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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />Froude Number of 1.0 is approached. There are natural processes at work that <br />limit the time to peak of a unit hydrograph as the drainageway becomes <br />steeper. To account for this phenomena, it is recommended that the slope used <br />in Equation 4-1 for natural drainageways and existing man made grass-lined <br />channels be adjusted using Figure 4-1. <br />Grass Lined UDFcD Criteria Channels: <br />Grass lined channels designed and built using Urban Drainage and Flood <br />Control District criteria, have a slope that limits maximum control <br />velocities. A typical range in longitudinal slopes for such channels is 0.004 <br />ft/ft to 0.007 ft/ft. It is recommended that for prel iminary estimating <br />purposes a longitudinal slope of 0.005 ft/ft be used for grass lined channels <br />that are to be designed using UOFCD criteria. <br />Riprap Lined UDFCD Criteria Channels: <br />The UDFCD criteria also limits the Froude Number to 0.8 for riprap lined <br />channel s. For this reason it is suggested that for prel iminary estimating <br />purposes where riprap channels are contemplated, a longitudinal slope of 0.10 <br />be used with Equation 4-1. When a riprap channel is in existence, use the <br />measured average channel profile slope. <br />Concrete Channels and Storm Sewers: <br />In concrete lined channels and buried conduits, the velocities can be <br />very high. For this reason, it is recommended the average ground slope (i.e. <br />not flow line slope) be used where concrete lined channels and/or storm sewers <br />dominate the basin drainageways. There is no upper limit recommended to the <br />slope for such basins. <br />Weighted Watershed Slope: <br />Where the flow line slope varies along the channel, calculate a weighted <br />basin slope for use with Equation 4-1. Do this by first segmenting the major <br />drainageway into reaches having similar longitudinal slopes. Then calculate <br />the weighted slope using Equation 4-5. <br />4.17 <br />[L S 0.24+L S 0.24+ +L S 0.2~ <br />11 22 .... nn J <br /> <br />S = <br /> <br />(4-5) <br /> <br />L1+L2+L3+....Ln <br /> <br />In Which, S = weighted basin waterway slope in ft/ft <br />Sl'S2'....Sn = slopes of individual reaches in ft/ft <br />(after adjustments using Figure 4-1) <br />L1,S2'....Ln = lengths of corresponding reaches. <br /> <br />5-1~84 <br /> <br />. <br /> <br />. <br /> <br />. <br />