COM 0176.041 2006-2008 - Laserfiche WebLink

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FLOOD POTENTIAL (PRESENT CONDITIONS) Each of the 25 watercourses within the study area has the potential to produce sufficient runoff to result in floodwater damages to urban and agricultural Lands. These damages have occurred almost annually from one or more of the watercourses. Flooding generally lasts only a few hours, but is long enough to cause water damages. A large storm such as the 100-year event would result in substantial floodwater damage to developed areas adjacent to the Mamalahoa Highway. The 100-year event was used as a reference flood for analyses of the flood hazards of the area and should be the basis for implementing flood plain and land use zoning ordinances. The results from the hydrologic and hydraulic studies provide elevation-discharge data for the 10-year, 50-year, 100-year, and 500-year flood events at each valley cross section (see Table 1). The elevation-discharge data for the 100-year and 500-year flood events were plotted on the topographic map at each section and the flood plain delineation was interpolated between cross sections. The flood pisin for each watercourse is shown on the flood hazard maps, plates 1 through 25. The flood hazard maps are located on the Index ` Map of the study area. In the reaches studied the flood plains ranged in width from 20 feet to 1,500 feet, depending on the topography (see maps). Table 2 lists the area flooded by the 100-year and 500-year flood event for each watercourse. This variation in flood plain width resulted in depth of flow variations for a given discharge. As shown on the maps, little difference in flood plain width is evident between the 100-year and 500-year flood events in many of the valley cross sections. Also, as a result of wide flood plains, at some valley cross sections small variations in depth occurred between different flood events. This small depth variation, combined with the steep profiles, made plotting profiles of more than one flood event impracticable. Therefore, only the 100-year flood event was selected to describe the water surface profiles. (See Appendices.) Typical valley cross sections are included in the appendices. The elevation-discharge data in Table 1, the water surface profiles in the Exhibits and the flood hazard maps (Plates 1 to 25) may be used to determine flood hazard potential at a specific location. The water surface elevation (Table I) determined at each specific valley section may be used directly in field evaluations of current flood potentials at that location. At points between cross sections, the water surface elevations can be estimated by examining the depth of flow at the upstream and downstream section and interpolating the depth that can be added to the channel bottom elevation (from flood hazard maps) at the point in question. -9- 4