FLOOD01653

• County: Larimer
• Community: Larimer County
• Stream Name: Big Thompson River
• Title: Evaluation of the Flood Hydrology in the Colorado Front Range Using Streamflow Records and Paleoflood Data for the Big Thompson River Basin
• Date: 5/1/1986
• Prepared For: USGS
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

Probable-maximum-flood (PMF) estimates based on rainfall-runoff

relations are determined by identifiying the drainage basin, distributing

the PMP by time, maximizing antecedent-moisture conditions and minimizing

loss rates, and using a mathematical model (usually the unit-hydrograph

method) to translate precipitation excess throughout the entire drainage

basin into its resulting flood hydrograph or PMF, The revised PMP values

(Miller et al., 1984) indicate that extremely large-magnitude rainfall

floods may occur at higher elevations in Colorado.

In Colorado, flood estimates based on streamflow records and rainfall-

runoff modeling are different. Design hydrology for flood-plain management

and hydraulic structures may be questionable because of the large differ-

ences in flood estimates in the foothills and mountains of Colorado.

Presently (1986), the U.S, Bureau of Reclamation is reevaluating the design

of the spillway for Olympus Dam on the Big Thompson River at Estes Park,

Colorado, The existing spillway is designed for a flood of 22,500 cubic

feet per second (637 cubic meters per second). However, a revised PMF

(U.S. Bureau of Reclamation, unpublished data, 1984), based on new PMP

estimates, is 84,000 cubic feet per second (2,380 cubic meters per second),

This revised design discharge would increase dramatically the size of the

spillway. Our studies of preliminary streamflow and regional analysis and

paleoflood data indicate that the largest natural floodflow in the Big

Thompson River at Estes Park is about 5,000 cubic feet per second (142

cubic meters per second) during the last 10,000 years.

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