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4 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS ~ " ...... " " " ----yO " " Yn " " '-- --Yc MILDER SLOPE Figure 3.-Backwater-curve transition for mild slope to milder slope. ~ ~ ~--- - ~Yn LOW" --_ - - -- MILD SLOPE HIGH: -Yo Figure 4.-Backwater-curve transition from small n to large n on mild slope. ~Yn .......... - - yo Figure 5.-Backwaterwcurve transition for mild slope to steeper mild slope. ~YO .... ..... --:/;,,----~v f'l" _ __ ,n e MILD SLOPE - LOW n - -Yo Figure 6.-8ackwater-curve transition from large n to small n on mild slope. normal depth lower than the critical depth. The water-surface profile must, in the transi- tion between the two normal-depth lines, pass through critical depth by means of M2 and S2 curves. Such a transition is known as a draw- down curve or a hydraulic drop. The transition from a steep slope (y, < y,) to a mild slope (y" > y,) in figure 8 requires a hydraulic jump in order for the water surface to pass through critical depth. The location of the jump depends on the relative elevations between the upstream and downstream nor- mal depths. Associated with the hydraulic jump could be a short segment of an Sl or an M3 curve, which is not ordinarily of concern because of its proximity to the jump. - Determination of normal depth Average prOfile In a long reach The simple transitions described in figures 3-8 are only a few of many possible combina- tions that could result from changes in channel yo --- Weter surface p88888 through critical depth MILO SLOPE Figure 7.-8ackwater-curve transition for mild slope to steep slope. e'