2017-02-21_REVISION - M1988112 (2)

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South Diversion Ditch Design Considerations The design basis for the existing South Diversion Ditch was the 100 -year, 24-hour storm. Improvements to the ditch and spillway structure are necessary to prevent extreme storm runoff in excess of a 100 -year event from escaping the ditch and flooding the reclaimed tailings impoundment area and/or washing out the overflow structure and eroding the left groin of the tailing dam. Selection of an appropriately conservative flood recurrence interval and defensible hydrologic methods as the design basis for the structural improvements is a critical first step. To successfully pass the most conservative (i.e., greatest) hydrologic loading (6 -hour Local Storm PMP with RMUH and Kurt methodology) through the South Diversion Ditch and spillway (without storage), a system hydraulic capacity of approximately 4,000 cfs is required. Alternatively, except for the combination of RMUH and Kul,, a system capacity of approximately 3,000 cfs would successfully pass all of the other hydrologic conditions evaluated. As an alternative to simply increasing the South Diversion Ditch and spillway system hydraulic conveyance capacity, detaining and releasing runoff at a controlled rate would allow for a lesser design standard (i.e., smaller peak design flow rate) for the ditch and spillway system. Storage could be incorporated directly in the upper reaches of the South Diversion Ditch or as an off-line pond in the reclaimed tailings impoundment area. The amount of detention storage provided vs. the magnitude of channel/spillway improvements constructed could be evaluated through a cost benefit analysis stemming from a more rigorous hydraulic flood routing analysis of the South Diversion Ditch and the surrounding topography. At the request of MGC, DOWL conducted preliminary analyses to evaluate scenarios for providing attenuating storage detention capacity versus ditch/spillway hydraulic capacity. Assuming 100% efficiency of the detention system, the following tables present an approximation of the minimum storage that would be required to contain the runoff in the ditch for a given system (spillway and ditch) capacity. As shown in the tables below, a system hydraulic conveyance capacity of between 1,500 cfs and 2,000 cfs will handle all but a PMF event (6 -hour Local Storm or 72 -hour General Storm) without additional storage. A system with hydraulic conveyance capacity of 2,500 cfs would still require some amount of attenuating storage to contain the 6 -hour Local Storm PMF. In reality the detention volume will not be 100% efficient (i.e., some detention volume will be consumed by the "rising" limb of the hydrograph), and the actual required volume will be larger than presented below. Determination of the actual storage volume needed for a specific ditch/spillway size will require more refined hydraulic routing analyses. These preliminary analyses are provided for purposes of discussions and to assist with layout of potential detention facilities through which the runoff hydrograph can be routed. Table 17: Minimum Storage Volume (Ditch/Spillway System Capacity =1,500 cfs) Runoff Storage Volume (ac -ft) (System Capacity = 1,500 cfs) Return Period Clark UH (CN Losses) Clark UH (Kset) RMUH (Kurt) RMUH (Ksat) 100 -Years -- - -- -- 500-Years -- -- 55.22 -- 1,000-Years -- 76.16 -- 10,000-Years -- - 118.17 -- PMF (6 -hr) 31.21 121.30 244.56 147.47 PMF (72 -hr) 156.65 169.50 \\BIL-FS\BIL-projects\26\21099-01\95Rpts\San Luis Flood Hydrology Study_R2.docx Page 9 of 10