FLOOD00850

• Designation Number: 31
• County: Arapahoe
• Community: Unincorporated Arapahoe County
• Title: Major Drainageway Planning - Little Dry Creek, Report, Volume I
• Date: 2/1/1974
• Designation Date: 6/1/1974
• Floodplain - Doc Type: Floodplain Report/Masterplan

I I
I I

I

valent hydro graphs in the Little Dry Creek sub-basins in which comparisons

were made. A sample of a print-out of the computer calculated hydro graph

is shown on Table 3 which follows.

I

I

The spillway design floods were derived in the same manner as the 10, 25 and

100-year frequency flood hydrographs, and they were checked by the method

described in the Second Edition, 1973 of Design of Small Dams, U.S. Bureau

of Reclamation. The Bureau of Reclamation method gave a substantially higher

peak discharge for the Englewood Dam spillway design flood. The higher peak

discharge resulted primarily from different rainfall distribution methods.

I

I

I

The effect of increasing imperviousness on both peak flood runoff and volume

of runoff was studied. Hydrographs were computed over a range of zero to one

hundred percent imperviousness of the sub-basin on Willow Creek above

hydrologic point 206 at the upper end of the Little Dry Creek Basin.

The following chart, Chart 4, shows that if this presently undeveloped

watershed was developed to a fully impervious condition, the peak runoff

rate could be expected to increase by 36 percent and the flood volume

35 percent. Residential areas do not normally exceed about 60 percent

imperviousness, and only about 20 percent imperviousness is expected above

point 206 under future conditions. This is expected to result in nominal

increases of four and six percent in runoff volume and peak rate, respectively.

Other sub-basins probably would react differently to increased impervious-

ness, but the foregoing relationship is indicative of the general tendency

for basin development to increase both peak runoff and runoff volume.

I

I

I

I

I

I

I

I

I

I

-31-

I