FLOOD01567

• County: Gunnison
• Community: Somerset
• Stream Name: Anthracite Creek
• Title: Floodplain Information Report Preliminary
• Date: 7/1/1977
• Prepared For: Gunnison County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

.

Another procedure often used to estimate flood magnitudes
for various frequencies employs a regional analysis method developed
by the U. S. Geological Survey. The details ,of this procedure are
described in USGS Water Supply Paper No. J68;3, entitled "Magnitude alL
and Frequency of Floods in the united States, Part 9, ColoradQ River
Basin." Under the procedure, the Anthracite Creek basin is within
Region A and Hydrologic Area 5. The mean annual flood for area 5
is estimated by the equation

Q2.33 = 0.506AO.76Hl.90

where Q2.33 is the mean annual flood, A is the area in square miles
and H is the mean elevation of the basin in thousands of feet above
sea level. The mean annual flood for Anthracite Creek'below Erickson
Springs calculated to be

Q2.33 = 1,068 cfs

for a tributary area of 85 square miles and a mean basin elevation
of 9,500 feet.

The relationship between the mean annual flood and other
frequencies for Hydrologic Region A is

.

Recurrence
Interval

Ratio

2.33
5
10
25
50
100

1.0
1.25
1.5
1.7
1.9
2.1 (extrapolated)

Using the extrapolated ratio of 2.1, the 100-year flood is estimated
at

Q100 = (2.1) (1,068) = 2,243 cfs

.

A third estimate of the 100-year discharge was made using
the CWCB Technical Manual No.1, entitled "Manual for Estimating
Flood Characteristics of Natural-Flow Streams in colorado." Under
this method, the study reach is defined as being an ungaged site)
.and the Anthracite Creek basin above the study reach falls partly
in the Southern Plateau Region (below elevation 7,500 feet) and
partly in the Mountain Region (above 7.500 feet). This is described
in the manual as a mixed population flood area, requiring the use
o'f Procedure 2 (page 30).

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