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JD -6 Mine — Drainage Design Plan 29 <br />The difficulty in quantifying the cumulative effects of very large numbers of small (i.e., <br />on -site) detention/retention facilities (Malcomb, 1982; Urbonas and Glidden, 1983) and the <br />virtual impossibility of assurance of their continued long -term performance or existence <br />(Debo, 1982; Prommersberger, 1984) requires the District to recognize in its floodplain <br />management only regional, publicly owned facilities. Nevertheless, upstream storage is <br />encouraged, such as with the 'Blue- Green" concept first described in Civil Engineering <br />magazine (Jones, 1967). <br />The runoff modeling performed for the JD -6 Mine using WinTR -55 conservatively does not take credit for <br />upstream storage. <br />4.6 Model Implementation of Stormwater Routing <br />Stormwater routing from the sub - basins, reaches, and pond described is shown schematically in Figure 14. <br />Although different symbols are used to identify three types of reaches (diversion ditches, borrow or <br />perimeter ditches, and naturally formed channels), the reaches are treated the same in the numerical model. <br />The outflow from the WinTR -55 model is broken into four outlets as shown in Figure 14, although, in the <br />physical system, all of the drainage from the site eventually reports to the same unnamed ephemeral <br />drainage. The stormwater modeling parameters (area, runoff curve number, T,, and receiving reach) for <br />each of the sub -basins shown in Figure 14 are summarized in Table 16. <br />A subset of the WinTR -55 model was also modeled in HydroCAD®, to more accurately evaluate pond <br />storage and spillway design for the planned SWRP. Figure 15 shows the four sub - basins and five reaches <br />reporting to the planned SWRP. HydroCAD® more accurately models the stage- storage relationships that <br />control pond discharge, spillway velocities, and scour. HydroCAD® can also be used to incorporate pond <br />and channel exfiltration, however, the exfiltration feature was not invoked in the planned SWRP model run <br />(JD6_PSWRP_02.hcp). <br />4148A.131028 Whetstone Associates <br />