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

To model this hypothesis, the contributing drainage area from each

1,000-foot (305-meter) part of each basin was calculated as shown in Figure

7 and Table 2. Beginning with the 13,000-foot (3,962-meter) elevation, the

contributing drainage area below this elevation was calculated and is

listed in Table 1. Regression analysis was done for each flood magnitude

against drainage areas below each elevation level. Using the decrease of

standard error of estimate (average) and the increase in the correlation

coefficient as criteria, the elevation level defining the contributing

drainage area was selected. The drainage area, mean basin elevation, and

gage datum were all significant but were so intercorrelated with each other

that mean basin elevation and gage datum were not used. For each de-

creasing (or increasing) elevation level, fewer sites were included in the

regression because the higher (or lower) sites would not have contributing

drainage area and were not used in the analysis.

Regression analyses were made on three drainage-area characteristics:

total drainage area, drainage area below a stated elevation level, and

drainage area above a stated elevation level. Regression models in the

form:

b
QRt = a(A)

(2)

where

QRt = rainfall flood magnitude, in cubic feet per second, for the

recurrence interval, t, in years;

a = regression constant;

A = drainage-area characteristic, in square miles; and

b = the regression exponent for the drainage-area characteristic.

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