Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods Waiakea Until recently, the existing drainage system was the result of uncoordinated development. Lacking a comprehensive plan, property owners have constructed and installed drainage facilities to protect their own interests. In many instances, these facilities have seriously concentrated flows and aggravated situations downstream. Many proposals of the "Hilo Drainage and Flood Control Report" have been completed in the Waiakea area reducing the flooding problems. Many culverts in upper Waiakea are inadequate. Roadside ditches, though small in cross- sectional area, are aided by the highly porous ground and are fairly effective even during heavy storms. One of the most serious problems faced by County maintenance crews is the frequent washout of cinder-gravel shoulders along road pavements. Another problem is the accumulation of vegetation growth and debris in waterways,which causes overflow. The Waiakea Stream Preliminary Investigation report prepared by Natural Resources Conservation Service in November, 1999 identified the channel constrictions at the Hoaka, Kupulau, and Kawailani bridges as a major factor in the flooding of the Waiakea Stream Watershed. Several recommendations in the report suggests an increase in the level of maintenance for the Waiakea stream channel, reconstruction of the three bridges to handle the 100-year flood, installation of a flood levee above the properties along Kupulau Road, and Stream channel improvements to manage the volume of a 100-year flood. In the lower Waiakea area, storm damage is minimal due to the effectiveness of the Wailoa and Waiakea-Uka Flood Control Projects. The Waiakea coastal area is subject to tsunami and hurricane storm surge inundation and has suffered considerable loss to life and property from tsunami activity. Kaumana-Ainako-Wailuku River Kaumana's drainage system consists of roadside ditches, culverts, and narrow channels. Except for the Ainako Avenue area, all of upper Kaumana's storm water runoff is discharged either through the Waipahoehoe or the Alenaio Streams. The Chong Street Diversion No. 3 and the Wailuku-Alenaio Diversion No. 4 along Akolea Road serves to reduce flooding in the lower areas and the Ainako Avenue sections. The Wailuku-Alenaio Watershed Reinvestigation report prepared by Natural Resources Conservation Service in December, 1999 identified a new flood diversion alternative for the watershed. The alternative will control stormwater in Waipahoehoe and Kaluiiki Streams and could provide 100-year flood protection to communities on the south side of Kaumana Drive in the vicinity of Akolea Road and Chong Street. The County has requested the Natural Resources Conservation Service's assistance to further plan and implement the new alternative. The drainage system in the Ainako-Wailuku River area is comprised of box culverts that pass the discharge of the Ainako River across Kokea, Koula, and Kapaa Streets. The residential areas bordering the Wailuku River have a system of collection ditches. Except during very intense storms,there are few problems in the area. Hilo Urban Area Between tsunamis and runoff from higher elevations, the commercial district has displayed amazing recuperative abilities. Prior to the completion of the Waiolama Canal in 1924 and 10-11 Hawaii County Multi-Hazard Mitigation Plan