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Comparison of City of Everett Standard Detention <br />Vault to Corrugated Metal Pipe Vault <br />December 2, 1994 <br />File No. 24-92-009-007-01 <br />Page 2 <br />The expression � is also known as the OverFlow rate. It was developed to describe actions in <br />0 <br />settling basin of uniform horizontal cross section. The following converu the expression to a <br />more useful form for pipe vaults and also shows the equivalence between equations (8.10) and <br />(8.11) in "Stom�water Management." (Copy of appropriate pages attached.) <br />AS =(LxW) <br />L = ]ength <br />W = width <br />Qo =(Ac)(Vo) <br />A� = crossectional area <br />_ (H)(W) for a rectat�gular conduit <br />H = height <br />Vo _ L <br />td <br />t d= time of detention or time to flow through length of pipe <br />AS _ (LxW) _ 1 _ ta <br />Qo (HxW)( a) e H <br />E =1- e�-W���� <br />=1- e(-w ��) <br />=1-e< <W �d� =1-e�-k�d� (Equation 8.11 "StormwaterManagement") <br />w <br />where k = — <br />H <br />Derivation of equadon 8.11 demonsuates that the settling e�ciency can be expressed as a <br />function of surface azea and flow rate, Q$ rime of detention and height, and these expressions <br />aze equivalent. The following restates the expression as a function of Q, L, A, H, and w. <br />