2025-04-09_PERMIT FILE - C1981019 ‎(2)‎

• Permit No: C1981019
• IBM Index Class Name: Permit File
• Doc Date: 4/9/2025
• Section_Exhibit Name: Exhibit 07 Methodologies & Assumptions for Sedimentation Pond Design Evaluations
• Media Type: D
• Archive: No

METHODOLOGIES AND ASSUMPTIONS FOR
SEDIMENTATION POND DESIGN EVALUATIONS
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 analyses are presented as attachments
in each of the individual pond design sections and are summarized in Table 6, Sedimentation Pond Design
Summary.
The general procedures described herein will be used for all future hydrologic designs for ponds, ditches,
and other facilities to be constructed at the Colowyo Mine. Also refer to a similar discussion presented in
Volume 13, Exhibit 7, Item 20 for the structures associated with the South Taylor area.
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 run -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 are developed on a sub -
watershed basis with the input of area, time of concentration, SCS Curve Number, and the selection of 1 of
3 dimensionless double triangle unit hydrograph shapes. Routing of hydrographs is accomplished by
Muskingum's Method.
Inputs to the hydrology component of the SEDCAD4 computer model include:
• Precipitation Distribution
• Storm Duration
• Return 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 hydrograph. SEDCAD4 allows the user to choose
between the SCS Type I, Type IA, Type I1, Type III, and user - defined storm unit mass curves. The Type II
distribution is input as the most accurate reflection of area storm patterns.
1.2 STORM DURATION
A storm duration of 24 hours is used for the design and evaluation of the Colowyo ponds.
Exhibit 7 Page 1 Revision Date: 3/18/13
Revision No.: TR -95