FLOOD06408

• County: Weld
• Community: Windsor
• Stream Name: Big Thompson/South Platte/Cach la Poudre
• Basin: South Platte
• Title: Drainage Basins and Areas of Past Flooding in the Winsor Area, Colorado
• Date: 1/1/1971
• Prepared For: University of Colorado
• Prepared By: James Paul Walsh, Jr.
• Floodplain - Doc Type: Flood Documentation Report

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Based on Calculations of Discharge

Another way to approach the problem of flood magnitude and

frequency is to calculate the discharges required to inundate the

mapped areas,l and compare these to known flood discharges. Un-

fortunately, gaged discharges during large floods are completely

absent in the thesis area. Ho~ever, calculated discharges can be

compared 'to floods gaged just outs~de the thesis area, and to a

statistical estimate of the largest probable flood for the Big

Thompson River at Loveland (U.S, Army, Corps of Engineers, Omaha

Engineer District, 1971),

The Hanning equation was used to calculate discharge.

Q = A 1.49 R2/3 Sl/2
n

where

Q = discharge

A = cross sectional area

R = hydralic radius (cross sectional
area/wetted perimeter)

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S = slope

n = a coefficient of roughness

Discharges necessary to inundate ,the areas mapped were

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calcula'ted a't those locations which had reasonable data on flood

depths, using the following procedure. Cross sectional areas and

wetted perimeters were determined from' topographic map data, con-

trolled by field observations. 1~1ere field obsetvations were care-

fully made, a single cross sectional area could be drawli; otherwise

IThe Rational Hethod (Chow, 1964, pp, 14-16) is a third ap-
proach. Q = CIA where: Q = discharge; C = coefficient of runoff;
I = rainfall intensity;- A = area of dtain~ge basin. A more strictly
geologic approach l"iS been used in this study,