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<br /> <br />B,8 <br /> <br />Step Remarks Modifyin~ values <br />1 Soil materials indicate min1mWn basic n, 0.,02 <br /> <br />2 Description indicates mod~rate irregularity. 0.01 <br /> <br />3. <br />4. <br /> <br />Changes in size and shapeijudged insignificant, <br />I <br />No obstructions indicated. <br /> <br />0,00 <br />0,00 <br />0.08 <br /> <br />5, <br /> <br />Description indicates very high effect of <br />vegetation, <br /> <br />Reach described as straight. <br /> <br />0,00 <br /> <br />6, <br /> <br />Total estimated n 0.11 <br /> <br />USDA Technical Bulletin No, 129, Table .9, page 96, gives the fOllowing <br />determined values for n for this chaQnel; for average depth of 4.6 <br />feet n = 0.095; for average depth of'7,3 feet n = 0,104, <br /> <br />ExamPle 2. Effect of subdividin~ cross sectiuns. <br /> <br />The sole purpose of this example is to illustrate the effect of <br />subdividing sections on the value of 'n for the complete section, It is <br />not an illustration of hydraulic computations for determining water sur- <br />face profiles or stage-discharge rel~tionships, <br /> <br />This illustration is based on t~e following; <br /> <br />1. An actual stream cross section for which curves <br />showing depth versus area and depth versus hydraulic <br />radius for the channel and flood plain subdivisions <br />and for the complete section are plotted on Figure B.3. <br />Values of n are; for the chahnel subdivision 0.04; for <br />the flood plain subdivision 0,08, <br /> <br />2. The conditions of uniform, ,s!teady flow are assumed. <br /> <br />Manning's formula is handled! in accordance with Leach's <br />method, See Handbook of Hydraulics, McGraw-Hill Book <br />Company, 3rd edition, page 534; 4th edition, page 8-65, <br /> <br />Notation; <br />Q = discharge - cfs <br />a = cross section area - ft.~ <br />r = hydraulic radius, ft. <br />p = wetted perimeter, ft. <br />so= channel slope, ft. per ft, <br />n = roughness coefficient <br />