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<br />Guide Por Approximate Zone A Areas <br /> <br />Developing BPEs <br /> <br />Hydraulic structures <br /> <br />As stated earlier, normal depth is the depth expected for a <br />stream when the flow is uniform, steady, one-dimensional, and <br />is not affected by downstream obstructions or flow changes. <br />However, there are s~tuations in which a physical structure <br />located downstream bf a particular site will cause an <br />obstruction or alteration of the flow, resulting in a flood <br />depth at the site higher than the normal depth. The <br />discussion below describes the appropriate methods for <br />determining BFEs for reaches that include hydraulic <br />structures. <br /> <br />Hydraulic structures that are common in approximate Zone A <br />areas include road and railroad crossings, including <br />embankments, dams, bridges and culverts, and canal crossings. <br />The flow over the road, railroad, embankment, dam or canal can <br />be described as weir flow. Weir flow can be calculated by hand <br />or by computer program in order to determine the BFE. When <br />flow passes through a bridge or culvert, the BFE can be <br />determined through the use of nomographs or computer programs. <br />The BFE at a structure where flow travels through a bridge or <br />culvert and over the crossinq can be determined by nomographs, <br />but is more easily determined with a computer program. <br /> <br />Weir Flow <br /> <br />Determination of the water-surface elevation for weir flow <br />requires at least two cross sections. The first cross section <br />represents the natural valley section downstream of the <br />structure, and the second cross section represents the road <br />profile and the opening of the structure (refer to Figure 18, <br />"Cross section Locations at structures." If the approach <br />velocity head is to be considered, then a third cross section <br />is required that represents the natural valley section <br />upstream of the structure. In most situations, however, the <br />velocity head can be assumed to be negligible, and a third <br />cross section is not necessary. <br /> <br />The water-surface elevation downstream of the structure should <br />be determined by using normal depth computations at the first <br />cross section, provided there are no structures further <br />downstream that can create backwater effects (refer to the <br />methods for determining normal depth described previously) . <br /> <br />The second cross section, which represents the profile along <br />the top of the structure including the road or the embankment, <br />should be used to determine the weir length for use in the <br />equation for weir flow, as shown on the following page. <br /> <br />V-29 <br />