REV04202

• Permit No: C1981071
• IBM Index Class Name: Revision
• Doc Date: 10/7/1996
• Doc Name: SEDIMENT YIELD DEMONSTRATION BLOCK C
• Type & Sequence: SL3
• Media Type: D
• Archive: No

July 1996 CYVCC -Sediment Yield Demonstration Rev.1- Black C • 3
2.0 METHODOLOGY AND ASSUMPTIONS
The required hydrology, sedimentology, and hydraulic calculations for Block C and the comparable
adjacent pastureland area were performed utilizing the SEDCAD+ computer model developed by Civil
Software Design SEDCAD+ is a third generation hydrologic and hydraulic calculation model designed
for use on IBM-compatible computers.
2.1 HYDROLOGY
The SEDCAD+model calculates nmofl and peak flow via a numerical modeling technique based on user
inputs of runoff basin information and design store. Design storm inputs include rainfall amount and
duration, selection of a rainfall distribution, and convolution increment. Hydrographs are developed on
a subwatershed basis with the input of area, time of concentration, SCS curve number, and selection of
one of three dimensionless double triangle unit hydrograph shapes. This infonnation is further discussed
in the SEDCAD+User's Manual (Schwab and Warner, 1987) and in the SEDCAD+ Inputs section that
follows. Routing of hydrographs is accomplished by Muskingum's methods.
Specific hydrologic output values calculated by the SEDCAD+ model are as follows:
• Peak Flows (cfs)
• Storm Runoff (inches)
• Runoff Volume (acre-feet)
• Total Storm Hydrograph
Inputs to the hydrology component of the SEDCAD+ computer model include:
• Precipitation Distribution
• Storm Duration
• Return Period/Precipitation
• Hydrograph Response Shape
• Time of Concentration
• Curve Number
Precipitation Distribution. A precipitation distribution is input to model the runoff hydrograph.
SEDCAD+ allows the user to choose between the SCS Type I or Type II storms. SCS Type ^ storm was
selected.
Storm Duration. A store duration of 24 hours was used for comparison.
Return Period/Precipitation. A precipitation amount is required for the appropriate return period. For
the CYVCC mine areas the following precipitation value was used:
0-year, 24-hour event: l.8 inches
The precipitation value was obtained Gom the Precipitation Frequency Atlas of the Western United
States, Volume 3, Colorado, (NOAA, 1973).
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