Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods Extensive property damage resulted from the event. Roofs were blown off of houses in downslope areas and downed power lines created widespread power outages. Estimates compiled from local authorities placed the damage cost for the event in the area of $3.4 million. 10.3 Probability of Occurrence The recurrence interval of a flood, or flood frequency, is the average time interval within which a flood of a given magnitude will be equaled or exceeded. Flood frequencies can be determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size may occur, or gathering hydrologic and hydraulic data from streams and calculating probabilities through models. The FIRM maps identify a flood hazard area as the area that would be inundated by a 100-year flood, or a flood with a 1% chance of occurring annually. The 100-year flood, also referred to as the base flood, is a national standard adopted by the NFIP that represents a compromise between minor floods and the greatest flood likely to occur in a given area (see Figure 10-4). The FIRM maps delineate the 100-year flood zones for rainfall flooding, coastal flooding, shallow flooding, and distinguish areas where detailed studies have been conducted to determine base flood elevations. Special Flood Hazard Area, ti - —- --(1oo-Year Floodplain) Flood Fringe Floodway Flood Fringe Base Flood Elevation Not rnal Water Level --Sttpm Channel Special Flood Hazard Area is the area that has a 1%chance of being flooded in any given year(100-year floodplain). The 100-year flood is also referred to as the base flood. Floodway is the stream channel and that portion of the adjacent flood plain that must remain open to permit passage of the base flood with- out raising the water surface elevation by more than one foot. Flood Fringe is the area within the 100-year floodplain other than the floodway. Base Flood Elevation(BFE)is the elevation of the water surface resulting from a 100-year flood in reference to a defined datum. Figure 10-4. Flood Insurance Rate Map Terminology Data compiled over the last 50 years indicate that, on average, a damaging flood event occurs on the Big Island with a annual probability of 0.5%. 10-7 Hawaii County Multi-Hazard Mitigation Plan