FLOOD04421

• County: Arapahoe
• County: Douglas
• Community: Arapahoe, Douglas
• Stream Name: Piney Creek, Cottonwood Creek, Lone Tree,
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 10/1/1975
• Prepared For: UDFCD
• Prepared By: Arapahoe, Douglas
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The Flood Insurance Study (FIS) included only the lower 5.0 miles

peak discharges for each storm frequency on each stream. The

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of Piney Creek, the lower 5.6 miles of Murphy Creek and the

lower 4.5 miles of Cottonwood Creek in the detailed study area.

calculations used rainfall data characteristic of the study area

and provided in the Urban Storm Drainage Criteria Manual. A

Lone Tree Creek and upper portions of the three other streams were

summary of the rainfall data used in the Study and the drainage

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mapped in the FIS by approximate flood plain outlines.

basin characteristics are provided in Tables 3 and 4 respectively.

Varying rainfall intensities were assumed to occur in each basin.

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The preliminary data on the Flood Hazard Area Delineation Study

for Sand Creek, as prepared by the Urban Drainage and Flood

Table 4 lists the maximum rainfall depths for each storm frequency

for a 2-hour duration storm. An infiltration rate of 0.5 inches

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Control District and the Corps of Engineers, Omaha District,

per hour was utilized in all of the study basins. The runoff

has been used in the Murphy Creek Study. Minor changes in the

computations included considerations of the percentage of watersheds

developed and planned future development, as determined by field

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Sand Creek data should have a minimal effect on the Murphy

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Creek flood plain limits.

reconnaissance and planning studies by Arapahoe and Douglas County

Planning Departments.

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Other flood plain studies, completed for various land develop-

ment projects along Piney Creek, have been reviewed and con-

Table 3

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sidered. No conflicting information has been found.

Rainfall Data

Basin

Total Rainfall Depth, Inches
10-Year 50-Year 100-Year

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- HYDROLOGIC AND HYDRAULIC DETERMINATIONS -

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Hydrologic Analysis

Piney Creek
Cottnwood Creek
Lone Tree Creek
Murphy Creek

2.0
2.7
2.6
1.6

2.6
3.4
3.1
2.1

2.9
3.7
3.3
2.4

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A hydrologic analysis was carried out to establish the peak

discharge-frequency relationships for floods of 10-, 50- and

Storm Duration, 2 hours

100-year recurrence intervals for each stream in the study area.

Ref: Rainfall/Runoff Information,
Project REUSE, UD&FCD,
May, 1972

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The Colorado Urban Hydrograph Procedure, as outlined in the

Urban Storm Drainage Criteria Manual, was used to determine the

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