REV00083

• Permit No: C1981071
• IBM Index Class Name: Revision
• Doc Date: 9/1/1993
• Doc Name: SEDIMENT YIELD DEMONSTRATION BLOCK C
• Type & Sequence: SL3
• Media Type: D
• Archive: No

1993 CYVCC -Sediment Yield Demanrtratian - Bbck C ~ 3
2.0 METHODOLOGY AND ASSUMPTIONS
The required hydrology, sedimentology, and hydraulic calculations for Block C and the
compazable adjacent pastureland area were performed utilizing the SEDCAD+ computer model
developed by Civil Softwaze 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 runoff and peak flow via a numerical modeling technique based
on user inputs of runoff basin information and design storm. Design storm inputs include rainfall
amount and duration, selection of a rainfall distribution, and convolution increment. Hydrographs
aze developed on a subwatershed basis with the input of area, time of concentration, SCS curve
number, and selection of one of three (3) dimensionless double triangle unit hydrograph shapes.
This information 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.( - et
• Total Sto~Hydrography
Inputs to the hydrology component of the SEDCAD+ computer model include:
• Precipitation Distribution
• Storm Duration
• RStu~ recipitation
• ` H~r{rn~r ^ espouse Shape
• Time of Concentration
• Curve Number
Precipitation Distribution. A precipitation distribution is input to model the runoff hydrography.
SEDCAD+ allows the user to choose between the SCS Type I or Type II storms. SCS Type II
storm was selected.
Storm Duration. A storm duration of 24 hours was used for comparison.
Return Period/Precipitation. A precipitation amount is required for the appropriate return
period. For the CYCC mine areas the following precipitation value was used:
]0-year, 24-hour event: 1.8 inches
The precipitation value was obtained from the Precipitation Frequency Atlas of the Western United
States, Volume 3, Colorado, (NOAA, 1973).
TERRAMATRIX • P.O. Box 774018 • Steamboa! Springs, Colorado 80477 •[phone]