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<br />3.1. LOW LEVEL OUTLET. <br /> <br />Flow through a low level outlet can be computed from the orifice <br />equation which is shown mathematically as follows: <br /> <br />Q = CA "2g H E <br /> <br />(8) <br /> <br />where: <br /> <br />Q = computed flow in cfs <br />C = orifice coefficient, typically 0.62 <br />g = acceleration of gravity, typically 32.174 <br />A = cross sectional area of conduit in square feet <br />H = head in feet above centerline of low level outlet <br />E = exponent, typically 0.5 <br /> <br />The orifice equation can be used accurately for conduits under inlet <br />control. For those in which outlet control decreases the capacity, alternate methods <br />should be used to develop an accurate stage-discharge rating curve. <br /> <br />3.2 SPILLWAY. <br /> <br />3.2.1. UNSUBMERGED RECTANGULAR SPILLWAY. For a rectangular <br />spillway unobstructed by backwater submergence from the outflow channel, the <br />spillway stage-discharge rating curve may be approximated with the weir equation if <br />the flow passes through critical depth at the outflow control section. The weir <br />equation is expressed as: <br /> <br />Q = CLH E (9) <br />where: <br />Q = outflow in cfs <br />C = weir coefficient, typically 2.6 to 3.1 <br />L = length of the spillway crest in feet <br />H = head above spillway crest in feet <br />E = exponent, typically 1.5 <br /> <br />For spillways in which the flow in the outflow channel does not pass <br />through critical depth but remains subcritical and is unobstructed by downstream <br />backwater effects or submergence, the stage-discharge relationship may be computed <br />from the normal depth Manning equation, which is expressed as follows: <br /> <br />7-77 <br />