FLOOD06956

• County: Huerfano
• Community: Walsenburg
• Basin: Arkansas
• Title: Drainage Ditch Evaluation: Walsenburg Colorado
• Date: 4/25/1996
• Prepared For: CWCB
• Prepared By: Colorado School of Mines
• Floodplain - Doc Type: Floodplain Report/Masterplan

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EXECUTIVE SUMMARY

After being asked by the Colorado Water Conservation Board to research the Holita Ditch
I in Walsenburg, Colorado, we concluded that the Holita Ditch is indeed capable of flooding due to

the storm drainage runoff that flows into the ditch from a 57" x 38" outlet pipe. The area of
drainage from the City ofWalsenburg into the ditch is approximately 8,103,600 square feet~I;;/
order to compute the worst case scenario, this area is considered to be all concrete and asphalt.
The Holita Ditch has a maximum capacity of 24 cubic feet per second with an average flow of 6
cubic feet per second flowing through the ditch at a given time.
We also used the Rational Method (Q=CIA) to calculate the storm drainage runoff that
will flow into the Holita Ditch at different rainfall intensities. For our calculations, runoff
coefficients for concrete or asphalt were used which have a minimum value of 0.80 and a
maximum value of 0.95. When the Intensity of Rainfall is 0.12 inches per hour and the runoff
coefficient is 0.80, the rate offlow into the ditch will be 18 cubic feet per second. When the
Intensity of Rainfall is 0.10 inches per hour and the runoff coefficient is 0.95, the rate of flow into
the ditch will be 18 cubic feet per second. These two cases are the maximum rainfall intensities
for the different runoff coefficients that will not flood the Holita Ditch,
Intensities of Rainfall that would exceed the ditch's capacity were found and are as
follows. When the Intensity of Rainfall is 0.5 inches per hour and the runoff coefficient is 0.80,
the rate of flow into the ditch will be 75 cfs. This amount of runoff flow is 288% the maximum
amount of flow that the Holita Ditch can handle. When the Intensity of Rainfall is 0.5 inches per
hour and the runoff coefficient is 0.95, the rate offlow into the ditch will be 89 cfs. When the
Intensity of Rainfall is 1.0 inches per hour and the runoff coefficient is 0.80, the rate of flow into
. the ditch will be 150 cfs. This amount of runoff flow is 600% the maximum amount of flow that

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the Holita Ditch can handle. When the Intensity of Rainfall is 1.0 inches per hour and the runoff

coefficient is 0.95 the rate offlow into the ditch will be 178 cfs From these calculations, when a

short, intense thunderstorm takes place, the Holita ditch will severely flood because the Intensity
of Rainfall would be more than 0.12 inches per hour.
In William C. Wittwer's report to Donald R. Martin, some rough storm runoff calculations
are given (See Appendix B). When a 10 year event occurs, the peak runoff will be approximately