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2 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS I I I I I I ~ I ." >1f I I I I I 4 :__~ 5 ' , '- 6 EXPLANATION Observation points Distance, in feet, from the initial point to the observation poi nt Depth 01 water, in leel. at the observation point Boundary of partial sections; one heavily outlined discussed in text Figure 1.-Definition sketch of midsection method of computing cross-section area for discharge measurements. bn b(n 1) Initial point b6 b5 b. b2 'fl b, .,,~ 3 1,2,3." n b" b2.b" . . . .bn d1. dz.d3. . ,dn Dashed lines Thus, for example, the discharge through partial section 4 (heavily outlined in fig. 1) is [b,-b'Ja 1l,=V, ~ ,. The procedure is similar when x is at an end section. The "preceding location" at the be- ginning of the cross section is considered coinci- dent with location 1; the "next location" at the end of the cross section is considered coincident with location n. Thus, [b,-b'Jd 1l,=V, ~ " and - [b.-b{,oIlJd q.-v. 2 "'. ( Water surface ~ I .,," oS ." n (n-l) For the example shown in figure 1, Il, is zero because the depth at observation point 1 is zero. However, when the cross-section boundary is a vertical line at the edge of the water as at location n, the depth is not zero and velocity at the end section mayor may not be zero. The formula for Il. or Il. is used whenever there is water only on one side of an observation point such as at piers, abutments, and islands. It usually is necessary to estimate the velocity at an end section as some percentage of the adja- cent section because it normally is impossible to measure the velocity accurately with the current meter close to a boundary, There also