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Hanalei Watershed Flood Study and Solution Evaluation

Funds secured through Act 12, Session Laws of Hawai‘i 2018 and Act 35, Session Laws of Hawai‘i, which were developed in direct response to the 2018 flood events, along with a grant from the National Fish and Wildilfe Foundation, are being used to help the North Shore communities recover from record rainfall and plan to mitigate the effects of future flooding. This project has been funded to specifically address flood mitigation design development in the Hanalei Basin.

The Hanalei Watershed Hui and our project partner Sustainable Resources Group Intn’l, Inc. are conducting a flood study using a hydrologic model to show the areas and depth of water that covers the land during various flow levels in the Hanalei and Waioli Rivers. The model uses highly accurate topographic maps that show all waterways (i.e. ditches, ‘auwai, streams, rivers, and ponds). Modeling will demonstrate, for the same flows, how strategies such as clearing hau bush or installing water holding basins on the floodplains change the area of water cover and depths.

The goal of the project is to evaluate solutions to reduce flood impacts in Hanalei. Various strategies will be modeled, solo and in combination, to see how they change flood waters, to identify which would be best for reducing flooding, and to determine where they should be located. The study will assess implementation options including permitting requirements, cost estimates, and maintenance needs. Land ownership along each water body will be identified, and rules, regulations, and responsible parties for maintaining water bodies and strategies will be outlined.

Community engagement will ensure that all issues are identified. Opportunities are available for the community to provide input and comment on project outputs. The results of the analysis will be communicated broadly to insure there is an understanding of how proposed solutions will benefit the Hanalei community. The study results will support the community in selecting actions to reduce flooding and assist in flood emergency planning.