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<br />Guidelines and Specificationsfor Flood Hazard Mapping Partners [April 2003] <br /> <br />important to verify that the new topographic data source is superior to the existing data and that <br />no changes in the hydraulic characteristics of the floodplain indicate that the existing study is no <br />longer appropriate. <br /> <br />Redeffneaffon--Coastal <br /> <br />This data update method involves no new analyses. This method combines effective information <br />from the FIRM and FIS report and the supporting analyses with new, more detailed, or more up- <br />to-date topographic data to redelineate coastal high hazard areas (V zones). <br /> <br />IdentifvinQ TOPoQraphic Data Needs and Sources <br /> <br />Topographic data are required for three of the methods of updating flood data: detailed study, <br />approximate study, and redelineation. Detailed studies require topographic mapping of <br />floodplain areas and surveys of bathymetry and structures. Approximate studies mayor may not <br />require bathymetry or structures. Redelineations require only topographic mapping of floodplain <br />areas. <br /> <br />Significant cost savings can be realized if existing topographic data sources are used because 50 <br />percent of the cost of a map update may be to acquire new topographic data. Possible sources of <br />existing topographic data include local planning departments, GIS coordinators Engineers, and <br />drrectors ot publ1c works, 1< EMA archIves (partIcularly tor cross-sectIOn data from etlectlve <br />hydrologic and hydraulic models); and State Departments of Transportation (e,g., bridge plans). <br />The Available Data Inventory template provided in Appendix I, Subsection 1.1.6 of these <br />Guidelines should be used to summarize the existing topographic data that may be available to <br />the Project Team. <br /> <br />Detailed specifications for topographic data and field surveys are contained in Appendix A of <br />these Guidelines. In evaluating the suitability of existing topographic data, the Project <br />Management Team shall consider the following factors: <br /> <br />Contour Map?ing or Digital Elevation Models <br /> <br />. Contour interval- should be 4 feet or less (2 feet in flat terrain). <br /> <br />. Currency of data-whether significant changes (e.g., highways, subdivisions, and <br />mining) have occurred since the data were developed. It may be possible to update only <br />"pockets" of the data. If a question about the currency of the data exists, "spot checks" <br />should be performed to verify the accuracy. <br /> <br />Bathymetric and Bridge/Structure Cross Sections Effective Study or Other Source <br /> <br />. Currency of data-Whether significant changes (e.g., new bridges, culverts, <br />geomorphologic changes) have occurred since the data were developed. If there is a <br />question about the currency of the data exists, "spot checks" should be performed to <br />verify the accuracy. <br /> <br />. Density of cross sections-whether an adequate number are located in the project area. <br /> <br />1-40 <br /> <br />Section 1.3 <br />