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<br />important. Overbank area near the bridge, which does not convey <br />flow until the embankment is overtopped. suddenly becomes effective <br />as the embankment is overtopped. Since one data set is used to ana- <br />lyze a wide range of discharges, it should be formulated in such a <br />way to permit this sudden change in ineffective flow area to occur. <br />The above discussion concentrated on only One type of bridge and <br />flow situation. All flow passed through a rather narrow opening and <br />then expanded to reoccupy a wide flood plain width. Many other bridge <br />crossing configurations and flow conditions are encountered. In <br />locating cross sections, it is important to understand what causes <br />the energy loss and to model that aspect of the problem. For ex- <br />ample. a bridge that spans the entire channel and flood plain and <br />has only a few piers does not cause a significant head loss. For this <br />type of bridge, special consideration in locating cross sections is <br />not required. This is the opposite extreme in geometric configura- <br />tion to the first case described. Bridge crossings that have the <br />highway approaches located down on the level of the flood plain such <br />that they cause little or no obstruction during flood flows represent <br />another case where cross section location is not extremely critical. <br />It is primarily the expanding flow, illustrated in the first case, <br />which causes the energy loss at bridges. Other losses that contribute <br />are form loss from bridge piers, the contracting flow, and friction <br />loss through the contracted reach. Often. flow. through a single <br />bridge crossing will exhibit all of the above types of behavior as <br />the discharge increases from a low in-channel flow to a very high <br />flow. In this case a model that is adequate for the case of extreme <br /> <br />4.06 <br />