PERMFILE71223

• Permit No: C1981071
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 13 SEDIMENTATION PONDS HYDROLOGIC INVESTIGATIONS
• Media Type: D
• Archive: No

-z-
a Type II Soil Conservation Service storm distribution was used to con-
vert total rainfall to design storm rainfall. The Type II storm distribu-
tion is shown on LRCWE Table 1 Design Rainfall, calrich also summarizes the
10, 25 and 100-year design storm rainfall.
Runoff Volumes
Runoff volumes were computed using equation 10.10 and the procedure outlined
in Chapter 10 of the Soil Conservation Service's National Engineering Hand-
book, Section 4 - Hydrology, dated 1971. Antecenent Poisture Condition - II
(AMC) curve numbers (CN) were estimated for the tributary areas of each pond
considering the worse case for the land use, and ground cover. AMC-II is
considered applicable for average conditions and allows for a total rain-
fa11 of 1.4 to 2.1 inches within the preceeding five days of the storm. For
the 10-year 24-hour precipitation event, the AMC-II curve numbers indicated
a relatively low runoff volume (less than 0.2 inches). In discussions with
Energy Fuels Corporation personnel, it was learned that for this area, the
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most cr
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umes occur
ur
ng t
e snowme
t season, especia
y i
rainfall accompanies the phenomenon. In the definition of precipitation
event, the OSM regulations include ". .water emanating from snow cover as
snowmelt in a limited period of time." Considering a water equivalent of
30%, the AMC-II curve numbers indicate a surface runoff volume equivalent to
approximately one-half inch of snowmelt. One-half inch of snowmelt within
a 24-hour period of time is expected to occur more frequently than once in
10 years on the average. The state of the art for computing snowmelt runoff
hydrographs is not well developed, and snowmelt with rainfall hydrographs
are beyond the scope of this investigation. The APfC-II condition was con-
sidered to give runoff volumes too low when snowmelt is considered. To
generate flood hydrographs with more conservative runoff volumes (greater
total volumes) the AMC-III condition was utilized. The AMC-III assumes a
greater antecedent moisture condition which exists if greater than 2.1 inches
of rainfall have fallen within the previous five days. This technique is not
intended as an accurate representation of the actual snowmelt hydrology.
LRCWE Table 2, Curve Numbers, is reproduced from Chapter 10 of the Soil Con-
'
servation Service
s National Engineering Handbook, Section 4, Hydrology.
Leonard Rice Consulting Waler Engineers.lnc.