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<br />HYDRAULICS OF STEEP-GRADIENT STREAMS <br /> <br />1 <br />By Robert D. Jarrett <br /> <br />INTRODUCTION <br /> <br />Hydraul ic calculations of the flow in channels and over-bank areas <br /> <br />of flood plains require an evaluation of roughness characteristics. <br /> <br />Most commonly, the Manning roughness coefficient n is used to describe <br /> <br />the flow resistance or relative roughness of a channel or over-bank <br /> <br />areas. The n appears in the general Manning equation for open-channel <br /> <br />flow, which has the following form: <br /> <br />1.49 R2/3S1/2 <br />V = <br /> <br />n <br /> <br />(1) <br /> <br />where: <br /> <br />Q = AV <br /> <br />(2) <br /> <br /> <br />V is the average cross-section velocity, in feet per second; <br />R is the hydraulic radius, in feet; <br />S is the energy gradient or friction slope; and <br />n is Manning's roughness coefficient. I <br /> <br />The Manning equation is often substituted into the continuity equation. which <br /> <br />has the following form: <br /> <br />where: <br /> <br />Q is the discharge, in cubic feet per second; and <br /> <br />A is the cross-sectional area, in square feet. <br /> <br />I <br />Hydrologist, U.S. Geological Survey, Lakewood, Colo. <br />