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<br />culate the energy loss based on friction using the standard step method <br />- normal bridge routine, or (3) calculate the energy loss by previously <br />discussed formulas of the special bridge routine. Each method should <br />be considered and the following discussion provides some basic guidelines. <br />Input Losses. The following are examples of when a change or known <br />water surface elevation might be read into the program: <br />1. If a structure is a control and a rating curve is available, <br />reading in the known water surface elevation is the easiest and surest <br />way to establish proper water surface elevations. <br />2. The use of observed data to estimate losses through a bridge <br />can also be an expeditious method of establishing the losses. <br />3. An alternate computation technique can be used such as the <br /> <br />Bureau of Public Roads procedure (reference j) for determining the Joss <br />for low flow conditions. The calculated loss can then be read in. Care <br />must be taken to insure the loss calculated by alternate methods are <br />properly used in the program. For example, the Bureau's technique pro- <br />vides the increase in water surface elevation above the normal water <br />surface elevation without the bridge. Therefore, it includes the effects <br />of contraction and expansion losses and the loss caused by the structure, <br />but it does not reflect the normal friction loss that would occur with- <br />out the bridge. <br />Normal Bridge Routine. The use of the standard step method for <br />computing losses is most applicable when friction losses are the pre- <br />dominate consideration. The following examples are some typical cases <br /> <br />26 <br />