2010-12-17_REVISION - C1981019 ‎(122)‎

• Permit No: C1981019
• IBM Index Class Name: REVISION
• Doc Date: 12/17/2010
• Doc Name: Exhibit 7 Item 23 Erosion and Sedimentation Control Plan
• Type & Sequence: PR3
• Email Name: JRS
• Media Type: D
• Archive: No

METHODOLOGIES AND ASSUMPTIONS FOR
SEDIMENTATION POND DESIGN EVALUATIONS
The hydrologic and hydraulic calculations used to model the proposed Colowyo Mine sedimentation
structures 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
facility design sections and are summarized in Table 6 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 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 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 10 -year, 25 -year and 100 -year storm event recurrence
intervals in the design and evaluation of the Colowyo sedimentation facilities.
1.3 RETURN PERIOD/PRECIPITATION
A precipitation amount is required for the appropriate return period. The following return periods and
precipitation events were used for the Colowyo Mine:
10 -year, 24 -hour storm event 1.8 in
25 -year, 24 -hour storm event 2.3 in
100 -year, 24 -hour storm event 2.7 in
Exh. 7 -23 -1 Revision Date: 12/15/10
Revision No.: PR -03