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1 system is modeled the same way as described in BMP C250: Construction Stormwater Chemical Treatment (p.396) and is as follows: While in the "Developed Mitigated" scenario, add a pond element under the basin element containing the post -developed land use areas. This pond ele- ment represents information on the available untreated stormwater storage and discharge from the filtration system. In cases where the discharge from the filtration system is controlled by a pump, a stage/storage/discharge (SSD) table representing the pond must be generated outside WWHM and imported into WWHM. WWHM can route the runoff from the post -developed condition through this SSD table (the pond) and determine compliance with the flow duration standard. This would be an iterative design procedure where if the initial SSD table proved to be out of compliance, the designer would have to modify the SSD table outside WWHM and re -import in WWHM and route the runoff through it again. The iteration will continue until a pond that enables compliance with the flow duration standard is designed. Notes SSD table on characteristics: . The pump discharge rate would likely be initially set at just below'/2 if 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 neces- sary to increase the pump discharge rate above Y2 of the 2-year. The increase(s) above'/2 of the 2-year must be such that they provide some ' relief to the untreated stormwater storage needs but at the same time they 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. b. The stormwater filtration system uses a storage pond/tank and the discharge from this pond/tank gravity flows to the filter. This is usually a slow sand filter system and it is possible to model it in WWHM as a Filter element or as a combination of Pond and Filter element placed in series. The stage/stor- age/discharge table(s) may then be generated within WWHM as follows: ' i. While in the "Developed Mitigated" scenario, add a Filter element under the basin element containing the post -developed land use areas. The length and width of this filter element would have to be the same as the bottom length and width of the upstream untreated stormwater storage pond/tank. 2014 Stormwater Management Manual for Western Washington Volume // - Chapter 4 - Page 407