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4 TECHNIQUE8 OF WATER-RESOURCES INVE8TIGATIONS fall over riflIes and boulders. The Manning equation is not applicable when free fall exists. However, free fall mayor may not be indicated by th8 high.water profiles or by inspection of the reach. Cross sections may be located to eliminate any part of a reach in which free fall is indicated; but when the reach includes stretches in which free fall might have occurred, the reliability of the computed discharge will be low. Channel bends often govern the length of a suitable reach. The influence of the bend on velocity distribution, slope, and water-sur- face elevations continues some distance down- stream from the bend. If a straight reach away from the influence of bends cannot be found, it is best to choose a long reach that includes one or more channel bends with ter- minal sections in straight portions of the channel. The reach should be long enough to develop a fall which is well beyond the range of error due to alternate interpretations of th8 high- water profile, or to uncertainties regarding the computation of velocity head. In general, the accuracy of a slope.area measurement will improve as the length of the reach is increased. However, the length of a desirabl8 reach is often governed by the geometry of the channel and the practical difficulties of surveying long reaches of river channel. One or more of the following criteria should be met, if possible, in selecting the length of a slope-area reach: 1. The length of the reach should be equal to or greater than 75 times the' mean depth in the channel. 2. The fall in the reach should be equal to or greater than the velocity head. 3. The fall in the reach should be equal to or greater than 0.50 foot. Selection of Cross Sections Cross sections represent samples of the geom. etry of the reach; thus, th8 accuracy of the measuremen t will to some extent depend on the numboc of sections tak,en. A minimum of three cross sections is recommneded. Criteria for location of cross sections are given in chapter Al by Benson and Dalrymple (1967). Computations e! In general, perform the computations as described by Benson ana: Dalrymple (1967) beginning on page 24. Specifically for slop.... area measurements the following considerations apply. Fall To compute the fall, Mo, average the eleva- tions on both banks at each cross section. Show the computations for fall on the profile sheet. There may be occasions when high- water marks show that a ridge in the middle of a stream divides the flow, so that different water.surface elevations are in effect for both banks, or a raised shelf may maintain overbank flow at a higher elevation for some distance. Under such conditions the fall may be obtained by weighting the separate falls in accordance with the conveyance in each portion. Length of reach For a reach in a straight or nearly straight channel, compute the length from the stationing of the ends of the cross section or scale the length from the plan. If the channel is curving and has nearly uniform d8pths, measure the length on the curved line along the center of the channel. If the main channel lies closer to the' outside of the bend, use the length along the center of the deep channel. The centroid of conveyance may be computed for each cross section and a line drawn along its approximate position between the sections. In some places a meandering main channel lies within a fairly straight flood plain. If the water is entirely within the main channel, use the main channel length, If the flood plain also carries water, weight the curving length along the main chan- nel and the shorter length along the flood plain in proportion to the approximate amount of water flowing in each portion. Show computa- tions of length for the individual subreaches on the profile sheet. e '- I Discharge Compute the conveyance, the velocity-head coefficient '" for each cross section, and the e