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<br />. <br /> <br />Memorandum/Issues/Rules <br />September 22. 1987 <br /> <br />In addition to ease of interpretation by the map users, the <br />drawing of flood contours provides the person preparing the <br />map with an efficient, systematic method for determining <br />the limits of the flood boundaries. This method is called <br />the "flood contour material" and it is preferred by many <br />engineers over the old "top width method". <br /> <br />Drawing flood boundaries using flood contours has many <br />advantages over the practice of drawing flood boundaries by <br />interpolating between end points of top width computed at <br />each cross section. The most important advantage is the <br />degree of confidence gained in the interpretation of the <br />100-year flood boundary by having many more plotting <br />points. Often. both large and small inlands. which would <br />previously have gone unnoticed. became apparent. Potential <br />erosion problems from high velocity flow will be indicated <br />where flood contours are spaced close together. The flood <br />contours become permanent documentation of the engineer's <br />judgment in interpreting flood hazards. By showing flood <br />contours in plan view. a local administrator can quickly <br />determine flood elevations and visualize flood depths <br />directly from the maps. <br /> <br />. <br /> <br />FEMA requires "base flood elevations" to be shown on their <br />work maps to "reconstruct the 100-year elevations shown in <br />the flood profile to an accuracy of +0.5 foot." <br /> <br />Drawing flood contours. the location of which have been <br />determined from the profiles relative to a common reference <br />line shown on the profiles and the maps. provides for <br />internal consistency in the document. The internal <br />consistency of a regulatory document is important in order <br />to maintain a credible. legally enforceable floodplain <br />management program. Discrepancies between the vertical <br />(profile) and horizontal (flooded area map) projections of <br />the delineated floodplain can be fatal to any study. <br /> <br />The rules are silent on the following hydraulic issues: <br /> <br />a. starting elevations for computer input <br />b. definition of floodways or high hazard areas <br />c. compaction standards for levees <br /> <br />. <br /> <br />Under PUblished Report and Map. proposed Rule 5.D.2. <br />Published Map Scale. requires a minimum published map <br />scale of 1 inch equals 1.000 feet. although a much larger <br />scale if preferred. This lower standard is a concession to <br />FEMA at this point in time in order to avoid controversy. <br />FEMA has declared "maps at a scale of 1 inch equals 1,000 <br />feet have proved to be sufficient for supporting floodplain <br />regulation in many urban areas in the United States". <br /> <br />-9- <br />