Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods the Ponahawai Storm Drain System in 1926, this area was a virtual "sea" during heavy rain. The construction of the canal and the storm drain system has since provided some degree of protection for the area. The Alenaio Stream Flood Control Project, completed in 1998, begins just below Kapiolani Street and ends below Kilauea Avenue with an earthen levee leading into the Waiolama Canal. The project consists of three flood walls, a 1,790-foot rectangular concrete-lined channel, a 200-foot concrete entrance transition, and an 830-foot earthen levee. The project also included the reconstruction of four bridges located at Kapiolani, Ululani,Kinoole Streets, and Kilauea Avenue. The Alenaio Flood Control project mitigated much of the flooding that occurred in the Alenaio flood plain. Except for the northern section of the business district, all of downtown Hilo falls within the Wailoa River basin and within the area tributary to the Alenaio Stream. The State Department of Transportation (DOT) has indicated that there are periodic shifts of beach material along the Hilo bay front shoreline. in addition, occasional storm events will close the roads at bay front due to storm surge. A study detailing the level of storm surge and the periodic shifts of beach material is needed before a solution can be developed. The shoreline areas south of the Wailuku River are subject to damage from tsunami and hurricane storm surge inundation. Paukaa, Papaikou, Pepeekeo, Honomu, Hakalau; these communities have no serious flood problems although Honomu and Papaikou have experienced minor flooding. These result from runoff from the areas above the communities. • North Hilo. The district is characterized by an average ground slope of approximately 10 per cent with scores of deep intermittent and perennial streams. Other than runoff from former cane lands, there is little record of flooding in urban areas. Each community is in close proximity to one or more gulches that carry flows from the upper watershed areas. The key to flood control within the district is to collect and divert surface runoff to the gulches. In addition, soil conservation practices are highly recommended. The flood hazard areas are extremely difficult to delineate. High intensity storms can produce localized flooding in almost any area. The only definite flood hazard area is Laupahoehoe School. The community of Ookala has not experienced heavy flooding although there are minor problems due to surface waters from the former cane fields above the town. There is no record of any flooding within the community of Ninole. The existing flood control system provided by the plantation is adequate. The community of Laupahoehoe has not experienced any extreme flood flows. However, there will be a need to supply flood protection for the community since Laupahoehoe School, which is located just to the south of the urban center, has experienced some flooding. Water flows from the former cane fields, when the natural vegetation does not form a complete cover. 10-12 Hawaii County Multi-Hazard Mitigation Plan