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1 Notes on SSD table characteristics: The pump discharge rate would likely be initially set at just below'/2 of the 2- year flow from the pre -developed condition. As runoff coming into the untreated stormwater storage pond increases and the available untreated stormwater storage volume gets used up, it would be necessary to increase the pump discharge rate above 1/2 of the 2-year. The increase(s) above '/2 of the 2-year must be such.that they provide some relief to the untreated storm - water storage needs but at the same time will not cause violations of the flow duration standard at the higher flows. The final design SSD table will identify the appropriate pumping rates and the corresponding stage and storages. When building such a flow control system, the design must ensure that any automatic adjustments to the pumping rates will be as a result of changes to the available storage in accordance with the final design SSD table. 5. It should be noted that the above procedures would be used to meet the flow con- trol requirements. The chemical treatment system must be able to meet the runoff treatment requirements. It is likely that the discharge flow rate of/2 of the 2-year or more may exceed the treatment capacity of the system. If that is the case, the untreated stormwater discharge rate(s) (i.e., influent to the treatment system) must be reduced to allow proper treatment. Any reduction in the flows would likely result in the need for a larger untreated stormwater storage volume. If the discharge is to a municipal storm drainage system, the allowable discharge rate may be limited by the capacity of the public system. It may be necessary to clean the municipal storm drainage system prior to the start of the discharge to pre- vent scouring solids from the drainage system. If the municipal storm drainage sys- tem discharges to a water body not on the flow control exempt list, the project site is subject to flow control requirements. Obtain permission from the owner of the col- lection system before discharging to it. If system design does not allow you to discharge at the slower rates as described above and if the site has a retention or detention pond that will serve the planned development, the discharge from the treatment system may be directed to the permanent reten- tion/detention pond to comply with the flow control requirement. In this case, the untreated stormwater storage pond and treatment system will be sized according to the sizing criteria for flow -through treatment systems for flow control exempt water bodies described earlier except all discharge (water passing through the treatment system and stormwater bypassing the treatment system) will be directed into the permanent reten- tion/detention pond. If site constraints make locating the untreated stormwater storage pond difficult, the permanent retention/detention pond may be divided to serve as the untreated stormwater storage pond and the post -treatment flow control pond. A berm or barrier must be used in this case so the untreated water does not mix with the treated water. Both untreated stormwater storage requirements, and adequate post -treatment 2014 Stormwater Management Manual for Westem Washington 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Volume // - Chapter 4 - Page 402 1