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

Exhibit 7 Page 2 Revision Date: 4/28/22
Revision No.: TR-154
1.3 RETURN PERIOD/PRECIPITATION
A precipitation amount is required for the appropriate return period. The following return periods and
precipitation events are 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

1.4 HYDROGRAPH RESPONSE SHAPE
A unit hydrograph is chosen for each drainage area or sub-area modeled to predict the run-off response.
The hydrograph responses available in the SEDCADTM4 model are slow, medium, and fast. A slow
response corresponds to a forested area or an area with a number of obstructions. A fast response
corresponds to an un-vegetated or poorly protected area. Medium and fast hydrograph responses are
considered as most representative for mine areas.

The internal convolution increment is 0.05 hours, and values are saved at the user-specified interval of 0.1
hours or greater. A convolution increment of 0.1 is specified. It should be noted that a time of
concentration less than 0.125 hours bypasses the unit hydrograph technique and instantaneous run-off is
assumed. As explained in the SEDCADTM4 User's Manual (Schwab and Warner, 1998), the time of
concentration restriction of 0.125 hours is dictated by a combination of array size restrictions, minimum
internal convolution interval, and the user-specified time increment for saving convolution values.

1.5 DRAINAGE BASIN AREA
Drainage basin areas are determined for each pond from scaled topographic drawings. The final post-
mining topography and or current mining topography is utilized to determine overall watershed for each
ponds. Watershed (or sub-watershed) areas are delineated consistent with four general categories
representing various stages of mining related disturbances and various stages of reclamation. The
watershed or sub-watershed delineations (including curve number applications) reflect mining and
reclamation status based on Colowyo’s current mining and reclamation plans as presented on the mine
plan (Maps 23 and 23B) and spoil grading timing maps (Maps 29 and 29B) in accordance with Rule
4.05.6(3)(a).

1.6 TIME OF CONCENTRATION, TC
The time of concentration is calculated using the SCS upland method (Soil Conservation Service, 1986)
as incorporated into the SEDCADTM4 software. The hydraulic lengths, drainage heights and associated
slopes are determined from topographic drawings. The values calculated for each structure are shown on
a sub-watershed basis in the SEDCADTM4 printouts.

1.7 MUSKINGUM ROUTING PARAMETERS, K, X
The Muskingum Routing Parameters are also calculated using the SCS upland method. The hydraulic
lengths, drainage heights and associated slopes are determined from topographic drawings. The values
calculated between each structure and/or sub-watershed are shown in the SEDCADTM4 printouts.

1.8 CURVE NUMBER, CN
The runoff curve number is a factor relating the amount of rainfall to the amount of runoff for a given
area. Curve numbers were jointly developed and agreed upon by the Division and Colowyo.

The curve number for the undisturbed lands is selected as Hydrologic Soil Group (HSG) “C” based on the
pre-mining mapped soils in the county SCS soil maps.

For disturbed lands the use of mapped SCS soil groups is not appropriate. Instead, Section 630.0702