PERMFILE67811

• Permit No: M1988112
• IBM Index Class Name: Permit File
• Doc Date: 11/8/1988
• Section_Exhibit Name: EXHIBIT G WATER INFORMATION
• Media Type: D
• Archive: No

I
Based upon precipitation maps for Colorado, prepared by
Miller, Frederick and Tracey (1973), the 100-yr, 24-hr point
precipitation used to estimate peak flood discharges at
selected locations in the Rito Seco watershed varies from 2.6
inches in the lower watershed to 3.8 inches ~n the upper
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n average value of 3.2 inches was used for
calculating the 100-yr flood.

The flood hydrograph resulting from a 100-yr, 24-hour
precipitation event of 3.2 inches falling on the Rito Seco
watershed was estimated using U.S. Soil Conservation Service
(SCS) unit hydrograph techniques (SCS, 1972). A Type-II
rainfall distribution and a runoff curve number of 70 were
used in the hydrograph calculations. Three locations along
' the Rito Seco were selected for hydrograph calculations, as
shown on Figure G-4. The first location is upstream from the
proposed mine site, with a contributing watershed area of 11.6
miz. The second location is downstream from the proposed mine
site, and has a contributing watershed area of 18.1 miz. The
third location is at the canyon mouth, with a ~:ontributing
watershed area of 23.2 miz.
Table G.2-5 shows the watershed characteristics used to
calculate the 100-yr flood hydrographs at the three locations,
as well as the peak discharge and runoff volume at each
' location. Using the unit-hydrograph method (SCS, 1972), the
estimated 100-yr flood peak discharge in Rito Seco varies from
2950 cubic feet per second (cfs) at the upstream location to
3840 cfs at the canyon mouth. Resultant flood hyorographs at
the three locations using the unit-hydrograph method are shown
on Figures G-5 through G-7. The estimated 100-yr flood volumes
in Rito Seco ranged from 510 acre-feet (ac-ft) at i.he upstream
location to 1020 ac-ft at the canyon mouth.

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