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DESIGN AND CONSTRUCTION STANDARDS AND SPECIFICATIONS 6-10.1 STORMWATER REQUIREMENTS FOR DEVELOPMENT IN THE COMBINED SEWER AREA Development projects draining to the combined sewer system are not required to meet the same stormwater management standards as projects which drain to a separated stormwater system. Instead, projects must comply with the following requirements for design of stormwater systems associated with development. Public Infrastructure development/redevelopment(such as road and utility construction)within the Public Right of Way is exempt from these requirements.Projects need not meet the standards included in Section 4 of this DCSS or the adopted Stormwater Management Manual,although the following section draws from portions of both. Unless otherwise stated below stormwater facilities constructed to meet the combined sewer area requirements should be constructed and maintained in accordance with the requirements of Section 4 and the Stormwater Management Manual. 6-10.1(1) FLOW CONTROL Projects shall employ Flow Control BMPs in accordance with the following thresholds, standards, and requirements to reduce the impacts of stormwater runoff from hard surfaces and land cover conversions. Thresholds The following projects require construction of Flow Control BMPs to achieve the Flow Control Performance Standard. If a project meets any of the following thresholds,Flow Control BMPs are required.The project proponent must demonstrate that the project-does not meet any of the following thresholds for Flow Control BMPs to not be required. • Projects that have a total of 200 square feet or more of netnew impervious surface,or • Projects that convert 1/4 acres or more of native vegetation,pasture, scrub/shrub, or unmaintained non-native vegetation to lawn or landscape. Flow Control Performance Standard The combined sewer collection system is designed to a goal of conveying the 25-year recurrence storm event without flooding. That analysis was based on the land cover in place at the time that the conveyance system was constructed. Any increases in runoff rates have the potential to cause or exacerbate flooding in the system. Therefore,projects which exceed the threshold for flow control shall comply with the following standard: • Stormwater discharges shall match developed discharge durations to existing site durations for the-range of-existing discharge-rates from the 10-year peak flow through the full 100-year peak flow. • Peak runoff rates from the developed site shall not exceed existing site runoff rates in the 25-year and 100-year return intervals. The existing condition to be matched shall be the site as it exists at the time of application. Calculations shall be performed using a continuous simulation modeling program approved by the Washington SANITARY SEWERS 6-15.