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<br />APPLICATIONS AND LIMITATIONS OF HEC-2 FOR SUPERCRITlCAL FLOWS <br /> <br /> <br />. <br /> <br />A3 type curves are controlled from upstream and can also be analyzed by HEC-2 as <br />supercritical flow profiles. The shapes and possible occurrences of these curves are <br />illustrated in Figure (1J. <br /> <br />The main assumptions used to develop the step-backwater model of HEC-2 are: (1) the <br />flow is steady and gradually varied; (2) the stream bed slope is less than 1 to 10; and (3) <br />the channel boundary is rigid. Although these assumption apply to most subscritical <br />flow profiles of rigid-boundary channels, they do not apply well to supercritical flow <br />profiles of natural channels, for the following reasons: <br /> <br />(1) The stability of supercritical flow in natural channels is not predictable. In the <br />proximity of critical depth, a relatively large change of depth may occur as the result of <br />a very small variation in specific energy, as, is shown in Figure (2). Flow in this region <br />is unstable and excessive wave action or undulations of the water surface may occur. <br />This instability cannot be simulated by the HEC-2 program. <br /> <br />(2) Another type of supercritlcal flow instability occurs at high Froude numbers that <br />. are larger than two. Formation of pulsating rapid flow, or "slugs," is observed on steep <br />slopes having shallow flow depths. The HEC-2 program is not capabie of detecting or <br />simulating this instability. <br /> <br />(3) Flow around channel bends is characterized by a rise in water surface elevation on <br />the outside bank and a depression of the water surface along the inside bank. This <br />elevation increase, referred to as superelevation, 'is attributed to the imbalance <br />between outward centrifugal and gravitational forces. Another problem characteristic <br />of channels carrying rapid flows is the generation of standing waves in simple curves. <br />These waves affect the curved-flow region and extend long distance downstream. The <br />total rise in water surface elevation for rapid flow has been found to be about twice <br />that for tranquil flow. Although superelevation cannot be simulated by HEC-2, it can <br />be estimated manually by the following equation: <br /> <br />. <br />