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1 1 sized using a hydraulic loading rate between 6-8 gpm/ft2.Other hydraulic loading rates may be more appropriate for other systems. Bypass should be provided around the chemical treatment system to accommodate extreme storms. Runoff volume shall be cal- culated using the methods presented in Chapter III-2 - Hydrologic Analysis (p.429). Worst -case land cover conditions (i.e., producing the most runoff) should be used for ana- lyses (in most cases, this would be the land cover conditions just prior to final land- scaping). Sizing Criteria for Flow Control Water Bodies: Sites that must implement flow control for the developed site condition must also control stormwater release rates during construction. Construction site stormwater discharges shall not exceed the discharge durations of the pre -developed condition for the range of pre -developed discharge rates from '/2 of the 2-year flow through the 10-year flow as pre- dicted by an approved continuous runoff model. The pre -developed condition to be matched shall be the land cover condition immediately prior to the development project. This restriction on release rates can affect the size of the storage pond and treatment cells. The following is how WWHM can be used to determine the release rates from the chem- ical treatment systems: 1. Determine the pre -developed flow durations to be matched by entering the existing land use area under the "Pre -developed" scenario in WWHM. The default flow range is from'/2 of the 2-year flow through the 10-year flow. 2. Enter the post developed land use area in the "Developed Unmitigated" scenario in WWHM. 3. Copy the land use information from the "Developed Unmitigated" to "Developed Mit- igated" scenario. 4. While in the "Developed Mitigated" scenario, add a pond element under the basin element containing the post -developed land use areas. This pond element rep- resents information on the available untreated stormwater storage and discharge from the chemical treatment system. In cases where the discharge from the chem- ical treatment 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 stand- ard. This would be an iterative design procedure where if the initial SSD table proved to be inadequate, 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 complies with the flow duration standard is correctly sized. 2014 Stormwater Management Manual for Western Washington Volume // - Chapter 4 - Page 401