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Comparison of City of Everett Standard D:rcntion <br />Vault to Corrugated Metal Pipe Vault <br />December 2, 1994 <br />File No. 24-92-009-007-0 l <br />Page 5 <br />� = 2(90° —sin-' ( r )) = 2(90° — sin-' (4)) <br />= 2(90° — 48.6° ) = 2(41.4° ) = 82.8° <br />X2�Radians _ 1.45R <br />360° <br />AseBme�� = Z[Z(�—sin�) = 242(1.45R —sin1.45R) <br />= 8(1.45R—.99) = 3.7ft'` <br />A� = A,ow — Asegmenc = 50.3ft2 - 3.7ft2 = 46.6ft2 <br />L= 340ft = 2=170ft (flow split 2 ways) <br />H=7ft <br />w =. 0004 �y <br />E =1— e� ��� =1— e�-�.�. �.' �,o > =1— e��� =1— e�-.a�is� =1—.624 <br />=.38 or 38% <br />The efficiencies aze less than 5 pereent different and are considered equivalent for this analysis. <br />These percentages indicate the efficiency of capturing silt particles over the life of the vault in <br />vazying storm conditions. Efficiency will be higher for faster setding pazticles. Actual <br />efficiency at the Sterling installation will be much greater since the average flow (maximum <br />pump capacity) will never be exceeded. Note that this portion of the system does not include the <br />wetpond treatment cell. <br />Since probability is not really a concern in the Sterline vault installation (due to site <br />configuration, elevation, and proximity to the Snohomish River, 6-month, 24-hour peak flows <br />are pumped to the vault and larger flows b��pass the pumps), the equation of concem is Stokes <br />Law for settling pazticles in low velocity conditions. <br />