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TABLE IV-5 MANNING ROUGHNESS COEFFICIENT - STORM SEWERS Pipe Material "n" Reinforced Concrete Corrugated Hetal Smooth flow 2 2/3" x ~" )11 X III 611 X 211 0.013 0.013 0.024 0.027 0.030 E. Storm Inlets Where physical constraints will allow, the DIO-R storm inlet shall be used (see Figure IV-5). Figures IV-6 and IV-7 are nomographs to determine the theoretical capacity of these inlets for continuous grades and sump conditions. Grated inlets may be used where extraordinary conditions dictate. Figure IV-8 may be used for determining capacity of a grated inlet for sump conditions. Due to differences in grate configurations, capacity analysis of these inlets on continuous grades should be made on an individual basis. This should be done by methodology out- lined in The Design of Storm-Water Inlets, Johns Hopkins University. Lateral pipes are sized in the same manner as culverts. The hydraulic grade line in the main line represents a tailwater depth and the depth of ponding in the inlet, the headwater depth. Figure IV-9 can be used for the sizing of inlet laterals. This figure assumes a minimum inlet box depth to allow for entrance losses and a 0.5 foot freeboard. Total minimum depth is: -- ~---=---=-=-= - ----- - . (V2) I = 0.5 + 1.2 2g + D where: I = inlet box depth, measured from bottom of curb opening, feet V = velocity in inlet lateral, feet per second acceleration of gravity, feet 2 g = per second D = lateral diameter, feet Due to potential clogging of inlets, the theoretical design capacity of the inlet shall be reduced to an allowable value by the factors listed in Table IV-6. 46 I ~ , I ~ ! ! ~ ! ! ,- I fI I ~. ~: I, "- . I' ., L. . ''': !I