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<br />assigned during the survey. Roughness coefficients should be defined for the channel, <br />right overbank and left overbank subsections. <br /> <br />5. COMPUTATION OF DISCHARGE. <br /> <br />Once the notes have been refined into three dimensional data, the computation <br />process can begin. The process described will be for a three section slope-area. <br />Reaches having more than 3 cross sections can be worked similarly, but will require that <br />the steps illustrated be repeated at a greater level of difficulty. The computation of peak <br />discharge by the slope-area method requires the solution of some complex equations. <br />Forms have been prepared by the U.S.G.S to organize and simplify the process(1). <br /> <br />5.1. COMPUTE THE REACH FALL BETWEEN CROSS SECTIONS. - This is the <br />difference in the total head between the sections. This can be done by averaging the <br />high water mark elevations of the ends of each cross section and calculating the <br />difference between the two section averages. <br /> <br />5.2. COMPUTE THE REACH LENGTH BETWEEN CROSS SECTIONS. - When <br />the reach is nearly straight, this is simply the distance between the cross sections as <br />scaled off of the plan view that was prepared when refining the field notes. If the channel <br />curves significantly, measure the distance along the center of the greatest flow at peak <br />stage. In most cases, this will be along the center of the deepest part of the main <br />channel. When there is significant flood plain flow on one bank, give weight to this flow <br />by offsetting the center of the greatest flow towards that bank. <br /> <br />5.3 COMPUTE THE MANNING'S PROPERTIES FROM CROSS SECTIONS. <br /> <br />5.3.1 COMPUTE THE CROSS SECTION AREAS AND WETTED <br />PERIMETERS. - The easiest method is to digitize or planimeter the cross sections that <br />were plotted from the survey notes. <br /> <br />5.3.2 COMPUTE THE HYDRAULIC RADII FOR EACH CROSS SECTION. <br /> <br />r=a/p . <br /> <br />(1) <br /> <br />Where: r= hydraulic radius of the cross section. <br />a= area of cross section. <br />p= wetted perimeter of cross section. <br /> <br />When there is overbank flow, compute a hydraulic radius for each part of the total <br />cross section (left overbank, channel, right overbank). <br /> <br />5.4 COMPUTE THE DISCHARGE. - Discharge may be solved for directly by use <br /> <br />1-7 <br />