Multi-Hazard Mitigation Plan: 10. Floods

Home

Chapter 10:Hazard Analysis—Floods CHAPTER 10 - FLOODS 10.1 Description of Hazard Floods are temporary inundation of land from excessive rainfall or wave action. Because flooding causes millions of dollars of damage each year, the federal government created the National Flood Insurance Program (NFIP) to assist those who suffer from flood disasters. Under the NFIP, each county has mapped flood hazard areas and established a permit system to regulate development within these flood hazard areas. The Flood Insurance Rate Maps (FIRMs) include areas prone to rainfall flooding(A zones) and high waves (V zones). In this County, the permit system is set forth in Hawaii County Code Chapter 27. The NFTP man- dates federal insured banks to require purchasing of flood insurance as a condition for financ- ing the construction of buildings in flood plain areas, thereby shifting the primary burden for flood disaster relief to those who choose to live or conduct business in flood hazard areas.48 Although the NFIP has significantly mitigated flood damages, major flood problems exist in older areas developed prior to flood control regulations and building standards, in areas that are subject to flooding but not identified on the FIRMs, and areas with flood control improvements that are inadequate to contain or control larger floods by present standards. Direct economic losses from flooding result from soaking, dislocation and destruction of property as well as erosion and scouring from the velocity of the flow, and deposition of sediment and debris transported by the water. Dams can exacerbate flooding should they fail; hence, a dam safety program is also an integral part of flood control. In the past, flood control meant engineered structural solutions such as dams, levees, stream channelization, diversion channels, and other costly improvements. In spite of these improvements, flood damage continued to escalate. A broader approach to flood management that attempts to work with nature instead of trying to control the awesome forces of nature has evolved. The federal government's flood management program embraces these non-structural measures by encouraging regulatory measures that direct development away from flood prone areas (e.g., zoning and subdivision codes), requiring flood-proofing building standards where structures are located in flood prone areas (e.g., flood control and building codes), allowing federal disaster assistance funds to be used to buyout and restrict future development in flood-prone areas where such acquisition is more cost-effective than heightened structural flood control improvements, and allowing upgrades to damaged structural flood control improvements (e.g., bridges) to resize inadequate structures rather than strict"in kind"replacements. 10.1.1 Rainfall Flooding Four types of storms produce heavy rainfall: e Frontal storms. Frontal storms usually occur during the period from December through 48 For an overview of the National Flood Insurance Program in Hawaii,see State of Hawaii Department of Land and Natural Resources,National Flood Insurance Program in Hawaii,Circular C90, 1994. 10-1 Hawaii County Multi-Hazard Mitigation Plan