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<br />Guide Por Approximate Zone A Areas <br /> <br />Developing BJ'Bs <br /> <br />Number of Cross Sections Required <br /> <br />If the determination of the BFE is for only one lot, one cross <br />section is required across the 100-year floodplain through the <br />property in question. For large parcels and multi-lot <br />subdivisions, at least one cross section is required at each <br />end of the parcel or subdivision. Additional cross sections <br />must be added if the difference in the computed lDO-year <br />water-surface elevations at the two cross sections is more <br />than one foot and the distance between the cross sections is <br />greater than 500 feet. <br /> <br />Proper Location of Cross sections <br /> <br />The following guidelines should be used to determine the <br />proper location for cross sections: <br /> <br />. Flow Path: Cross sections must be oriented perpendicular <br />to the anticipated flow path of the 10D-year flood, as <br />shown in Figure 16, "Cross Section Orientation." <br /> <br />Channel Characteristics: Cross sections should be <br />located where changes in channel characteristics, such as <br />slope, shape, and roughness, occur. <br /> <br />Discharqe: Cross sections should be located at points <br />along a stream where changes in flood discharge occur, <br />such as upstream of tributaries, as shown in Figure 17, <br />"Locate Cross Sections at Points of Flood Discharge <br />Changes." <br /> <br />Structures: A minimum of two cross sections are required <br />to compute a BFE at or near a structure, such as a bridge <br />or dam. If the floodplain configurations upstream and <br />downstream of the structure are similar, two cross <br />sections may be used. One cross section should represent <br />the structure profile including the profile of the road <br />or embankment. When obtaining the structure profile in <br />the field" measurements of the structure opening, if <br />there is one, and any piers should also be obtained. The <br />other cross section should represent the natural valley <br />cross section downstream of the structure and should not <br />include any part of the structure or embankment. The <br />natural valley cross section should be located at a <br />distance equal to the width of the structure opening, W, <br />measured from the downstream foot of the embankment or <br />wing walls, as shown in Figure 18, "Cross Section <br />Locations at Structures." <br /> <br />l <br /> <br />V-13 <br />