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CONVEYANCE CALCULATIONS <br />The conveyance system was designcd to provide capacity for the 25-year design storm <br />event with a minimum of six inches of freeboard within each catch basin. The capacity of <br />the system was checked by perfortning hydraulic grade line calculations for the <br />conveyancc system. Runoft was estimated using the Rational Method with an initial time <br />of wncentration of 63 minutes and weighted runofi coefficients respective of the <br />impervious (C=0.90) and pervious (C = 0.25 landscapad) ereas tributary to each catch <br />basin. Rainfall intensities were taken from the 25-year I.D.F. curve for the Ciry of Everett. <br />The hydraulic grade line calculations were perfortned using Eagle Point Software's <br />computer program, Storm Sewers. A brief description of Storm Sewers and a summary <br />of the hydraulic grade line calculations for the conveyance system follows. A copy of the <br />actual output from the calculations, which includes the Rational Method flow estimates, is <br />included in the Appendix. <br />Storm Sewers determines the flow rate in each pipe and then performs a standard step <br />hydraulic analysis on the pipe network. The methodology used for non-uniform flow <br />analysis is the standard step energy balance. This procedure is used to determine the <br />hydraulic grade line throughout the network and is identical to that used for any open <br />channel water surface profile. The steady state energy equation (Bernoulli's Equation) is <br />used between the upstream and downstream sections of each pipe. The friction slope is <br />calculated by applying Manning's equation at the upstream and downstream points and <br />averagin� the slope between them. The program then pe:forms threc iterations to pinpoint <br />the hydraulic grade line. The computations begin at the most downstream pipe in the <br />network and continue in an upward direction. After three iterations, the program has <br />established the hydraulic grade line at the upstream and downstream end of each pipe in <br />the network. <br />S(ornr Drninnge Re�rt For Iling Extrusion - Page !* <br />