2013-10-31_REVISION - M1977284 (3)

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Mineral Joe Mine — Drainage Design Plan 29 The difficulty in quantifying the cumulative effects of very large numbers of small (i.e., on -site) detention/retention facilities (Malcomb, 1982; Urbonas and Glidden, 1983) and the virtual impossibility of assurance of their continued long -term performance or existence (Debo, 1982; Prommersberger, 1984) requires the District to recognize in its floodplain management only regional, publicly owned facilities. Nevertheless, upstream storage is encouraged, such as with the 'Blue- Green" concept first described in Civil Engineering magazine (Jones, 1967). The runoff modeling performed for the Mineral Joe Mine using WinTR -55 conservatively does not take credit for upstream storage. 4.6 Model Implementation of Stormwater Routing Stormwater routing from the sub - basins, reaches, and pond described is shown schematically in Figure 14. Although different symbols are used to identify three types of reaches (diversion ditches, borrow or perimeter ditches, and naturally formed channels), the reaches are treated the same in the numerical model. The outflow from the WinTR -55 model is broken into four outlets as shown in Figure 14, although, in the physical system, all of the drainage from the site eventually reports to the same unnamed ephemeral drainage. The stormwater modeling parameters (area, runoff curve number, To, and receiving reach) for each of the sub -basins shown in Figure 14 are summarized in Table 16. A subset of the WinTR -55 model was also modeled in HydroCAD®, to more accurately evaluate pond storage and spillway design for the planned SWRP. Figure 15 shows the four sub -basins and five reaches reporting to the planned SWRP. HydroCAD® more accurately models the stage- storage relationships that control pond discharge, spillway velocities, and scour. HydroCAD® can also be used to incorporate pond and channel exfiltration, however, the exfiltration feature was not invoked in the planned SWRP model run (PS WRP- 02.hcp). N -0 MDS OU7 2 N•E OU7 3 LDS -E EXPLANATION N•A Sub -Basin Reach. Naturally Formed >rhahnei V V OUT 1 Outlet NHR Reach: Borrow Ditch sub•Basin Within SWRP Pond U S Reach? Diversion Ditch Figure 14. Schematic of Stormwater Routing at the Mineral Joe Mine 4148B.131029 Whetstone Associates •