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1 ii. In cases where the length and width of the filter is not the same as those for the bottom of the upstream untreated stormwater storage tank/ - pond, the treatment system may be modeled as a Pond element fol- lowed by a Filter element. By having these two elements, WWHM would then generate a SSD table for the storage pond which then grav- ity flows to the Filter element. The Filter element downstream of the untreated stormwater storage pond would have a storage component through the media, and an overflow component for when the filtration capacity is exceeded. WWHM can route the runoff from the post -developed condition through the treatment systems in 4b and determine compliance with the flow duration standard. This would be an iterative design procedure where if the initial siz- ing estimates for the treatment system proved to be inadequate, the designer would have to modify the system and route the runoff through it again. The iteration would continue until compliance with the flow duration standard is achieved. 5. It should be noted that the above procedures would be used to meet the flow con- trol requirements. The filtration 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 storm - water 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 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 requirements. 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 waterbodies described earlier except all discharges (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 discharge 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 flow control must be achieved. The post -treatment flow control pond's revised dimen- sions must be entered into the WWHM and the WWHM must be run to confirm com- pliance with the flow control requirement. 2014 Stormwater Management Manual for Western Washington 1 1 1 �I G 1 Ij 1 1 1 1 1 1 1 1 Volume 11 - Chapter 4 - Page 408 1