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b. The Permittee must divert off-site stormwater(run-on)or ground water away <br /> from slopes and disturbed areas with interceptor dikes,pipes, and/or swales. <br /> Off-site stormwater should be managed separately from stormwater generated <br /> on the site. <br /> c. At the top of slopes, collect drainage in pipe slope drains or protected channels <br /> to prevent erosion. <br /> i. West of the Cascade Mountains Crest: Temporary pipe slope drains must <br /> handle the peak 10-minute flow rate from a Type 1A, 10-year, 24-hour <br /> frequency storm for the developed condition. Alternatively,the 10-year, <br /> 1-hour flow rate predicted by an approved continuous runoff model, <br /> increased by a factor of 1.6,may be used. The hydrologic analysis must <br /> use the existing land cover condition for predicting flow rates from <br /> tributary areas outside the project limits.For tributary areas on the project <br /> site,the analysis must use the temporary or permanent project land cover <br /> condition,whichever will produce the highest flow rates. If using the <br /> Western Washington Hydrology Model(WWHM)to predict flows,bare <br /> soil areas should be modeled as "landscaped area." <br /> ii. East of the Cascade Mountains Crest: Temporary pipe slope drains must <br /> handle the expected peak flow rate from a 6-month, 3-hour storm for the <br /> developed condition,referred to as the short duration storm. <br /> d. Place excavated material on the uphill side of trenches, consistent with safety <br /> and space considerations. <br /> e. Place check dams at regular intervals within constructed channels that are cut <br /> down a slope. <br /> 7. Protect Drain Inlets <br /> a. Protect all storm drain inlets made operable during construction so that <br /> stormwater runoff does not enter the conveyance system without first being <br /> filtered or treated to remove sediment. <br /> b. Clean or remove and replace inlet protection devices when sediment has filled <br /> one-third of the available storage(unless a different standard is specified by the <br /> product manufacturer). <br /> 8. Stabilize Channels and Outlets <br /> a. Design, construct and stabilize all on-site conveyance channels to prevent <br /> erosion from the following expected peak flows: <br /> i. West of the Cascade Mountains Crest: Channels must handle the peak <br /> 10-minute flow rate from a Type 1A, 10-year, 24-hour frequency storm <br /> for the developed condition. Alternatively,the 10-year, 1-hour flow rate <br /> indicated by an approved continuous runoff model, increased by a factor <br /> of 1.6,may be used. The hydrologic analysis must use the existing land <br /> Construction Stormwater General Permit <br /> Page 32 <br />