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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />Construction of Flood-Control Structures <br /> <br />Some flood-control structures (e.g., levees, diversion channels) are designed to protect certain <br />areas from inundation or otherwise reduce flood elevations.. Therefore, the Mapping Partner <br />performing the Mapping Needs Assessment shall evaluate carefully any flood-control structures <br />constructed since the hydraulic analyses were performed to determine whether they have a <br />significant effect on the floodplain boundary delineation and/or flood elevations. Levee systems <br />and diversion channels are typically, but not always, built by the USACE. The agency or <br />organization that built and/or administers the structure should have information about that <br />structure. Specific procedures for evaluating and mapping levees are provided in Appendix H of <br />these Guidelines. <br /> <br />Assessina Factors That Affect Stillwater Analyses <br /> <br />The analyses of coastal flood hazards can be broadly categorized into two components: analyses <br />of the stillwater elevations and analyses of the effects of waves. When determining whether the <br />stillwater conditions in a coastal area require reevaluation, the Mapping Partner performing the <br />Mapping Needs Assessment shall consider (1) whether any major storm events have occurred <br />that may provide data; and (2) increased length of tide gage record. Each of these factors is <br />discussed below. <br /> <br />Occurrence of Major Storm Events <br /> <br />Surveys of high-water marks taken from the insides of structures can provide data on stillwater <br />elevations for comparison to the stillwater elevations shown in the FrS report. An indicator of <br />map update needs is when a relatively minor storm event causes stillwater elevations well above <br />those in the published PIS report. <br /> <br />Increased Length of Tide Gage Record <br /> <br />An increase in the length of a tidal gage record may also affect the stillwater elevation estimate. <br />If the effective stillwater elevation was estimated by conducting a frequency analysis of a <br />relatively short record of tidal gage data, the stillwater elevation estimate may be sensitive to <br />newly added data. If tidal gage data with a relatively long record were used in the effective <br />analyses, however, a few additional years usually will not cause significant changes in the <br />stillwater elevation estimation, unless a large-magnitude event occurred since the analyses were <br />conducted. <br /> <br />Assessina Factors That Affect Wave Heiaht Analyses <br /> <br />The second broad category of analyses to be considered in coastal areas is the analyses of wave <br />heights, which include the effects of erosion. When determining whether the stillwater <br />conditions in a coastal area require new analyses, the Mapping Partner performing the Mapping <br />Needs Assessment shall consider the following factors: <br /> <br />. When the previous wave height analyses were conducted (if they were included in the <br />previous analysis); <br /> <br />1-20 <br /> <br />Section ],2 <br />