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1 <br />Sizing Criteria for Flow -Through Treatment Systems for Flow Control <br />Exempt Water Bodies: <br />When sizing storage ponds or tanks for flow -through systems for flow control exempt <br />water bodies the treatment system capacity should be a factor. The untreated stormwater <br />storage pond or tank should be sized to hold 1.5 times the runoff volume of the 10-year, <br />24-hour storm event minus the treatment system flowrate for an 8-hour period. For a <br />chitosan-enhanced sand filtration system, the treatment system flowrate should be sized <br />using a hydraulic loading rate between 6-8 gpm/ft2.Other hydraulic loading rates may be <br />more appropriate for other systems. Bypass should be provided around the chemical <br />treatment system to accommodate extreme storms. Runoff volume shall be calculated <br />using the methods presented in Chapter III-2 - Hydrologic Analysis (p.429). Worst -case <br />conditions (i.e., producing the most runoff) should be used for analyses (most likely con- <br />ditions present prior to final landscaping). <br />Sizing Criteria for Flow Control Water Bodies: <br />Sites that must implement flow control for the developed site condition must also control <br />stormwater release rates during construction. Construction site stormwater discharges <br />shall not exceed the discharge durations of the pre -developed condition for the range of <br />pre -developed discharge rates from 1 /2 of the 2-year flow through the 10-year flow as <br />predicted by an approved continuous runoff model. The pre -developed condition to be <br />matched shall be the land cover condition immediately prior to the development project. <br />This restriction on release rates can affect the size of the storage pond, the filtration sys- <br />tem, and the flow rate through the filter system. <br />The following is how WWHM can be used to determine the release rates from the fil- <br />tration systems: <br />1. Determine the pre -developed flow durations to be matched by entering the land <br />use area under the "Pre -developed" scenario in WWHM. The default flow range is <br />from '/2 of the 2-year flow through the 10-year flow. <br />2. Enter the post developed land use area in the "Developed Unmitigated" scenario <br />in WWHM. <br />3. Copy the land use information from the "Developed Unmitigated" to "Developed Mit- <br />igated" scenario. <br />4. There are two possible ways to model stormwater filtration systems: <br />a. The stormwater filtration system uses an untreated stormwater storage <br />pond/tank and the discharge from this pond/tank is pumped to one or more fil- <br />ters. In -line filtration chemicals would be added to the flow right after the <br />pond/tank and before the filter(s). Because the discharge is pumped, WWHM <br />can't generate a stage/storage /discharge (SSD) table for this system. This <br />2014 Stormwater Management Manual for Western Washington <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />Volume 11- Chapter 4 - Page 406 <br />1 <br />