PERMFILE43062

• Permit No: C1981026
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: TR 7 NOV C-94-024 CANADIAN STRIP MINE C-81-026 POND 1 REVIEW OF EMERGENCY SPILLWAY RIPRAP REQUIREMEN
• Section_Exhibit Name: APPENDIX Q
• Media Type: D
• Archive: No

• analysis is limited to hydrology parameters; thus CP Factors have no direct influence on analysis results, and
have not been presented in this analysis.
2.9 Other Parameters
Principal spillway parameters were taken from permit documents and Feld observations. The water level in
the pond was assumed, at the time of a storm, to be at the level of the principal spillway intake.
3. SEDCAD+ ANALYSIS INFORMATION
Enclosure 1 is a SEDCAD+ demonstration fora 10-year, 24-hour storm event. Enclosure 2 is a SEDCAD+
demonstration for a 25-year, 24-hour storm event.
The runoff volume from a 10-year, 24-hour storm event is 4.14 acre-feet (Enclosure 1, Subwatershed/Structure
inpuUOutput Table). As demonstrated in the Elevation-Area-Capacity-Discharge Table (Enclosure 1), the
peak stage elevation for this runoff would be 8199.2 (below the crest of the emergency spillway). The runoff
would be discharged through the principal spillway. There would be no flow over the emergenry spillway
(Figure Q-3, 1129P94, attached).
The runoff volume from a 25-year, 24-hour storm event would be 6.41 acre-feet (Enclosure 2,
Subwatershed/Structure Input/Output Table). As demonstrated in the Elevation-Area-Capacity-Discharge
Table (Enclosure 2), the peak stage elevation (level of the pond) would be 8200.23 with a peak stage discharge
of 15.41 cubic feet per second (cfs). To obtain a close approximation of the distribution of this discharge
between the principal and emergency spillways, the performance of the pond was analyzed between elevations
• 8196 and 8202 at 0.2-foot intervals (Enclosure 3). This more precise definition demonstrates that the total
discharge, at the peak stage elevation of 8200.23 would be 15.55 (Elevation-Area-Capacity-Discharge Table,
Enclosure 3). Straight-line proration of discharges through the principal spillway, and over the emergenry
spillway, are developed in the following table.
Spillway Discharges
Stage Elevation Principal Spillway Emergency Spillway Total
8200.20 9.3' S.4' 14.7'
8200.23 9.42= 6.21= 15.63=
8200.30 10.1' 10.8' 20.9'
' Elevation-Discharge Table, Enclosure 3
2 Calculated, straight-line proration
The straight-line approximation raises the total peak stage discharge to 15.63 from 15.55. The wnservative
maximum value of 6.21 cfs over the emergency spillway is used to review the requirement for riprap.
Enclosure 4, SEDCAD+ Erodible Channel Design, demonstrates that riprap in the emergency spillway is not
required. The maximum allowable velocity over the colloidal alluvial silt substrate is 5.00 feet per second (fps).
Anticipated velocities are not expected to exceed 2.35 fps (Enclosure 4). Incidental to the riprap review, the
spillway analysis also demonstrates that the as-built emergenry spillway meets depth criteria (the distance from
the maximum water surface elevation to the top of the emergenry spillway is 1.77 fee[, greater than the
required 1 foot).
(Canadian Strip Mine, Pond 1 Riprap Review, 27 Dec 94, Page 4 of 5)