FLOOD02366

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Big Thompson Flood of 1976 Field Trip Guidebook
• Date: 1/1/1976
• Prepared For: Annual meeting of Geological Society
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

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HYDROLOGIC ASPECTS OF THE BIG THOMPSON FLOOD

Based on eyewitness acounts, the Big Thompson Flood began about

7 pm and was most pronounced in an area centered a few miles above

Drake. It is known with certainty that by 7:30 pm, U. S. Highway 34

was blocked by rocks and debris that had been carried off the slopes

above. The high intensity rain falling on soils with a low storage

capacity and residing on steep slopes resulted in rapid runoff to the

slope channels as well as high rates of erosion. Because of the high

spatial variability of the rainfall, tributary and mainstream discharges

were variable and non-synchronous. By 11 pm flood peaks had passed

through the canyon and flow was diminishing.

The peak discharge data listed in Table 1 have been obtained in

two ways. Some data were derived from records of gaging stations that

withstood the flood. Other data were estimated by identifying high water

marks, measuring cross sectional area and solving the Manning equation

for velocity. The spottiness of rainfall and runoff can be verified by

comparing peak unit area discharge for Locations 1 and 2 of Table 1.

The upper two square miles of Dry Gulch produced over 1,600 cubic feet

per second per square mile while the entire six square mile watershed

produced at less than half that rate. Decrease in the peak unit area

discharge continues in a downstream direction until the Big Thompson

enters an area of high precipitation above Drake. In this area an un-

named tributary (Location 4, Table 1) is thought to have produced

6,305 c.s,m. with an estimated peak velocity as high as 45 f.p.s. Long

Gulch, (Location 5, Table 1) which we will visit on the field trip, has

an estimated peak unit area discharge of 9,246 c.s.m. The Big Thompson

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