2017-02-21_REVISION - M1988112 (3)

Home Browse Search

February 17, 2017 Mr. Wallace Erickson Page 2 of 3 Analyses of the South Diversion Ditch determined that minimal reshaping and regrading of the channel would allow the Ditch to convey runoff events exceeding the 500 -year event and up to 1522 cfs. The historic Ditch bottom had rounded corners and an adverse grade in some locations. The South Diversion Ditch capacity could be substantially increased by reshaping the channel geometry to widen the channel bottom to 15 feet in all areas and removing undulations to allow for free drainage toward the drop structure. In addition, to reduce erosion of the Ditch during designed overflow events, the west bank (impoundment side) of the upper reach of the South Diversion Ditch required regrading so that it could serve as a weir to allow excess runoff to be routed to the reclaimed area of the tailing impoundment during high flood events. In the fall of 2016, routine maintenance was performed on the South Diversion Ditch to remove accumulated sediments and vegetation. Due to the minimal amount of regrading required, the identified Ditch improvements were made as part of the maintenance activities at that time. Those improvements did not change the existing function of the South Diversion Ditch System, and did not alter the existing drop structure. Improvements to the drop structure are the subject of this proposed Technical Revision. PROPOSED DROP STRUCTURE IMPROVEMENTS The existing drop structure was designed to handle upland runoff from a 100 -year event that would be conveyed through the South Diversion Ditch; however, the closed -conduit design of the structure made it susceptible to clogging by debris during a large flood event. The proposed new drop structure is designed to pass the 500 -year flow (792 cfs), and up to 1522 cfs of flow associated with the Probable Maximum Flood (PMF), with the remainder of the PMF flow passing over the upstream overflow weir to the impoundment. Consistent with the recommendations of the Detailed Inspection Report, the new drop structure is designed as an open structure to reduce the risk of debris blocking the structure during a flood event. The drop structure includes an approach section, a chute, a stilling basin, and a discharge channel. These design features are intended to further improve the hydraulic performance of the drop structure during major flood events. A full basis of design for the drop structure and South Diversion Ditch, including calculations, criteria, and design references for construction of the improvements is attached as Appendix A — Design Report, South Diversion Ditch Drop Structure. Design specifications and drawings, for the drop structure, are attached as Appendix B and C, respectively. SUMMARY Final design analyses have been prepared for proposed drop structure improvements as recommended by the Detailed Inspection Report (MGC 2014). Drop structure drawings and specifications are attached for Division of Reclamation Mining and Safety (DRMS) review. BMRI is requesting a Technical Revision for replacing the existing drop structure with a larger open conduit drop structure to improve capacity and reduce the chance of blockage.