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<br /> <br />The following table establishes coefficients for <br />the rational formula. Rainfall intensity is <br />obtained from records of nearby weather stations, <br />reduced to a graph showing rainfall intensity vs. <br />rainfall duration for various recurrence <br />intervals. Rainfall intensity is based on <br />estimates of the acceptable frequency of <br />occurrence and the time required for water to <br />reach the outlet from the most remote point in the <br />basin. <br />Twee Of Drainage Area C <br />Concrete or Bituminous Pavement .8 - .9 <br />Gravel Roadways .4 - .6 <br />Bare Earth (high values for steep slope) .2 - .8 <br />Turf Meadow .1 - .4 <br />Cultivated Fields .2 - .4 <br />Forest .1 - .2 <br />II. Culverts shall be located at each natural draw or <br />watercourse as conditions warrant to prevent <br />excessive accumulation of flow in roadside <br />ditches or along toe of slope. Draws and <br />watercourses shall be cleared of debris for a <br />distance of 100 feet upstream from all culvert <br />inlets, where such clearing doesn't conflict with <br />the provisions of Section 5-22. Inverts at the <br />inlet shall be elevated slightly above the normal <br />flow line in steep or natural draws to avoid <br />plugging by debris. Inlets shall not be elevated <br />where ponding or backwater curves would cause <br />stagnation or obstruct irrigation ditches. <br />The culvert shall slope downward in the direction <br />of natural flow and be self-cleaning whenever <br />possible. The outlet shall not discharge on <br />unprotected fills or unstable material or at <br />adverse angles to streams or open channels. <br />Headwall, riprap, or other means of protection <br />shall be required at inlets or outlets where <br />erosion might occur. Velocities of flow in <br />culverts shall be calculated using acceptable <br />design charts or formulas. Where Manning's <br />Equation is used, the following "n" values shall <br />apply: <br />Material "n" <br />Corrugated Steep Pipe .027 <br />Reinforced Concrete Pipe .013 <br />Concrete (smooth-rough) .013 to .020 <br />Asphalt .016 <br />Page 5-5-25 <br />