Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods sugar to other crops may increase runoff. In this regard, Soil and Water Conservation District conservation programs can help lessen the potential problem. Some of the flood hazard areas for the Puna district are difficult to delineate due to the lack of defined drainage ways. Recorded flood damage has mainly been caused by surface sheet flows that are likely to occur anywhere when heavy storms strike. Examples of this are found in Fern Forest, Eden Rock, Fern Acres, Orchidland, and Hawaiian Paradise Park. In addition to these subdivisions, flooding occurs in certain areas of Pahoa. Other areas, such as Hawaiian Acres, may be more defined. The flooding below Mt. View may be the result of diversion of the Mt. View watershed into some of the substandard subdivisions. Systems that incorporate diversion channels to intercept sheet flows and main channels to transport the flows away or through the area have been proposed for the communities of Kcaau and Pahoa. Along the Kcaau-Pahoa Road, the State Department of Transportation (DOT)has installed culverts to facilitate the movement of water and minimize overtopping of the road in certain sections. In addition, the DOT plans to replace those culverts that are ineffective or inadequate. Drainage systems incorporating the use of diversion channels to collect and transport surface flows safely through the area are also proposed for Mt. View. A portion of this system has been constructed. • South Hilo. The South Hilo District is divided into two watershed study areas divided by the Wailuku River. North of the river, the coastline has abrupt cliffs 30 to 80 feet high that are broken by deep stream channels. Flooding problems in this area are primarily caused by runoff from former sugar cane fields situated above the communities situated closer to the coast. South of the Wailuku River is a relatively flat plain that extends towards High- way 11 with slopes gradually increasing in southerly and westerly directions. Development in the upper section of the Waiakca Stream Watershed has been susceptible to flooding. Two recent studies by NRCS examine the problem areas of the Waiakea and Wailuku-Alenaio watersheds.54 With the Wailuku.River as a dividing line,the South Hilo district can be separated into two watershed study areas. To the north of the river, the coastline has abrupt cliffs 30 to 80 feet high that are broken by deep stream channels. Usable land areas have a ground slope of six to twelve per cent. Above the 4,000 foot elevation,the stream channels diminish in number and depth and have all but disappeared above the 7,000 foot elevation. Flooding problems in this area are primarily caused by local water runoff from former sugar cane fields situated above the communities. South of the Wailuku River is a relatively flat plain of less than one per cent slope that extends towards Highway 11. Above Highway 11, the slope steepens to approximately six to twelve per cent. Stream channels are poorly defined and disappear at elevations above 2,500 feet. 54 Natural Resources Conservation Service,The Waiakea Stream Preliminary Investigation report, November, 1999;NRCS,The Wailuku-Alenaio Watershed Reinvestigation report, December, 1999(identified a new flood diversion alternative that could provide 100-year flood protection to development on the south side of Kaumana Drive in the vicinity of Akolea Road and Chong Street). 10-10 Hawaii County Multi-Hazard Mitigation Plan