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Table 8: Summary of WQv Provided Step 5 Step 3 Reduced WQv to be Step 2 WQv Reduction by Treated by Standard WQv Required (CF) Standard SMPs w/ RRv SMPs or Alternative Capacity (CF)1 Practices (CF)2 8,672 3,338 6,627 Footnotes: 1Step 3: WQv Reduction = RRv Provided + WQv Treated by Standard SMP with RRv Capacity 2Step 5: Reduced WQv to be Treated = WQv Required – WQv Reduced Based upon the results listed in the above Table, the entire WQv has not been treated by application of RR techniques and standard SMPs with RRv capacity. As such, the standard SMPs or the Alternative Practices (without RRv capacity) described in the following sections, have been incorporated into the stormwater management plan for this project, to meet the WQv objective. Practice-specific calculations are presented in Appendix C. 3.5.1 Underground Stormwater Detention System Underground stormwater detention systems store and detain stormwater runoff in order to meet water quantity control requirements. Stormwater is stored in subsurface vaults and/or a system of large diameter interconnected storage pipes. Stored water is then released at rates designed to reduce peak run-off flows during post-development storms. Underground stormwater storage provides minimal stormwater quality benefits, but can be an effective component of a development’s overall stormwater management plan. 3.5.2 Hydrodynamic Separators Hydrodynamic separators accelerate the separation of floating and settling pollutants from stormwater through the use of a vortex. These pre-fabricated devices come in the form of an underground manhole or vault. The devices have no moving parts and are typically fabricated from concrete and marine grade aluminum. During operation, stormwater runoff enters the unit tangentially to promote a gentle swirling motion in a treatment chamber. As stormwater circles within the chamber, settable solids fall into a sump and are retained. Buoyant debris, oil, and grease rise to the surface and are separated from the water as it flows under a baffle wall. Finally, treated water exits the treatment chamber through a flow control orifice located behind the baffle wall. During low-flow conditions all runoff is diverted into the treatment chamber by a flow partition. At higher flow rates, a portion of the runoff spills over the flow partition and is diverted around the treatment chamber to prevent re-suspension and washout of previously trapped pollutants. Water that spills over the partition flows into a head equalization chamber above the treatment chamber outlet. As the head equalization chamber fills, the head differential driving flow through the treatment chamber collapses. The result is that flow rates in the treatment chamber remain relatively constant even as total flow rates increase substantially. This configuration further reduces the potential for re-suspension or washout. Stormwater Pollution Prevention Plan Page 10 2241067 Page 97 of 359