Denver, Colorado

2015

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U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Contents

Introduction...................................................................................................................................................................................... 1 Data for U.S. Geological Survey Streamgage 09358550 Cement Creek at Silverton, Colorado ..................................................... 1 Calculation of the Volume of Water for the August 5, 2015, Streamflow Event ............................................................................... 3 Quality Assurance for U.S. Geological Survey Streamgage 09358550 Cement Creek at Silverton, Colorado ............................... 5 References Cited ............................................................................................................................................................................. 5

Figures

Figure 1. Location of U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, Colorado. .......................... 2

Figure 2. Hydrograph of 15-minute discharge data for U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, and the duration of the streamflow event assuming a base flow of 26 cubic feet per second, August 5, 2015. .............. 3

Figure 3. Discharge and water volume for each 15-minute period greater than base flow for the August 5, 2015, streamflow event at the U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, Colorado. ............................ 5

Table

Table 1. Calculation of the total volume of water greater than base flow for the August 5, 2015, streamflow event at the U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, Colorado. .................................................................. 4

1

Introduction

On August 5, 2015, a streamflow event lasting several hours and reaching a maximum instantaneous

discharge of 120 cubic feet per second (ft3/s) was recorded at the U.S. Geological Survey (USGS) streamgage

09358550 Cement Creek at Silverton, Colorado. On August 8, 2015, the USGS was asked by the U.S.

Environmental Protection Agency Region 8 (EPA) to calculate the volume of water generated by the event. Using

provisional data collected at the streamgage, the USGS estimated the volume of water produced by the event to

be about 3 million gallons greater than the base flow that existed in the stream on August 5. On August 9, 2015,

the USGS provided this volume estimate to the EPA.

The discharge data for August 5, 2015, for USGS streamgage 09358550 Cement Creek at Silverton,

Colorado, were approved by the USGS Colorado Water Science Center on November 13, 2015. This report

summarizes the procedures and calculations used to estimate the volume of water contained in the August 5,

2015, streamflow event using the approved data. Using approved data produced an estimate of water volume that

was the same as what was originally estimated using the provisional data. That is, using approved data collected

at the streamgage, the USGS estimated the volume of water produced by the event to be about 3 million gallons

greater than the base flow that existed in the stream on August 5, 2015.

Data for U.S. Geological Survey Streamgage 09358550 Cement Creek at Silverton, Colorado

The USGS streamgage 09358550 Cement Creek at Silverton, Colorado, is located 0.1 mile north of

Silverton, Colorado, and 0.8 mile upstream from the confluence with the Animas River (fig. 1). The streamgage

records observations of stream stage (water level) every 15 minutes. Derivation of discharge from stage data

requires direct measurements of discharge at various stages for constructing a relation that converts stage to

discharge (hereafter referred to as a rating curve). Onsite direct measurements of discharge are calculated by

USGS personnel using physical measurements of stream velocity, depth, and width (Turnipseed and Sauer,

2010). Periodic measurements of discharge are required to maintain calibration of the rating curve. The rating

curve allows for the determination of discharge from the stage data when USGS personnel are not physically

present at the streamgage to make a discharge measurement.

Stage data recorded at the streamgage on Cement Creek are transmitted by satellite to a USGS office

where the rating curve is automatically applied to determine a discharge for each corresponding value of stage. In

turn, the stage and discharge values are posted on NWISWeb, the USGS online water database which is

publically available at: http://waterdata.usgs.gov/co/nwis/uv/?site_no=09358550. The stage and discharge data

are considered provisional until reviewed and approved by USGS personnel.

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Figure 1. Location of U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, Colorado.

3

Calculation of the Volume of Water for the August 5, 2015, Streamflow Event

The first step in calculating the volume of water in the August 5, 2015, streamflow event was to

determine the duration of the event above base flow. From 12:00 a.m. to 12:30 p.m. Mountain Daylight Time on

August 5, 2015, discharge at the Cement Creek streamgage was fairly consistent and represents base flow ranging

between 24 and 28 ft3/s (fig. 2). At 12:30 p.m., discharge was 26 ft

3/s before rising to a peak of 120 ft

3/s at 12:45

p.m. Discharge returned to 26 ft3/s at 7:15 p.m. on August 5. The duration of the streamflow event was

determined by the period of discharge that was greater than an assumed base flow of 26 ft3/s (fig. 2). This resulted

in a start time of about 12:30 p.m. and an end time of about 7:15 p.m.

Figure 2. Hydrograph of 15-minute discharge data for U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, and the duration of the streamflow event assuming a base flow of 26 cubic feet per second, August 5, 2015.

