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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />conducted. For example, changes in the operating flood stages of a reservoir may affect how <br />floodwaters are routed through the reservoir. Any changes in operating procedures may affect <br />how the structure is considered in future mapping efforts. <br /> <br />AssessinQ Factors That Affect Hvdraulic Analvses <br /> <br />Another primary component in riverine flooding analyses is the hydraulic analysis. The <br />methodology for hydraulic analyses is discussed in Section 3.0 of the FIS report, detailed in <br />Appendix J of these Guidelines. Floodplain conditions can change that would affect these <br />analyses. The Mapping Partner performing the Mapping Needs Assessment shall consider the <br />following factors that affect the hydraulic conditions in evaluating the community's flood data <br />update needs: <br /> <br />. New bridges and culverts; <br /> <br />. Changes in stream morphology; and <br /> <br />. Construction of flood-control structures. <br /> <br />These factors are discussed in more detail below. <br /> <br />New Bridges and Cffl-verts <br /> <br />If a discharge exceeds the capacity of a bridge opening or culvert, floodwaters can back up, <br />thereby increasing flood levels upstream. Although most bridge openings and culverts are <br />designed to allow streamflows associated with frequent storm events to pass without such <br />backwater effects, they may not be designed to carry the I-percent-annual-chance flood <br />discharge. Therefore, the Mapping Partner performing the Mapping Needs Assessment shall <br />evaluate any bridges or culverts that have been constructed since the effective FIRM and FIS <br />report were completed to determine the potential effect of the bridges and culverts on the 1- <br />percent-annual-chance flood and the associated regulatory floodway. The Mapping Partner may <br />obtain information regarding the date of construction a.nd other details of roads, bridges, and <br />culverts from the state Department of Transportation or local public works departments. <br /> <br />Changes in Stream Morphology <br /> <br />Any significant change in the stream channel or floodpJlain geometry, particularly regrading or <br />the placement of fill, can affect the I-percent-annual-chance floodplain and the associated <br />regulatory floodway. Another consideration is any change in the stream location, either through <br />natural processes (e.g., stream migration, erosion, or deposition) or through manmade changes <br />(e.g., channelization, stream widening, stream straightening, or dredging). Additionally, any <br />significant change in the vegetation or structural encroachments in the floodplain may affect a <br />stream's hydraulic characteristics. Aerial photographs are useful tools in evaluating changes in <br />stream channels and floodplains. The Mapping Partner performing the Mapping Needs <br />Assessment shall evaluate all of these factors that may result in changes in stream morphology. <br /> <br />1-19 <br /> <br />Section 1.2 <br />