FLOOD01898

• County: Larimer
• Community: Estes Park
• Stream Name: Big Thompson, Fall Rivers
• Title: Hydrology Report for Estes Park and Larimer County FIS
• Date: 2/1/1984
• Prepared For: FEMA
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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guidelines. A probability adjustment to the rest of the record was found to

be insignificant due to the very large recurrence interval of the excluded

event (Reference 6).

After computing frequency relationships for both snowmelt and rainfall

peaks at each station, the relationships were plotted. Where streams were

found to have intersecting rainfall and snowmelt frequency curves, the two

frequency curves were assumed to represent independent events and were com-

bined. The composite flow frequency curve was developed from the following

equation (Reference 4):

P(q) ~ P (q ) + P (q ) - P (q ) P (q )
s r s r

(1)

where peg) ~ Probability of a given flow being equaled or exceeded from
either snowmelt or rainfall flooding.

P(q )
s

probability of that flow being equaled or exceeded due to
snowmelt flooding.

P(q ) ~
r

Probability of that flow being equaled or exceeded due to
rainfall flooding.

The developed frequency curves for each of the stations utilized in the

analysis are presented in Appendix B.

A regression analysis was then made between peak 10-, 50-, 100-, and

500-year recurrence interval flows from the developed frequency relationships

and the drainage areas for the gaging stations utilized. This was done to

develop a regional means of computing peak flows. Plots of data points and

the developed regressions are presented in Appendix C.

The regressions fit the plotted data well. For the 500-year regression,

however, two data points, those for Fish Creek and the North Fork of the Big

Thompson River, plot significantly higher than the developed relation. From

Table 3.1, the elevation for these points are two of the lower values. In

Appendix B, it is seen that the frequency relationship developed for these

points are the only ones in which rainfall produces the larger peak events,

probably due to elevation. These data points were included in the study since

the peak flows for the other recurrence interval floods were consistant with

the regional regression.

The developed regressions were then used to develop peak 10-, 50-, 100-,

and 500-year peak flows for each of the study streams. Confidence intervals

for 50 and 90 percent confidence were also determined. The flows and con-