2017-02-21_REVISION - M1988112 (3)

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Between June and August 2015, under the direction of MGC, DOWL completed updated hydrologic (rainfall -runoff) studies for the major contributing watersheds above the tailing facility. A report documenting those studies is attached as Appendix B to this report. The updated hydrology resulted in revised estimates for storm -water runoff volumes and peak discharges reporting to the main South Diversion Ditch system for major events up to PMF-level storms. For PMF magnitude events, DOWL presented a range of results, varying due to different assumptions used in the hydrologic modeling. The results produced were similar to previous calculations (MGC, 2014) for the most conservative combination of hydrologic parameter assumptions (unit hydrograph and soil infiltration rates). However, other methodologies and assumptions provided progressively significant reductions in estimated peak PMF discharges. A decision was made to use the set of assumptions and results based on the Rocky Mountain Unit Hydrograph with Constant Loss Method (National Engineering Handbook, Part 630 - Hydrology) with saturated soil hydraulic conductivity rates (Ksat) by hydrologic soil group to estimate infiltration. These procedures are considered reasonable, but not overly conservative, and are generally acceptable practice for dam safety risk evaluation. Using the recommended revised PMF calculations, DOWL conducted additional channel routing analyses to refine the calculations, and developed preliminary (concept -level) design options for re -configuring the South Diversion Ditch conveyance channel (channel size and slope), in combination with varying size and capacity of a new drop structure at the left dam abutment, and ditch overflow capacity. Inflows in excess of the channel/drop structure conveyance capacity will overflow at an upstream weir and spill from the ditch into the impoundment area. In close consultation with BRMI, an agreement was reached as to the basic design parameters for hydraulic sizing and arrangement of the system, and selection of type of drop structure, as follows: • Size the South Diversion Ditch improvements (channel and Drop Structure) such that there will be no overflows into the tailing impoundment area for floods up to approximately a 500 -year event; impoundment storage would be utilized only for rare flood events; • Drop Structure sized to handle maximum peak discharge of approximately 1,500 cfs during the PMF, with the remainder of the PMF flow passing over the upstream overflow weir to the impoundment; and • Drop Structure will be an open overflow design (no conduit) to avoid potential for clogging with debris, and will be a straight drop chute into a hydraulic jump stilling basin. This report documents the project goals, design criteria and basis of design for the San Luis Project, South Diversion Ditch improvements. 1.2 Project Goals and Design Criteria The specific goal for the project was to design structural upgrades to the facility that will reduce risk of severe erosion at the Drop Structure and South Diversion Ditch during major flood event, which could potentially result in an unacceptable release of flood water or tailing material stored in the impoundment. -------------------- - - - - - - — - - -- -- ----- ------------- ------ - San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 3