves

MSE3 unit inches Type 2 unit inches



23

Table 2-5. Tabular Rainfall Distribution from NOAA Atlas 14.

Time MSE3 Type II

Time MSE3 Type II

Time MSE3 Type II

hours unit inches

unit inches

hours unit

inches unit inches

hours unit

inches unit inches

0.0 0.0000 0.0000

4.0 0.0261 0.0480

8.0 0.0837 0.1200

0.1 0.0003 0.0010

4.1 0.0271 0.0494

8.1 0.0856 0.1222

0.2 0.0006 0.0020

4.2 0.0282 0.0508

8.2 0.0874 0.1246

0.3 0.0009 0.0030

4.3 0.0293 0.0523

8.3 0.0893 0.1270

0.4 0.0012 0.0041

4.4 0.0304 0.0538

8.4 0.0912 0.1296

0.5 0.0015 0.0051

4.5 0.0316 0.0553

8.5 0.0931 0.1322

0.6 0.0019 0.0062

4.6 0.0327 0.0568

8.6 0.0951 0.1350

0.7 0.0023 0.0072

4.7 0.0339 0.0583

8.7 0.0970 0.1379

0.8 0.0027 0.0083

4.8 0.0351 0.0598

8.8 0.0990 0.1408

0.9 0.0031 0.0094

4.9 0.0363 0.0614

8.9 0.1010 0.1438

1.0 0.0036 0.0105

5.0 0.0375 0.0630

9.0 0.1031 0.1470

1.1 0.0040 0.0116

5.1 0.0388 0.0646

9.1 0.1063 0.1502

1.2 0.0045 0.0127

5.2 0.0401 0.0662

9.2 0.1096 0.1534

1.3 0.0050 0.0138

5.3 0.0414 0.0679

9.3 0.1129 0.1566

1.4 0.0055 0.0150

5.4 0.0427 0.0696

9.4 0.1163 0.1598

1.5 0.0061 0.0161

5.5 0.0440 0.0712

9.5 0.1197 0.1630

1.6 0.0067 0.0173

5.6 0.0454 0.0730

9.6 0.1231 0.1663

1.7 0.0072 0.0184

5.7 0.0467 0.0747

9.7 0.1266 0.1697

1.8 0.0078 0.0196

5.8 0.0481 0.0764

9.8 0.1302 0.1733

1.9 0.0085 0.0208

5.9 0.0495 0.0782

9.9 0.1338 0.1771

2.0 0.0091 0.0220

6.0 0.0510 0.0800

10.0 0.1374 0.1810

2.1 0.0098 0.0232

6.1 0.0524 0.0818

10.1 0.1411 0.1851

2.2 0.0104 0.0244

6.2 0.0539 0.0836

10.2 0.1449 0.1895

2.3 0.0111 0.0257

6.3 0.0554 0.0855

10.3 0.1487 0.1941

2.4 0.0119 0.0269

6.4 0.0569 0.0874

10.4 0.1525 0.1989

2.5 0.0126 0.0281

6.5 0.0584 0.0892

10.5 0.1564 0.2040

2.6 0.0134 0.0294

6.6 0.0600 0.0912

10.6 0.1621 0.2094

2.7 0.0141 0.0306

6.7 0.0615 0.0931

10.7 0.1686 0.2152

2.8 0.0149 0.0319

6.8 0.0631 0.0950

10.8 0.1759 0.2214

2.9 0.0158 0.0332

6.9 0.0647 0.0970

10.9 0.1839 0.2280

3.0 0.0166 0.0345

7.0 0.0664 0.0990

11.0 0.1927 0.2350

3.1 0.0175 0.0358

7.1 0.0680 0.1010

11.1 0.2023 0.2427

3.2 0.0183 0.0371

7.2 0.0697 0.1030

11.2 0.2126 0.2513

3.3 0.0192 0.0384

7.3 0.0714 0.1051

11.3 0.2236 0.2609

3.4 0.0202 0.0398

7.4 0.0731 0.1072

11.4 0.2354 0.2715

3.5 0.0211 0.0411

7.5 0.0748 0.1093

11.5 0.2480 0.2830

3.6 0.0221 0.0425

7.6 0.0765 0.1114

11.6 0.2663 0.3068

3.7 0.0230 0.0439

7.7 0.0783 0.1135

11.7 0.2906 0.3544

3.8

0.0240 0.0452

7.8 0.0801 0.1156

11.8 0.3245 0.4308

3.9 0.0250 0.0466

7.9 0.0819 0.1178

11.9 0.3725 0.5679



24

Table 2-5. Tabular Rainfall Distribution from NOAA Atlas 14 (continued).

