FLOOD07738

• Designation Number: 538
• County: Arapahoe
• Community: Arapahoe County
• Basin: South Platte
• Title: Hydrology Addendum to FIS Arapahoe County and Incorporated Areas
• Date: 1/1/2001
• Prepared For: Aarapahoe County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrololrlc Analvsis
The Arapahoe County and Incorporated Areas PIS presents detailed and approximate floodplain
delineations, Hydrologic information used for the FEMA approximate floodplain delineations are
not available or were not documented by FEMA. The CWCB has developed acceptable
hydrologic information for undeveloped basins using regional regression methodologies for
computing IOO-year flood-flow values.

The CWCB methodology is fully described in a publication by the CWCB entitled "Guidelines
for Determining lOO-year Flood Flows for Approximate Floodplains in Colorado". The South
Platte Southern Foothills Subregion (SPL-2) was used for streams west of the Box Elder Creek-
Kiowa Creek basin divide, while the South Plalle Eastern Plains Subregion (SPL-l) was used for
streams east of the Box Elder Creek-Kiowa Creek basin divide. The CWCB equations are as
follows:

Sub-Region
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SPL-1

Sub-Re ion Name
South Platte, Eastern Plains

2 to 1090

SPL-2

South Platte, Southern Foothills

Q,oo = 1005,5(A)o,638

1 to 170

For the equations shown in Table 2, the QIOO is the lOO-year peak flow in cubic feet per second,
and A is the upstream (contributing) drainage area in square miles. The regression equations are
only valid for the range of drainage areas indicated.

The lOO-year discharge for approximate floodplains in Arapahoe County were estimated by
utilizing the regional regression equations shown above. The estimated loo-year peak discharges
are shown in Table 3.

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