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<br />. <br /> <br />. <br /> <br />. <br /> <br />(6) Flow changes from the supercritical to the subcritical regime usually occur in the <br />form of hydraulic jumps. Flow depths before and after the jump are less than and <br />greater than critical depth, respectively. HYdraulic jumps in a channel may occur either <br />in the vicinity where the channel slope decreases from steep to mild, or in the vicinity <br />of channel constrictions caused by abrupt, converging transitions or by bridge piers; by <br />increased channel roughness in the downstream reaches due to sediment, debris, and <br />vegetation accumulation caused by the high-velocity flows; and by confluencing with <br />another channel where supercritical flow occurs in the tributary and subcritical flow <br />occurs in the main channel. <br /> <br /> <br />Hydraulic jumps and sudden drops represent cases of suddenly varied flows that the <br />HEC-2 program is not capable of solving, due to the equations and assumptions inherent <br />in the development of the program. The standard-step backwater model that is based on <br />the one-dimensional energy equation will not allow profile computations to pass from <br />one flow regime to another. The subcritical profiles computed by the program are <br />limited to critical depth and above, while supercritical profiles are limited to critical <br />depth and below. The only exception occurs in the area of special bridge models, where <br />the momentum and continuity equations are used. In cases where the flow regime <br />changes, the flow profile should be computed twice, alternately assumIng subcrltical and <br />supercritical flow regimes. An analysis of the hydraulic jumps should be performed <br />separately because HEC-2 does not have the capability to estimate the location, length, <br />and energy losses of a hydraulic jump. The requirement for three separate operations is <br />a circumstance that is frequently overlooked by most analysts. <br /> <br />CASE STUDY AND HEC-2 ANALYSIS <br /> <br />The natural stream chosen for this study is the Leele Stream in American Samoa (see <br />Figure 3). This stream is characterized by a steep slope and a meanerlng flow pattern. <br />The 100-year flood discharge of 820 cfs flows at supercrltlcal velocities with a Froude <br />number ranging from 1.7 to 4.8. Both critical and supercrltlcal depths were calculated <br />using the HEC-2 program, and the resulting profiles are shown in Figure (4). Table (1) <br />shows the top widths corresponding to the critical flow profile, In addition to the top <br />widths, velocities, Froude numbers, and energy grade line for the supercritlcal flow <br />profile. Figure (5) represents cross-section A-A, chosen in the vicinity of a bend in <br />Leele Steam. The energy grade line and the three water surface depths illustrate that <br />