Time MSE3 Type II

Time MSE3 Type II

Time MSE3 Type II

hours unit inches

unit inches

hours unit

inches unit inches

hours unit

inches unit inches

12.0 0.4629 0.6630

16.0 0.9163 0.8800

20.0 0.9739 0.9520

12.1 0.6276 0.6820

16.1 0.9181 0.8823

20.1 0.9750 0.9533

12.2 0.6756 0.6986

16.2 0.9199 0.8845

20.2 0.9760 0.9546

12.3 0.7094 0.7130

16.3 0.9217 0.8868

20.3 0.9770 0.9559

12.4 0.7337 0.7252

16.4 0.9235 0.8890

20.4 0.9780 0.9572

12.5 0.7520 0.7350

16.5 0.9252 0.8912

20.5 0.9789 0.9584

12.6 0.7646 0.7434

16.6 0.9269 0.8934

20.6 0.9798 0.9597

12.7 0.7764 0.7514

16.7 0.9287 0.8955

20.7 0.9808 0.9610

12.8 0.7874 0.7588

16.8 0.9303 0.8976

20.8 0.9817 0.9622

12.9 0.7977 0.7656

16.9 0.9320 0.8997

20.9 0.9825 0.9635

13.0 0.8073 0.7720

17.0 0.9337 0.9018

21.0 0.9834 0.9647

13.1 0.8161 0.7780

17.1 0.9353 0.9038

21.1 0.9842 0.9660

13.2 0.8241 0.7836

17.2 0.9369 0.9058

21.2 0.9851 0.9672

13.3 0.8314 0.7890

17.3 0.9385 0.9078

21.3 0.9859 0.9685

13.4 0.8379 0.7942

17.4 0.9401 0.9097

21.4 0.9866 0.9697

13.5 0.8436 0.7990

17.5 0.9416 0.9117

21.5 0.9874 0.9709

13.6 0.8475 0.8036

17.6 0.9431 0.9136

21.6 0.9881 0.9722

13.7 0.8513 0.8080

17.7 0.9446 0.9155

21.7 0.9889 0.9734

13.8 0.8551 0.8122

17.8 0.9461 0.9173

21.8 0.9896 0.9746

13.9 0.8589 0.8162

17.9 0.9476 0.9192

21.9 0.9902 0.9758

14.0 0.8626 0.8200

18.0 0.9491 0.9210

22.0 0.9909 0.9770

14.1 0.8662 0.8237

18.1 0.9505 0.9228

22.1 0.9916 0.9782

14.2 0.8698 0.8273

18.2 0.9519 0.9245

22.2 0.9922 0.9794

14.3 0.8734 0.8308

18.3 0.9533 0.9263

22.3 0.9928 0.9806

14.4 0.8769 0.8342

18.4 0.9547 0.9280

22.4 0.9934 0.9818

14.5 0.8803 0.8376

18.5 0.9560 0.9297

22.5 0.9939 0.9829

14.6 0.8837 0.8409

18.6 0.9573 0.9313

22.6 0.9945 0.9841

14.7 0.8871 0.8442

18.7 0.9587 0.9330

22.7 0.9950 0.9853

14.8 0.8904 0.8474

18.8 0.9599 0.9346

22.8 0.9955 0.9864

14.9 0.8937 0.8505

18.9 0.9612 0.9362

22.9 0.9960 0.9876

15.0 0.8970 0.8535

19.0 0.9625 0.9377

23.0 0.9964 0.9887

15.1 0.8990 0.8565

19.1 0.9637 0.9393

23.1 0.9969 0.9899

15.2 0.9010 0.8594

19.2 0.9649 0.9408

23.2 0.9973 0.9910

15.3 0.9030 0.8622

19.3 0.9661 0.9423

23.3 0.9977 0.9922

15.4 0.9049 0.8649

19.4 0.9673 0.9438

23.4 0.9981 0.9933

15.5 0.9069 0.8676

19.5 0.9684 0.9452

23.5 0.9985 0.9944

15.6 0.9088 0.8702

19.6 0.9696 0.9466

23.6 0.9988 0.9956

15.7 0.9107 0.8728

19.7 0.9707 0.9480

23.7 0.9991 0.9967

15.8 0.9126 0.8753

19.8 0.9718 0.9493

23.8 0.9994 0.9978

