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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fidence limits as determined for each of the study streams by the above proce-

dure, along with flows determined in previous studies are presented in Table

3.2.

The effect of storage on the Fall River peak flows was considered.

Cascade Dam is assumed to be rebuilt at its former location and capacity for

power generation, but flood control storage would not be provided. Also,

there would be no flood control operation policy.

A recommendation for further analysis is based on visual inspection of

the data used for the 100- and SOO-year regression lines shown in Appendix C.

Rainfall floods generate higher peak flows per square mile and were the predo-

minant factor of extreme floods for the Fish Creek and North Fork Big Thompson

gage records. It is not recommended to adjust flows based on two-gage

records. Hundred-year peak flows on Fall River would double if only these

gages were used. Precipitation data and HEC-1 could be used to generate an

estimate of peak flows for extreme events due to rainfall. A review of floods

in the area shows most frequently flooding is due to snowmelt. This is due to

the infrequent number of thunderstorms at high elevations (greater than 7,SOO

ft, msl) and the direction in which the storms move along the front range.

But the less frequent extreme flooding, not adequately represented by gage

data, is due to rainfall events, or perhaps rain on snow. AS mentioned above,

a physically based model, such as HEC-1, could be used to evaluate flood peaks

due to rainfall and perhaps define a recurrence interval at which rainfall

would be the predominant cause of floods.

3.2 Conclusions

The peak flows determined from the regional regression analysis compare

well with the flows determined from previous studies. For the Big Thompson

River above Estes Park, the flows determined for the 10- and 50-year return

periods are slightly less than those determined from previous studies. For

the 100- and SOO-year events, the flows determined from the regressions fall

between values previously determined. Flows determined for the Fall River are

slightly lower for the 10-year event compared to the previous studies and

intermediate to values from the two previous studies for the 50-, 100-, and

SOO-year flows. Flows determined for Black Canyon Creek are almost identical

to those determined previously.

FEMA guidelines advise: