FLOOD05894

• Designation Number: 342
• County: Jefferson
• Community: Arvada
• Stream Name: Moon Gulch Watershed
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Moon Gulch Watershed
• Date: 8/1/1992
• Designation Date: 9/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydrograph shape depends on a number of watershed characteristics, The configuration

of the watershed and its slope are two of the most important characteristics, A short, wide

watershed can be expected to have a high rate of peak discharge that develops quickly, while

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a long, narrow watershed will have a relatively lower peak discharge that develops more

slowly. A steeply sloped watershed will have a relatively higher peak rate of discharge that

develops more quickly than would a more gently sloping watershed. Peak rates from

upstream water shed areas may be reduced as they pass through long, narrow, gently sloping

watersheds because of the effect of natural channel and flood plain storage capacity. A

synthetic unit hydrograph method was used to develop flood hydrographs for the Moon

Gulch basins. The unit hydrograph method used is the Colorado Urban Hydrograph

Procedure as described in the Drainage Criteria Manual. In this method, it is first necessary

to determine the area, stream length, location of the centroid, and the average slope of each

sub-basin drainage area. These physical characteristics of each sub-basin plus two empirical

coefficients were used to develop the unit hydro graphs. The empirical coefficients, Ct and

Cp' are coefficients of time to unit hydrograph peak and peak rate of runoff, respectively.

These coefficients are given in the Drainage Criteria Manual for 60, 40 and 20 percent of

watershed imperviousness, and it was necessary to interpolate to obtain coefficients for the

estimated percent imperviousness for future land use conditions of the basins.

Figure 3- shows the 100-year hydrographs for existing and-future basin development at the

Ralston Valley Detention Cell at 72nd Avenue and Eldridge Street.

The projected peak discharges along the Moon Gulch channel are shown in Figure 4.

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