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

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Runoff

Surface runoff from a basin equals total rainfall minus basin retention

and losses. Basin retention includes temporary ponding, temporary

interception of rainfall and the like that delay surface runoff. Rain-

fall losses include phenomena such as infiltration and evaporation

that reduce the amount of water that reaches the drainage channel.

An important consideration in estimating basin retention and losses

is the appraisal of antecedent moisture conditions on the watershed.

Denver area rainfall records indicate that major rainfall events are

frequently preceded by rains, so that the design storm losses to ground

absorption and temporary retention will be small. Basin retention and

losses also depend on basin soil characteristics and the extent of ur-

banization. The soils in the Little Dry Creek basin have low rates of

absorption. Typical rates used are as follows:

1. Infiltration, 0.48 to 0.60 inches per hour, depending on extent

of grass cover and storm intensity.

2. Surface Retention, 0.20 to 0.40 inches average retention depth

depending on ground slope and density of

ground cover.

The degree of urbanization in any sub-basin is of major significance

in hydrograph analysis. A high degree of urbanization results in exten-

sive impervious areas and greater runoff than would occur from unmodi-

fied watershed areas. A composite building density versus percent

impervious area curve, Chart 3, was developed from a study of 16 test

areas in the basin. A study of the entire Little Dry Creek basin shows

that at present impervious surfaces cover some 10 percent of the area.

Under future fully urbanized conditions, it is expected that 32 percent

of the basin will be covered by impervious surfaces.

Hydrographs. Hydrograph shape depends on a number of watershed charac-

teristics. 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 a long narrow watershed will have a relatively lower peak discharge

that develops more slowly. A steeply sloped watershed will have a rela-

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