FLOOD09600

• County: Grand
• Community: Granby
• Stream Name: Fraser River, Tenmile Creek
• Basin: Colorado Mainstem
• Title: Silvercreek Floodplain Study
• Date: 2/1/1984
• Prepared For: Grand County
• Prepared By: Wright Water Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SEcn ON I V
HYDROLOGY

FRASER RIVER
The Fraser River near Granby, just downstream of the Tenmile Creek con-
fluence, has a tributary area of 28S square miles and a mean basin elevation
of approximately 9,720 feet above sea level. There are 23 years of avai 1-
able records including 5 years of staff gage records from 1905 to 1909, and
18 years of water stage records from 1938 to 1954. The maximum flood record
during this period was 2,500 cfs which occurred on June 8, 1952. The high
percentage of peak floods occurred in the month of June (Figure IV-I),
together with high basin elevation suggested that peak floods are due pri-
marily from snowmelt.

Four different methods have been used for estimating the Fraser River 100-
year flood. The first two methods are a result of generalized regional
analysis. Technical Manual No.1 of the Colorado Water Conservation Board
provide a simplified method for calculating peak discharges and flood depths
for natural-flow streams. The manual uses the log-Pearson Type III method
for fitting a frequency curve to gaging station data and multiple-regression
techniques for regionalization or transferring the results to ungaged
basins. The U.S.G.S. Water Supply Paper, 1683, "Magnitude and Frequency of
Floods in the U.S. "provides another regional evaluation approach based on
an index flood (or mean annual flood) concept.

The third method analyzed the annual flood peaks according to the log-
Pearson Type III procedures described by the Water Resources Council in
tiulletin 17A and 17B. The 1981 version of the 17B procedure takes into con-
sideration the fact that the skewness coefficient of the station record is
sensitive to extreme events. Thus, it is difficult to obtain skew estimates
from small samples. The accuracy of the estimated skew coefficients can be
improved by weighing the station skew with generalized skews estimated by
pooling information from nearby sites. The end results is that the 17B pro-
cedure gives an adjusted skew coefficient comparatively closer to the
,

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