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<br />I <br />. <br />---~ <br /> <br />, <br />_ i <br />----0 <br /> <br />_J <br />-, <br /> <br />When the water surface profile calculatlons are to be made with <br /> <br />an electronic computer, it i.s most convenient to calculate geometric <br /> <br />properties directly from cross section coordinates as each new trial <br />water surface is analyzed. A less desirable approach is to interpolate <br />values from a table because of the necessity of generating new tables <br />for each change in cross section shape. The least desirable approach <br />is to curve fit geometric properties versus elevation because dis- <br /> <br />continuities are common. <br /> <br />For example, the area-elevation curve for the subsection in fig. <br />5.03 would be smooth because flow is shown in areas al. a2. a3, and <br />a4 when the WS is lower than YC in the main channel. However, cases <br />frequently occur where no flow can enter those areas until the WS <br />becomes higher than YC. When this occurs, the resulting area-elevation <br />curve would be ~dified as shown in fig. 5.04. From YA to YB all <br />area is usable and a smooth curve results. Between YB and YC the <br /> <br />amount of usable area is determined by whether water can physically <br /> <br /> <br />enter all areas before the WS exceeds YC. If water can enter areas <br /> <br />al, a2. a3, and a4 the curve is smooth. but its slope changes at <br />point YB. However. if flow cannot get into areas al. a2. a3, and <br />a4 until WS exceeds YC. a discontinuity results at WS = YC. This is <br />difficult to account for in prograumi ng the computer to util i ze curves. <br />However, when the water surface profiles are to be calculated <br />manually, curves of geometric properties are most desirable. These <br />are defined by selecting elevations at the major break points in the <br /> <br />5.09 <br />