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Step 8. Outlet Structure <br />In almost all cases the outlet structure required will be per the detail <br />shown in Figure 3. The orifice shown in Figure 4 can be sized by the <br />following equations: <br />D = 6.12 <br />D = 4.83 h <br />sharp -edged orifices (inches) <br />rounded orifice (inches) <br />Although the units are not compatible, the above equations yield the correct <br />value and save some calculation time. Normally, orifices should be no <br />smaller than 2.0 inches. <br />The riser must be sized to pass the design storm in the developed condition, <br />assuming the orifice is plugged, without causing the water surface to <br />rise higher than the top of the restrictor catch basin or in the case of <br />ponds, to within 1 foot of the top of the berm. Figure S shows the capacity <br />of various risers as a function of head. The time of concentration for <br />sizing the riser is the sum of the time of concentration in the developed <br />state and the time of filling for the detention facility. <br />The outlet pipe must also be sized to accommodate the design storm in <br />the develoed state. This can be done by use of a nomograph such as that <br />shown in Figure 6, or by Alanning's equation: Q - 1.49 D� 2/3(s) 1/2n �2 <br />n f <br />Where S is tslope of the pipe in feet per foot, D the diameter of the pipe, and <br />n the pipe roughness. For concrete pipe n is 0.012, CMP with annular <br />corrugations is .024, and CMP with helical corrugations is 0.02. <br />