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I . . . . . I :. . I I I I I I I . I I I Advance Warning. The length of warning time available, based of flood forecasts such as provided by the National Weather Service, is important where flood-proofing measures require considerable time for implementation. Streams with small drainage areas and steep slopes are often subject to flash floods having short duration with high peak flows. Streams with larger drainage areas usually have a longer period of rise with more advance warning opportunities. A closure device requiring human interven- tion is not acceptable if the advance warning time is insufficient to permit implementation of a complete closure system. Soil Characteristics. The permeability of local soil is an important factor to consider in flood-proofing applications such as use of land fills for building elevations, back-filling around basement walls, and construction of protective berms; and in the determination of the need for flood protect- ive measures in Secondary Flood Hazard Areas. Soil conditions can vary widely over a relatively small area and this possibility should be thoroughly evaluated. SOURCES OF INFORMATION Flood Plain Maps. These may include aerial photos or orthophotos, or topographic, planimetric, or zoning maps on which are depicted the limits of various floods such as historic, lOO-year, or 500-year. The flood plain limit may have been established by aerial photo interpretation of an actual flood event and/or by plotting the runout line of the computed lOO-year or 500-year flood (or some other flood). Flood plain maps may be adopted by reference as part of the local zoning ordinance, or the information contained on the maps may be redrawn on the official zoning map. The provisions of the ordinance then apply to the mapped flood plain areas. As stated previously, local ordinances and accompanying flood plain zoning maps may divide the flood plain into floodway and flood fringe areas, and with separate regulations for each. These maps also determine the areas of application of flood-proofing regulations contained in the State Building Code. Water Surface Profiles. The line depicting the continuous 'plotting of the water levels along the centerline of the stream, as shown in Figure 4, is known as the water surface profile. The elevation of a given point on the profile determines the water surface elevation at right angles to the direction of flow from the centerline of the stream to the limits of the flooded area. The water surface profile can thus be used to determine flood depths at any point by com- paring the ground elevation at a site to the corresponding elevation of the stream profile. Water surface profiles are usually adopted by reference as part of zoning ordinances containing flood plain provisions. It is important to note that the profiles may not always correspond to the Regulatory Flood Datum (RFD). Where they do not, the RFD must be determined by the zoning administrator or the city engineer using available technical data. 17 ...