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<br />v2 <br /><l"* = velocity head <br /> <br />HL = head loss <br /> <br />Defining the head (H) on the orifice as the difference between the upstream <br />energy elevation and the downstream water surface elevation (the defini- <br />tion used in HEC-Z) produces: <br /> <br />H = (Yl + Zl + <br /> <br />Z <br /><llVl <br />2g ) - (Yz + Zz) <br /> <br />(Z) <br /> <br />Substituting H from equation 2 into equation 1 produces: <br /> <br />H = <br /> <br />Z <br /><l2VZ <br />Zg + HL <br /> <br />(3) <br /> <br />Head loss (HL) through the bridge can be defined in terms of the bridge <br />velocity head and loss coefficient~. The ,expansion to a point just <br />downstream can be defined by an expansion coefficient K and the change <br />e <br />in velocity head. <br /> <br />VZ <br />HL = Kb ~ + Ke ( <br /> <br />VZ <br />b <br />Zg <br /> <br />Z <br /><lzVz <br />2g <br /> <br />) <br /> <br />(4) <br /> <br />where: <br /> <br />b c subscript designating the bridge <br /> <br />The head loss equation 4 then can be used to define HL in equation 3: <br /> <br />H = <br /> <br />Z <br /><lzV2 <br />2g + Kb ( <br /> <br />l <br />b <br />2g <br /> <br />) + K ( <br />e <br /> <br />~ <br />zg <br /> <br />Z <br /><l2VZ <br />2g <br /> <br />) <br /> <br />( 5) <br /> <br />/ <br /> <br />31 <br />