PERMFILE69489

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 26 SEDIMENTATION POND F - DESIGN ADEQUACY
• Media Type: D
• Archive: No

. Getty Mining Company
March 26, 1985
Page 2
Getty Mining Company provided In-Situ, Inc. with the
characteristics of. Sedimentation Pond F, including the
elevation-area-capacity curve up to elevation 99.5 (relative
elevation), and previous calculations associated with the
original design. Sedimentation Pond F consists of an earth
embankment with a crest elevation at 101.3 Eeet and has a
principal spillway consisting of an 18-inch diameter
corrugated metal pipe riser whose crest elevation is 99.5
feet. A 2-inch diameter dewatering hole is located in the
18-inch diameter riser at elevation 98.0 feet. Sedimentation
Pond F has an open channel emergency spillway whose crest
elevation is at 100.0 feet. The open channel spillway has a
trapezoidal cross-section with 1:1 (horizontal to vertical)
side slopes, an 18-foot bottom and a longitudinal slope of
3 percent. More detail on the elevation-capacity and
elevation-discharge curves for the outlet structures of
Sedimentation Pond F may be found in Appendix 1.
SEDIMENTATION POND F AS-BUILT DESIGN
• The as-built design of Sedimentation Pond F was based on
38 acres of watershed all of which were assumed disturbed.
Design parameters included the SCS runoff curve number of 71
Eor Antecedent Moisture Condition II (AMC-I I), the 10-year,
24-hour design storm of 1.8-inches of rainfall and the 3-year
sediment yield of 0.02 acre-feet per acre of disturbed area.
The resulting design to completely store runoff from the
10-year, 24-hour design storm plus 3 years of sediment volume
resulted in storage of 0.60 acre-feet of runoff and 0.76 acre-
feet of sediment for a total of 1.36 acre-Feet of water and
sediment. These quantities would be stored in Sedimentation
Pond F from its lowest elevation of approximately 95.0 feet,
to an elevation of approximately 98.4 Eeet. Excess water
would then be released at a very low rate through the 2-inch
diameter dewatering hole until the water level returned to
elevation 98.0.
We reviewed the hydrologic, hydraulic and sediment yield
calculations provided to us for the as-built design. In our
judgment the point precipitation values of 1.8 inches and
2.2 inches, respectively, for the 10-year and 25-year, 24-hour
storms are acceptable based on the National Weather Services'
NOAA Atlas 2, Volume III-Colorado. The runoff curve number of
71 seems reasonable although we could not check it because we
had no soil or vegetation maps of the area. The 3-year
sediment yield of 0.02 acre-feet per acre of disturbed area