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<br />for sections 2 and 3 to eliminate the portion of the overbank area <br />considered ineffective even under weir flow conditions. <br />Cross section 3, just upstream from the bridge, is usually coded <br />in the same manner as section 2. In many cases the cross sections are <br />identical. The only difference generally is the elevation to use for <br />the effective area option. For the upstream section, the elevation <br />would be the low point of the top-of-road (ELTRD). <br />Using the effective area option in the manner just described for <br />the two sections on either side of the bridge provides for a constricted <br />section when all of the flow is gOing under the bridge. When the water <br />surface is higher than the control elevations used, the entire cross sec- <br />tion is used. The program user should check the computed solutions on <br />either side of the bridge section to insure they are consistent with <br />the type of flow. That is, for low flow or pressure flow solutions, the <br />printout should show the effective area restriwted to the main channel. <br />When the bridge routine indicates weir flow, the solution should show <br />that the entire cross section is effective. <br /> <br />Selection of Methods <br /> <br />When selecting the method of computing the water surface profile <br />through a bridge, there are three basic choices: (1) determine the <br />change in water surface elevation or the water surface elevation by <br />an "external" technique and input the results into the program, (2) cal- <br /> <br />25 <br />