Multi-Hazard Mitigation Plan: 10. Floods

Home

Chapter 10:Hazard Analysis—Floods areas and thick tropical vegetation in the upper forest reserves. The ground slope is steep, averaging approximately 15 per cent. The steep slopes, shallow soils, frequent high intensity rains, and the lack of well-defined drainageways make many areas in the North Kona district susceptible to flooding and overland flows. Flash floods,primarily from overflows of the Keopu/Hienaloli, Waiaha, Kaumalumalu and the Holualoa/Horseshoe Bend drainage ways, have been identified by the Natural Resources Conservation Service's "North Kona Flood Plain Management Study." Flood water and sediment damage occurs along the entire coffee belt with the Kainaliu, Holualoa and Kailua village areas experiencing the heaviest damage. • South Kona. Being geographically young, there are few well-defined drainage ways in the district. Overland and stream flows are rare and can only be detected when the rainfall intensity exceeds the rate of infiltration. The district is subject to sudden high intensity rainstorms that can strike anywhere and cause localized flooding. Flood prone areas have been identified by the Natural Resources Conservation Service's "South Kona Flood Hazard Analyses." Coffee and other agricultural lands are subject to erosional damage and roads and culverts are sometimes damaged by high flows and sediment deposition. The Sunset Coffee Mill Flood Prevention Project has provided substantial relief in the Napoopoo area. There are also records of minor flooding from Kiilae, South Keokea, Honaunau and Wailapa Streams. In general, an area within 150 feet of the stream channels can be considered subject to flooding. Other areas with records of minor flooding include the areas along the Belt Highway in the area of the 1950 lava flows and at Hookena Road. • Ka'u. The Ka'u district can be divided into three separate regions. The northeastern region is dominated by the Ka'u desert. The average annual rainfall here is approximately 20 inches. There are few defined stream channels, none of which are perennial. The soils are very shallow, covering rough lava flows that are extremely permeable. The southwestern region that extends westerly from the South Point Road is characterized by moderate slopes, extremely permeable soils, and relatively young lava flows. The median annual rainfall varies from less than 20 inches at South Point to 75 inches at the 5,000-foot elevation. There is little evidence of stream flow within this region and no record of damage from flood flows other than the flooding of roads within the Hawaiian Ocean View Estates subdivision. The central region contains the communities of Pahala, Naalehu, and Waiohinu. There are several streams within the region, none of which are perennial. Flood flows occur when the soils are saturated and rainfall intensity exceeds the rate of infiltration. Storm runoff descends steep slopes behind the communities and causes flooding and deposition of sediment and debris in the communities. 10-15 Hawaii County Multi-Hazard Mitigation Plan