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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SECT! ON II I
FLOOD HISTORY

During the period of record of the U.S.G.S. gage on the Fraser River at
Granby (1905-1909, 1938-1955) the largest peak discharge recorded was 2,500
cfs which occurred on June 8, 1952. Based on the hydrology done for this
study the lOa-year flood on the Fraser River is 3,300 cfs and the 10-year
flood on the Fraser River is just under 2,000 cfs. If these values are
plotted on probability paper it is seen that a flow 2,500 cfs is equivalent
to approximately a 33-year flood.

During the spring of 1983, high flows occurred on the Fraser River. The
U.S.G.S. gage at Winter Park, which is the nearest oEerating gage to Granby
on the Fraser River, recorded discharges of 450 cfs on June 28. An analysis
was performed based on the period of common record between the Winter Park
gage and the Granby gage on the Fraser River in order to correlate these
flows. Based on this information, it was shown that a peak flow of 450 cfs
at Winter Park would equate to approximately 2,300 cfs at Granby. This
correlation is shown graphically in Figure III-I. A flow of 2,300 cfs at
Granby is equivalent to approximately a 1.5-year'return interval flood.
Thus, the high water which occurred on (the Fraser River during the spring of
1983 was the result of approximately 25-year runoff event. It should be
kept in mind that the relationship between runoff magnitude and frequency is
not linear. Hence, even though a 25-year runoff event is only about 1/4 of
the return frequency of the lOa-year event, the 100-year runoff rate is not
4 times that of the 25-year. The 100-year event is only about 1.5 times the
size of the 25-year event. ,. .' 'J /,,,..,.

During the flooding which occurred during the spring of 1983, there was a
large area of sheet flow on the west side of Highway 40 in an area not
designated as floodplain from either the Fraser River or Tenmile Creek. The
reason that this flooding occurred was that a series of irrigation and
drainage culverts crossing Highway 40 discharged to the west side of the
highway in an uncontrolled manner. Flow from these culverts crossed the