FLOOD02983

• County: Arapahoe
• Community: Littleton
• Stream Name: Lee Gulch, Little Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 9/1/1978
• Prepared For: Littleton
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III. HYDROLOGY

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The storm hydro graphs of the sub-basins were routed and combined using

A hydrological analysis of both basil.s '.as completed to determine the

the Convex Method of Stream Routing of the Soil Conservation Service and the

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peak discharge-frequency relationship for floods of the 2, S, 10, and 100-

Modified PuIs Method of Reservoir Routing to obtain peak discharges at the

year recurrence intervals. These peak discharges were predicated upon full

various hydrological points. It was assumed that all trans-basin conveyance

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development of the basins with future land use determined from the City of

of storm runoff by agricultural canals would be eliminated by spilling storm

Littleton and Arapahoe County master plans.

waters out of the canals at the channel crossings.

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The Colorado Urban Hydrograph Procedure as described in the "Urban

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Storm Drainage Criteria Manual" (revised 5/15/75) and interim guidelines

SUB-BASIN DELINEATION

furnished by the UDFCD were used to obtain storm hydrographs for each sub-

Each drainage basin was divided into sub-basins. This division was

basin.

based upon topography land use similarities, major crossings, and juris-

Rainfall data was obtained from the Rainfall-Depth-Duration-Frequency

dictional boundaries. The sub-basin boundaries are shown on Sheet 1 of

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Maps in the nUrban Storm Drainage Criteria Manual". Deviations between the

the master plan drawings.

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upper and lower portions of basins were between 0 and 0.2 inches and did not

Each sub-basin was physically described in terms of its area, length of

"arrant using separate rainfall intensities over the basin. Consequently,

stream, length to the centroid, degree of imperviousness, and slope. The

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the upper basin, or slightly higher values, were used. A three hour duration

percent of imperviousness of each sub-basin was determined by sampling seven

storm was used. An infiltration rate of 0.5 inch per hour, impervious de-

"typical" blocks within the study area and plotting this information versus

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tention storage of 0.05 to 0.1 inches, and pervious detention storage of

housing density. From this information the hydrograph coefficients Ct

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0.3 inches were used to determine the storm hydrographs. The rainfall data

is summarized below:

(time coefficient) and Cp (peak rate of flow coefficient) were determined.
The hydrograph parameters for each sub-basin are shown in Table I.

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Return Frequency
2 Year
10 Year
100 Year

1 Hour
1.0
2.1
2.7

Rainfall Duration
6 Hour
1.6
3.3
4.3

24 Hour
2.1
3.7
4.7

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These values were used in the proceedure outlined in the "Urban Storm

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Drainage Criteria Manual" to determine the depth-duration relationship for

the 5, 10, 50 and laO-year storms.

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