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5.3 Flow Computation Uniform flow and critical flow computations shall bE! in accordance with the USDCM) sections 2.2.3 and 2.2.4, "Major Drainage", and shall use the Manning's equation as follows: Q = ~ (AR1liS'/2) n (Equation 5.1) Where: Q = flow rate (cfs) n = Manning's roughness coefficient A = area (fe) R = AlP '" hydraulic radius (ft) P = wettE',d perimeter (ft) S = enerc,W grade line slope (ft/ft) 5.4 Design standards fc:,r Major Drainageways These standards cover the design of major draina~reways. The design standards for open channels cannot be presented in a step-by-step fashion because of the 1,dde range of design ,options available to the design engineer. Certain planning and conceptual design criteria are particularly useful in the preliminary design of a channel. These criterii~., which have the grE!at:est effect on the performance and cost of the channel, are discussed below. 5.4.1 Natural Channe,ls IMa;or DrainagewaYIll The design criteria and evaluation techniques fot" natural channels are: 1. The channel and overbank areas shall have adequate capacity for the 100-year storm runoff. 2. Natural channel segments greater than 0.8 for protected from erosion. which have a calculated Froude Number the 100-year flood peak shall be 3. The water surface profile shall be d,~fined so that the floodplain can be zoned and protected. 4. Roughness factors (Manning's n), which are representative of unmaintained channel conditions, shall be used for the analysis of water surface profiles. 5. Roughness factors (::-lanning's n), which are representative of maintained channel conditions, shall bE' used to determine velocity limitations. 6. Erosion control structures, such as drop st;ructures or check dams, may be requin~d to control flow vE!IClcities, including the initial storm runoff. 5-4 I I I I I I I I I I I I I I I I I I I