Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods surface water and sediment runoff. In addition, there are the improvements to natural channels to increase their capacity. The North Kohala district has recorded runup from tsunami and hurricane storm surge activity. Areas subject to inundation include Pololu Valley, Upolu Point Airport, and the Mahukona Harbor areas. • South Kohala. The South Kohala district can be divided into two separate watershed areas. The Waimea Village watershed extends into the Kohala Mountains. Heavy rainfall occurs in these mountains and several intermittent streams flow through the Waimea area. Upon reaching the Waimea plains, these streams turn to the west and flow toward Kawaihae across the extremely permeable lava flows of Mauna Kea. The Waikoloa stream has caused flooding within the town of Waimea during high intensity storms when waters overflow due to sharp stream bends and generally inadequate flow-carrying capacities. In addition, there is some flooding concern around the area abutting the Kawaihae road. The second watershed area above the Kawaihae to Anaehoomalu shoreline extends from the coast to the peaks of Mauna Kea to Mauna Loa. The area is semi-arid with few well- defined channels and infrequent stream flows. High intensity storms have caused flooding along the Queen Kaahumanu Highway from Kawaihae to Puako, and at Puako. These storms are very infrequent and tend to create flash floods. High flows have been experienced in the Hapuna Beach and Spencer Beach Park areas due to the flash floods. The Puako Beach lots have also been subject to flooding. During the evening of September 8, 1996, heavy rains generated a flash flood along Auwaiakeakua Stream. The floodwaters overtopped the existing drainage ways causing damage to private properties, particularly the Fairway Terrace Condominium at Waikoloa Village, County roads and drainage facilities. In 1997, construction was completed on the Parker Ranch drainage improvements. The improvements diverted the Kamuela and Lanimaumau Streams toward open pasture lands. Flooding has done substantial damage to the residential subdivision at the Mauna Kea Beach Resort. To mitigate the flooding, the developer has constructed flood control measures off site and the State Department of Transportation intends to install three sets of new culverts on site. The State Department of Transportation also intends to realign and replace Waiaka Road. An added threat to the coastal areas results from fire that leaves the upper slope areas with limited ground cover and thus more susceptible to flooding. • North Kona. The North Kona district can be divided into two watershed areas. The area north of Keahole Point and the summit of Hualalai have very low rainfall and runoff. Rainfall for this area reaches a maximum average of 40 inches per year, but most of the area receives less than 20 inches per year. The soils in the area are extremely permeable and there is no record of hazardous flooding in this area. The southern area, extending southward from Keahole Point, contains most of the urban development and is subject to increasing hazards from floodwater damages as land is more intensively utilized. The area is characterized by dry vegetative growth along the coastal 10-14 Hawaii County Multi-Hazard Mitigation Plan