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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Oc-(-tJA,f
v station skewness. J.F. Sato & Associates made a study of th~ Winter Park
~ ,eflood in 1984. In that study they found that use of l7B instead of l7A pro-
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cedure reduced thevpeak flows by 7 percent. In the case of the 100-year
flooding of Fraser River at Granby, the difference is nearly a 15 percent
reduction.

, IV-3
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The fourth method used was the multiple regression analysis. This method
correlates actual annual peak flood data of the tributary basins to the
Granby station. The 100-year flood at Granby is then able to be calculated
based on the multiple regression formula using the "known" 100-year flood at
the tributaries. Not all of the tributary drainage sub-basins have recorded
flood data. The five drainage sub-basins that have been used for this mul-
tiple regression analysis were studied by Worrall Engineering in 1983, and
Sato & Associates in 1984. They include the Fraser River near Winter Park,
Vasquez Creek at Winter Park, St. Louis Creek near Fraser, Ranch Creek near
Fraser and Meadow Creek near Tabernash (Fi9ure IV-2).

~mmation of these sub-drainage areas gives a total area of 115 square miles
which accounts for about 40 percent of the total Fraser drainage basin at ,'.'
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Granby. Most of these sub-drainage basins have longer flood records~ Thus, 1
this technique in effect extended the data base of the Granby station. The
use of the previously determined 100-year floods at tributaries as data
input also guarantees the consistency in the level of 100-year flood protec-
tion within the same drainage basin.

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Detailed information of the gaged sub-basins regarding its drainage area
size, mean basin elevation, year of records, etc. are listed in Table IV-I.
In this table the gage station number is related to the number shown on
Figure IV-2.

Tabulated in Table IV-2 are the annual flood peaks used in the regression
analysis. The derived regression equation takes the form of

Qg = 26.562 + 1.793 x Ow + 1.096 x Qs + 1.875 x Ou - 0.308 x Or + 1.065 x Om
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