15.9 0.9144 0.8777

19.9 0.9729 0.9507

23.9 0.9997 0.9989

24.0 1.0000 1.0000



25

Table 2-6. Roughness Coefficients (Manning’s n) for Sheet Flow.

Surface Description n

Smooth surfaces (concrete, asphalt, gravel, or bare soil) 0.011

Fallow (no residue) 0.05

Cultivated Soils:

Residue cover < 20% 0.06

Residue cover > 20% 0.17

Grasses:

Shortgrass prairie 0.15

Dense grasses1 0.24

Bermuda grass 0.41

Range (natural) 0.13

Woods:

Light underbrush 0.40

Dense underbrush 0.80

1 Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass,

and native grass mixtures.

2 When selecting n, consider cover to a height of about 0.1 foot. This is the only part of the

plant cover that will obstruct sheet flow.



26

Table 2-7. Runoff Curve Numbers – Urban Areas.

Cover Type and Hydrologic Condition Average Percent Impervious Area1

Curve Numbers for Hydrologic Soil Groups

A B C D

Fully developed urban areas (vegetation established)

Open space (lawns, parks, golf courses, cemeteries, etc.)2

Poor condition (grass cover <50%) 68 79 86 89

Fair condition (grass cover 50% to 75%) 49 69 79 84

Good condition (grass cover > 75%) 39 61 74 80

Impervious areas:

Paved parking lots, roofs, driveways, etc. (excluding right-of-way)

98 98 98 98

Streets and roads:

Paved; curbs and storm drains (excluding right-of-way)

98 98 98 98

Paved; open ditches (including right-of-way) 83 89 92 93

Gravel (including right-of-way) 76 85 89 91

Dirt (including right-of-way) 72 82 87 89

Urban districts:

Commercial and business 85% 89 92 94 95

Industrial 72% 81 88 91 93

Residential districts by average lot size:

1/8 acre or less (town houses) 65% 77 85 90 92

1/4 acre 38% 61 75 83 87

1/3 acre 30% 57 72 81 86

1/2 acre 25% 54 70 80 85

1 acre 20% 51 68 79 84

2 acres 12% 46 65 77 82

Developing urban areas

Newly graded areas (pervious areas only, no vegetation)

77 86 91 94

Idle lands (curve numbers [CNs] are determined using cover types similar to those in Table 2-9).

Average runoff condition and initial abstraction such as surface storage, interception and infiltration prior to runoff are assumed to be 20% of the potential maximum retention, Ia = 0.2S.

1 The average percentage of impervious area shown was used to develop the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the SCS method has an adjustment to reduce the effect.

2 CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type.

Source: TR-55



27

Table 2-8. Cultivated Agricultural Land.

