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i. West of the Cascade Mountains Crest:Temporary pipe slope drains must handle <br /> the peak 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 /> predicted by an approved continuous runoff model, increased by a factor of 1.6, <br /> may be used. The hydrologic analysis must use the existing land cover condition <br /> for predicting flow rates from tributary areas outside the project limits. For <br /> tributary areas on the project site, the analysis must use the temporary or <br /> permanent project land cover condition, whichever will produce the highest flow <br /> rates. If using the Western Washington Hydrology Model (WWHM)to predict <br /> flows, bare soil areas should be modeled as "landscaped area." <br /> ii. East of the Cascade Mountains Crest:Temporary pipe slope drains must handle <br /> the expected peak flow rate from a 6-month, 3-hour storm for the developed <br /> condition, referred to as the short duration storm. <br /> d. Place excavated material on the uphill side of trenches, consistent with safety and <br /> space considerations. <br /> e. Place check dams at regular intervals within constructed channels that are cut down <br /> a slope. <br /> 7. Protect Drain Inlets <br /> a. Protect all storm drain inlets made operable during construction so that stormwater <br /> runoff does not enter the conveyance system without first being filtered or treated <br /> to remove sediment. <br /> b. Clean or remove and replace inlet protection devices when sediment has filled one- <br /> third of the available storage(unless a different standard is specified by the product <br /> manufacturer). <br /> 8. Stabilize Channels and Outlets <br /> a. Design, construct and stabilize all on-site conveyance channels to prevent erosion <br /> from the following expected peak flows: <br /> i. West of the Cascade Mountains Crest: Channels must handle the peak 10- <br /> minute flow rate from a Type 1A, 10-year, 24-hour frequency storm for the <br /> developed condition.Alternatively, the 10-year, 1-hour flow rate indicated by <br /> an approved continuous runoff model, increased by a factor of 1.6, may be <br /> used.The hydrologic analysis must use the existing land cover condition for <br /> predicting flow rates from tributary areas outside the project limits. For <br /> tributary areas on the project site,the analysis must use the temporary or <br /> permanent project land cover condition, whichever will produce the highest <br /> flow rates. If using the WWHM to predict flows, bare soil areas should be <br /> modeled as"landscaped area." <br /> ii. East of the Cascade Mountains Crest: Channels must handle the expected peak <br /> flow rate from a 6-month, 3-hour storm for the developed condition, referred <br /> to as the short duration storm. <br /> b. Provide stabilization, including armoring material, adequate to prevent erosion of <br /> outlets,adjacent stream banks,slopes, and downstream reaches at the outlets of all <br /> conveyance systems. <br /> Construction Stormwater General Permit Page 28 <br />