FLOOD01491

• Designation Number: 433
• County: Prowers
• Community: Unincorporated Prowers County
• Title: Floodplain Information Report - Flood Hazard Boundary Map - Prowers County, CO
• Date: 6/1/1996
• Designation Date: 7/1/1996
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Community Response to the CWCB Desimation and Approval Process

A letter from the CWCB was transmitted to Prowers County regarding the Board's proposed action
for designation and approval of this Floodplain Information Report and the Prowers County FHBM.
The County was receptive to the Board's proposed actions, and sent an official letter requesting that
the CWCB designate and approve the floodplain information currently available to the community,
A copy of the request letter is included in the Appendix at the end of this report,

ENGINEERING METHODS

Hvdrololrlc Analvsis

Hydrology information for the streams with approximate floodplain delineations on Prowers
County's FHBM has been estimated in accordance with the CWCB's Guidelines for Determining
IOO-Year Flood Flows for Approximate Floodplains (Guidelines). The lOO-year flow for each of
the streams with approximate floodplains (in all of Prowers County) can be estimated by utilizing
the regression equation for Arkansas River Basin, Subregion Ark-I. The equation is QIOO =
1564.98(A)o539 , where Ql00 is the lOO-year peak flow in cubic feet per second, and A is the
upstream (contributing) drainage area in square miles, Figure I, presented in the Appendix at the
end of this report, displays the Arkansas River Basin with Subregions Ark-I through Ark-6. The
regression curve for subregion Ark-I is also presented in the Appendix. The regression equation is
only valid for tributary streams having drainage areas between 25 square miles and 1125 square
miles, and is not valid for the Arkansas River mainstem. Table I presents drainage areas and
estimated lOO-year flows, using the subregion Ark-I regression equation, for selected streams in
Prowers County,

Table 1

Drainage Area 1oo-year Peak
Stream Name Location (SQ. mi.) Flow (cfs)
Buffalo Creek Confluence with Arkansas River 73 15,800
Clav Creek Confluence with Arkansas River 230 29,400
Two Butte Creek Confluence with Arkansas River 920 62,100
Wild Horse Creek Confluence with Arkansas River ISO 23,100
Willow Creek at Lamar 40* 12,800*

* Obtained from the Lamar, Colorado Flood Insurance Study

There are numerous other streams within the County having drainage basins of various sizes, Peak
flows may also be estimated for those streams using the subregion Ark-I regression equation,
providing that their drainage areas fall between 25 and 1125 square miles, The lOO-year flow for
approximate floodplains of streams that have drainage areas between 0.5 and 25 square miles can
be estimated using the USGS publication entitled "Rainfall-Runoff Modeling and Regional Flood
Characteristics of Small Rural Watersheds in the Arkansas River Basin in Colorado," That
publication is available from the USGS publication center in Denver, Colorado,

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