FLOOD09767

• County: Huerfano
• Community: Walsenburg
• Basin: Arkansas
• Title: Flood Hazard Mitigation Feasibilty Study
• Date: 2/1/1997
• Prepared For: Walsenburg
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

VII. ENGINEERING ANALYSES

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Design drawings and technical backup data for the existing storm drain are unavailable,
Therefore, the peak flows used for this study were estimated using the Rational Method,
The lO-year and lOO-year peak flows for the existing storm drain outfall at Holita ditch
near Russell A venue were computed for the purposes of this study. The Rational Method
is an acceptable methodology for estimating runoff from small basins in developed areas.
The equation for the Rational Method is:

Q=CIA

Where:

Q = peak discharge (cfs)
C = runoff coefficient (varies with land use)
I = rainfall intensity (inches per hour) for the specific frequency of interest for a
specific time duration which is equal to the time of concentration (Ie) of the
watershed
area of watershed (acres)

.

A =

The watershed area for the existing storm drain was planimetered to be lOS acres, The
runoff coefficients for the lO-year and lOO-year storms were estimated to be 0,70 and
0.80 based on mixed residential and commercial development of the watershed. The time
of concentration for the watershed was estimated using an equation published in the
Urban Drainage and Flood Control District's storm drainage criteria manual. The time of
concentration was computed using Ie = (U180) + 10, where L is the length of the flow
path (in feet) from the hydrologic point of interest to the most distant point in the
watershed. For this study, L was measured to be 3,000 feet, and thus Ie was computed to
be approximately 27 minutes, Finally, the rainfall intensity was estimated to be 2,8 inlhr
for the 10-year event and 4.5 inlhr for the lOO-year event. The hydrologic parameters for
the 1O-year and lOO-year events are shown in the table below.

Watershed Parameter' , , 10.yeariEvenl~TjT itoo-Year Event
Peak Flow (cfs) 20S 380
Area (acres) 10S IOS
Runoff Coefficient 0.70 0,80
Rainfall Intensity (inlhr) 2.8 4.S
Time of Concentration (min) 27 27

In addition to the flows contributed by the storm drain, the NRCS estimated additional
flow amounts that would enter the Holita Ditch between Russell Avenue and Interstate
2S, The additional flows were estimated to be 230 cfs and 430 cfs for the lO-year and
lOO-year events, respectively,

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