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<br />A discussion of the logic sequence is provided to assist the user in in- <br />terpreting the solutions he obtains. <br />The first step in the special bridge routine is to assume low flow <br />conditions and estimate the water surface elevation on the other side of <br />the bridge. How that estimate is made depends on whether the bridge has <br />piers. If there are bridge piers, the program goes through the momentum <br />equations to determine class of flow and water surface elevation. With- <br />out piers, the program temporarily assumes the water depth is the same <br />on both sides of the bridge. <br />The program then checks for weir flow by comparing the estimated <br />water surface elevation to the minimum top of road elevation (ElTRD). <br />If it is possible that weir flow exists, the program estimates an energy <br />elevation based on the velocity head at the previous section. <br />The program then compares the estimated low flow energy elevation <br />to the maximum elevation of the bridge low chord (EllC). If the low <br />flow energy elevation (EGlWC) is greater than the low chord elevation <br />(ELLC) the program will calculate an energy elevation assuming pressure <br />flow (EGPRS). If the low flow energy elevation is less than ELLC, the <br />program concludes that low flow controls and checks again to determine <br />if weir flow exists. If there is weir flow, the program will check <br />for piers. With piers, a trial and error solution will be made for <br />low flow (by the Yarnell equation) and weir flow (by the weir equation). <br />Without piers, the normal bridge solution (standard step calculation with <br />adjustments in area and wetted perimeter) will be used to compute the <br /> <br />12 <br />