The total volume of water above base flow for the August 5, 2015, event in Cement Creek was calculated

by initially subtracting the assumed base flow of 26 ft3/s from each 15-minute discharge value for the duration of

the event. The resulting discharge values were then used to compute the volume of water above base flow, in

cubic feet, for each 15-minute period. This entailed multiplying each reported discharge minus base flow (in ft3/s)

by 900 seconds (the number of seconds in each 15-minute period). The volume of water for each 15-minute

period was converted from cubic feet to gallons by using a multiplier of 7.48. Lastly, the volumes of water above

base flow for each 15-minute period during the event were summed resulting in approximately 3 million gallons

of total water volume above base flow (table 1). A graph of discharge and water volume for each 15-minute

period greater than base flow for the event is presented in figure 3.

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Table 1. Calculation of the total volume of water greater than base flow for the August 5, 2015, streamflow event at the U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, Colorado.

Date/time Discharge,

in cubic feet per second

Discharge greater than base flow of 26 cubic feet per second

Volume of water greater than base flow

for a 15-minute period, in cubic feet

Volume of water greater than base flow

for a 15-minute period, in gallons

8/5/15 12:30 p.m. 26 0 0 0

8/5/15 12:45 p.m. 120 94 84,600 633,000

8/5/15 1:00 p.m. 85 59 53,100 397,000

8/5/15 1:15 p.m. 67 41 36,900 276,000

8/5/15 1:30 p.m. 67 41 36,900 276,000

8/5/15 1:45 p.m. 62 36 32,400 242,000

8/5/15 2:00 p.m. 61 35 31,500 236,000

8/5/15 2:15 p.m. 43 17 15,300 114,000

8/5/15 2:30 p.m. 38 12 10,800 81,000

8/5/15 2:45 p.m. 39 13 11,700 88,000

8/5/15 3:00 p.m. 38 12 10,800 81,000

8/5/15 3:15 p.m. 38 12 10,800 81,000

8/5/15 3:30 p.m. 36 10 9,000 67,000

8/5/15 3:45 p.m. 34 8 7,200 54,000

8/5/15 4:00 p.m. 34 8 7,200 54,000

8/5/15 4:15 p.m. 33 7 6,300 47,000

8/5/15 4:30 p.m. 31 5 4,500 34,000

8/5/15 4:45 p.m. 33 7 6,300 47,000

8/5/15 5:00 p.m. 30 4 3,600 27,000

8/5/15 5:15 p.m. 30 4 3,600 27,000

8/5/15 5:30 p.m. 30 4 3,600 27,000

8/5/15 5:45 p.m. 29 3 2,700 20,000

8/5/15 6:00 p.m. 29 3 2,700 20,000

8/5/15 6:15 p.m. 30 4 3,600 27,000

8/5/15 6:30 p.m. 29 3 2,700 20,000

8/5/15 6:45 p.m. 28 2 1,800 13,000

8/5/15 7:00 p.m. 29 3 2,700 20,000

8/5/15 7:15 p.m. 26 0 0 0

Total volume, in gallons (rounded) 3,000,000

5

Figure 3. Discharge and water volume for each 15-minute period greater than base flow for the August 5, 2015, streamflow event at the U.S. Geological Survey streamgage 09358550 Cement Creek at Silverton, Colorado.

Quality Assurance for U.S. Geological Survey Streamgage 09358550 Cement

Creek at Silverton, Colorado

The calibration of the rating curve that was in effect on August 5, 2015, for the Cement Creek streamgage

was verified with streamflow measurements on July 31 and August 8, 2015. The measurements of 25.8 ft3/s on

July 31 and 26.4 ft3/s on August 8 were within 2.8 percent of the corresponding values from the rating curve.

Also, high water marks at the Cement Creek streamgage left by the August 5 event were documented by a USGS

technician on August 8, 2015. The values of the high water marks were within 0.06 foot of the maximum stage

recorded by instrumentation at the streamgage. Therefore, it is assumed the USGS streamgage at Cement Creek

recorded the peak of the runoff event on August 5, 2015.

Accuracy of discharge data depends on stability of the rating curve, frequency of direct discharge

measurements, accuracy of stage and discharge measurements, and interpretation of the records. The degree of

accuracy of the records at the USGS streamgage at Cement Creek in August, 2015 is considered fair, which

means that about 95 percent of the daily-mean discharges are within 15 percent of the actual value (Rantz and

others, 1982)

References Cited

Rantz, S.E., and others, 1982, Measurement and computation of streamflow—Volume 1, Measurement of stage

and discharge, and Volume 2, Computation of discharge: U.S. Geological Survey Water-Supply Paper 2175, p.

1–631. [Also available at http://pubs.usgs.gov/wsp/wsp2175/.]

Turnipseed, P.D., and Sauer, V.B., 2010, Discharge measurements at gaging stations: U.S. Geological Survey

Techniques and Methods, book 3, chap. A8, 87 p.