FLOOD09514

• County: Grand
• Community: Ouray Ranch
• Stream Name: Colorado River
• Basin: Colorado Mainstem
• Title: Ouray Ranch Project Phase II Preliminary Plat
• Date: 12/13/1984
• Prepared For: Grand County
• Prepared By: RME
• Floodplain - Doc Type: Educational/Technical/Reference Information

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6-12

HRS WATER CONSULTANTS, INC.
1350 INDEPENDENCE STREET ~
SUITE 3-A
LAKEWOOD. COLORADO 80215
[phone]

fill

ONSUL T ANTS IN
YO OGEOLOGY AND
WATER RESOURCES

HARLAN W, ERKER
ROBERT 0, TAFELSKI
STEPHEN R. PALMER

December 11, 1984

Mr. Ed Opitz
Rocky Mountain Engineers
P.O. Box 445
Kremmling, CO 80459

84039-01

Dear Mr. Opitz:

As you requested, we have conducted an analysis of annual instantaneous
peak discharge data for U.S.G.S. Station 09019500 on the Colorado River near
Granby to determine the 100-year flood discharges for the periods before and
after the initiation of operation of Lake Granby in late 1949.

The enclosed table lists the annual instantaneous peak discharge data
available for these two periods and includes a total of 20 years of discontinuous
data for the period prior to lake impoundments, and a total of 28 years of
discontinuous data after the start of the lake impoundments. As can easily be
seen, impoundment of water in Lake Granby has had a marked effect on peak
discharge at this station.

Two commonly recognized probability distribution methods were used to
estimate the 100-year flood: the Log Pearson Type 1II distribution and the Gumbel
Extreme Value Type I distribution. The Log Pearson method is an analytical
method utilizing statistical analysis of the logarithms of the data and involving
the skew of the data as an important component in the analysis. The Gumbel
method is a basic graphic probability technique involving the fitting of a straight
line through the data.

For the 20 years of record prior to the operation of Lake Granby (1908
- 1911 and 1934 - 1949), the following estimates for the 100-year instantaneous
peak discharge were obtained:

Log Pearson Type 1II method - 4,380 cfs

Gumbel Extreme Value method - 4,650 cfs

It should be noted that these values are only estimates based on statistical analysis
of the available data and that 20 years of record is considered a minimum for
estimation of a 100-year event. Analyzing the Log Pearson estimate of 4,380
cfs based on 20 years of record and a 90% confidence interval, there is a 90%
probability that the 100-year flood is between 3,700 and 5,800 cfs.

Using the 28 years of record following the operation of Lake Granby, an
attempt was made to estimate the 100-year peak discharge using the two methods.
However, the nature of the data did not lend itself to these statistical methods.
The operation of the lake has created an unnatural, and therefore no longer
random, set of flow data (see enclosed figure). The statistical methods available
for analyzing flood frequencies assume a random distribution of data. The results
of the analysis are indicative of this problem.