Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods The community of Papaaloa has not experienced any serious flooding problems. With the projected expansion of the community, there will be a need to provide flood protection for the area. • Hamakua. The Hamakua District can be divided into two major watershed areas. The northern watershed, which affects the Waipio Valley area, extends upward into the Kohala Mountains. The second watershed extends to the peak of Mauna Kea and affects the communities of Kukuihaele,Honokaa,Paauhau,Paauilo, and Kukaiau. The majority of the flood damage in this area is felt by the State or the County through damage to improvements within former cane fields,roads, ditches, and bridges. Streams originating above and flowing through Honokaa have caused flooding in the town. The existing culverts within the town also do not have adequate capacity to handle volume flows. The communities of Paauhau and Kukaiau have not been subject to any high flood flows in the past and the only recommended flood prevention measures would be the construction of diversion channels above the communities to divert water from former sugar cane fields and into the surrounding gulches. Occasional flooding along the Hawaii Belt Road between Ahualoa and Waimea occurs when rainwater comes down from the pastures and overtops the road. Although there are no mitigation measures planned at this time, road improvements have alleviated some of the flooding and improved sheet flow. Localized drainage problems exist within the limits of Paauilo. These problems are caused by allowing surface waters to collect from large areas within the town and flow down narrow roadways. The problems could be eliminated if this water was intercepted and transported to the gulches for disposal. • North Kohala. The North Kohala district is subject to occasional heavy rainfall that creates heavy runoff. Streams collect water from the upper watershed and convey most flows safely through the urban centers. Although the gulches are generally smaller than those on the Hamakua coast, they have adequate capacity to handle storm flows. Other than damage to highway culverts, there is no record of any flood damage to structures. There are areas, however, which are subject to flooding problems. These include the town of Hawi which has experienced surface sheet flows concentrating along the highway within the town, the highway and road culverts at Lipoa Gulch, and Halelua and Pueka gulches. The community of Kapaau has problems similar to those of Hawi. The existing highway culverts are inadequate to handle peak flood flows and have caused minor flooding problems in the past. On each side of the highway, the Makapala area is relatively flat and is susceptible to flooding by the Niulii and Waikani Streams. The solution to the flood control and drainage problems of this district lies in the practice of proper soil conservation in agricultural lands and forest areas to help reduce and retard 10-13 Hawaii County Multi-Hazard Mitigation Plan