Cover Description Curve Numbers for Hydrologic Soil Groups

Cover Treatment1 Type Hydrologic Condition2 A B C D

Fallow Bare soil - 77 86 91 94

Crop residue Poor 76 85 90 93

Cover (CR) Good 74 83 88 90

Row straight row (SR) Poor 72 81 88 91

Crops Good 67 78 85 89

SR + CR Poor 71 80 87 90

Good 64 75 82 85

Contoured (C) Poor 70 79 84 88

Good 65 75 82 86

C + CR Poor 69 78 83 87

Good 64 74 81 85

Contoured & Terraced (C&T) Poor 66 74 80 82

Good 62 71 78 81

C&T + CR Poor 65 73 79 81

Good 61 70 77 80

Small grain (SR) Poor 65 76 84 88

Good 63 75 83 87

SR + CR Poor 64 75 83 86

Good 60 72 80 84

C Poor 63 74 82 85

Good 61 73 81 84

C + CR Poor 62 73 81 84

Good 60 72 80 83

C&T Poor 61 72 79 82

Good 59 70 78 81

C&T + CR Poor 60 71 78 81

Good 58 69 77 80

Close-seeded SR Poor 66 77 85 89

or broadcast Good 58 72 81 85

Legumes or C Poor 64 75 83 85

Rotation Good 55 69 78 83

Meadow C&T Poor 63 73 80 83

Good 51 67 76 80


1 Crop residue cover applies only if residue is on at least 5 percent of the surface throughout the year.

2 Hydrologic condition is based on a combination of factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas; (b) amount of year-round cover; (c) amount of grass or closed-seeded legumes in rotations; (d) percentage of residue cover on the land surface (good > 20 percent); and (e) degree of roughness.

Poor: Factors impair infiltration and tend to increase runoff. Good: Factors encourage average and better than average infiltration and tend to decrease runoff.

Source: TR-55



28

Table 2-9. Other Agricultural Lands.

Cover Description Curve Numbers for Hydrologic Soil

Group

Cover type Hydrologic condition

A B C D

Pasture, grassland, or range-continuous forage for grazing1

Poor 68 79 86 89

Fair 49 69 79 84

Good 39 61 74 80

Meadow – continuous grass, protected from grazing and generally mowed for hay

30 58 71 78

Brush – brush-weed-grass mixture with brush the major element2

Poor 48 67 77 83

Fair 35 56 70 77

Good 330 48 65 73

Woods – grass combination orchard or tree farm)4

Poor 57 73 82 86

Fair 43 65 76 82

Good 32 58 72 79

Woods5

Poor 45 66 77 83

Fair 36 60 73 79

Good 430 55 70 77

Farmsteads – buildings, lanes, driveways, and surrounding lots

59 74 82 86


1 Poor: < 50 percent ground cover or heavily grazed with no mulch Fair: 50 to 75 percent ground cover and not heavily grazed

Good: > 75 percent ground cover and lightly or only occasionally grazed

2 Poor: < 50 percent ground cover Fair: 50 to 75 percent ground cover Good: > 75 percent ground cover

3 Actual curve number (CN) is less than 30; use CN = 30 for runoff computations.

4 CNs shown were computed for areas with 50 percent grass (pasture) cover. Other combinations of conditions may be computed from CNs for woods and pasture.

5 Poor: Forest litter, small trees and brush are destroyed by heavy grazing or regular burning. Fair: Woods grazed but not burned, and some forest litter covers the soil. Good: Woods protected from grazing, litter and brush adequately cover soil.

Source: TR-55



29

Table 2-10. Conversion from Average Antecedent Moisture Conditions to Dry and Wet Conditions.

Curve Number (CN) for Average Conditions

Corresponding CNs

Dry Wet

100 100 100

95 87 98

90 78 96

85 70 94

80 63 91

75 57 88

70 51 85

65 45 82

60 40 78

55 35 74

50 31 70

45 26 65

40 22 60

35 18 55

30 15 50

25 12 43

15 6 30

5 2 13

Source: USDA Soil Conservation Service TP-149 (SCS-TP-149) 1973.



30

Table 2-11. Rainfall Groups for Antecedent Soil Moisture Conditions during Growing and Dormant Seasons.

Antecedent Conditions Growing Season 5-day Dormant Season 5-day

Condition Description Antecedent Rainfall Antecedent Rainfall

Dry

An optimum condition of watershed soils, where soils are dry but not to the wilting point and when satisfactory plowing or cultivation takes place

Less than 1.4 in. Less than 0.5 in.

Average The average case for annual floods

1.4 – 2.1 in. 0.5 – 1.1 in.

Wet

When a heavy rainfall, or light rainfall and warm temperatures, have occurred during the five days before a given storm

Over 2.1 in. Over 1.1 in.

Source: Soil Conservation Service

Table 2-12. Composite Curve Number Calculations.

Group A Group B Group C Group D Group E

Land Use Curve Number

Area % of Total Curve Number

Composite Curve Number (Column B x Column D)

Composite curve numbers for a drainage area can be calculated by entering the required data into a spreadsheet (see this exhibit for column headings): The composite curve n

2.0 hydrology 2.1 introduction

Documents