2007-06-01_PERMIT FILE - C1981019A

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 6/1/2007
• Section_Exhibit Name: Exhibit 07 Methodologies & Assumptions for Sedimentation Pond Design Evaluations
• Media Type: D
• Archive: Yes

METHODOLOGIES AND ASSUMPTIONS FOR
SEDIMENTATION POND DESIGN E\/ALUATIONS
The hydrologic and hydraulic calculations used to model all 10 of the Colowyo Mine sedimentation ponds
and the Taylor Pump Holding Pond, were performed using the SEDCAD4 computer modeling program
developed by Civil Software Design. The results of the SEDCAD4 aztalyses are presented as attachments in
each of the individual pond design sections and are summarized in Table G, Sedimentation Pond Design
Summary.
1.0 HYDROLOGIC AND HYDRAULIC CALCULATIONS
SEDCAD4 is a comprehensive, computer-based model used to evaluate storm water, erosion, and sediment
control management techniques. The SEDCAD4 model calculates tun-off and peak flow via a numerical
modeling technique based on user inputs of a design storm event (i.e., rainfall amount and duration, selection
of rainfall distribution, and convolution increment). Hydrographs aze developed on asub-watershed basis
with the input of area, dme of concentration, SCS Curve Number, and the selection of 1 of 3 dimensionless
double triangle unit hydxograph shapes. Routing of hydxographs is accomplished by Muskingum's Method.
Inputs to the hydrology component of the SEDCAD4 computer model include:
• Precipitation Distribution
• Storm Duration
• Retum Period/Precipitation
• Hydrograph Response Shape
• Drainage Basin Area
• Time of Concentration
• Muskingum Routing Parameters
• Curve Number
• Base Flow
Input values are shown on the SEDCAD4 printouts included as attachments in each of the individual pond
design sections, and explained in the following sections.
1.1 PRECIPITATION DISTRIBUTION
A precipitation distribution is input to model the run-off hydxograph. SEDCAD4 allows the user to choose
be[ween the SCS Type I, Type IA, Type II, Type III, and user-defined storm unit mass curves. The Type II
distribution was input as the most accurate reflection of area storm patterns.
1.2 STORM DURATION
A storm duration of 24 hours was used for the design and evaluation of the Colowyo ponds.
7.3 RETURN PERIOD/PRECIPITATION
A precipitation amount is required for the appropriate return period. The following xemrn periods and
precipitation events were used for the Colowyo Mine:
10-year, 24-hour storm event 1.8 in
25-year, 24-hour storm even[ 2.3 in
100-year, 24-hour storm event 2.7 in
Exhibit 7 1 Revision Date 10/23/03
Revision No. TR-56