FLOOD02174

• County: Denver
• Community: Lakewood
• Stream Name: Dry Gulch
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 7/1/1979
• Prepared For: Denver
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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7

TABLE III-l

Five-Year
Ten-Year
Fifty-Year
One Hundred-Year

l.69 inches
2.08 inches
2.6l inches
2.92 inches

Flow rates at various points along Dry Gulch and North Dry Gulch

were determined presuming that the channels would remain in their

present conditions. A hydraulic analysis was then performed to

determine the levels of flooding in each of the four storms.

This analysis indicated that flows on both major channels diverge

at several locations and enter different basins or follow separate

courses. For this reason, the hydrology of the basin was closely

interrelated with the basin hydraulics. On several instances,

the hydraulic analysis indicated that a flow split occurred, thus

changing the flow rates downstream. The hydraulic analysis with

the adjusted flows sometimes indicated a need for a further re-

finement of the flow rates. The results of this iterative pro-

cedure were compiled into a figure of peak flow rates in each

flood at all stations along the two gulches. These flow rates

for existing channel conditions (Before Construction of Recom-

mended Facilities) are presented in Figures III-l and 1II-2.

TOTAL RAINFALL IN STORMS OF 200 MINUTE DURATION ON DRY GULCH

Return Period

Rainfall

Computation of Runoff for Existing Channel Conditions

Information was acquired from the cities of Lakewood and Denver

which forecast land use within the basin at future dates. The

information was presumed to represent "fully developed" land use.

Parameters such as infiltration rate, depression storage, deten-

tion storage, and percentage of impervious ground cover were

,utilized to determine the amount of "effective rainfall" which

would be discharged from each sub-basin. Parameters such as

tributary area, channel length, distance to basin centroid, and

several coefficients were selected and used to compute the hydro-

graphs of runoff which would be formed as the result of the effec-

tive precipitation on each sub-basin. The hydrograph flows were

then combined in order to develop flood flows along each channel.

Computation of Runoff after Implementation of the Proposed

Master Plan

As previously stated, the flows which would presently occur in

major storms are significantly affected by detention ponding,

flow splitting, and inadvertent trans-basin diversions.