Hanalei Watershed Flood Study
The Hanalei Watershed Flood Mitigation Study evaluated potential flood mitigation strategies to determine their effectiveness at reducing the onset, duration, water depths, and lateral extent of flood waters in the Hanalei Basin. This analysis was conducted within 1,800 acres of the coastal plain area of Hanalei River and Wai‘oli Stream. The strategies were evaluated on the two waterways and their adjacent floodplains. A hydraulic model was used to calculate hydraulic variables for both rivers, considering existing conditions and potential future conditions that represent flood mitigation strategies, with the goal of assessing the extent to which flood attenuation can be achieved.
FINAL REPORT
Project Objective
The primary objective of the project was to evaluate potential flood mitigation strategies to determine if they would reduce riverine flooding of Hanalei River and Wai‘oli Stream. The evaluation focused on assessing whether strategies, if implemented, would reduce the number of closings of Kūhiō Highway between Hanalei Bridge and Hanalei Town.
Methodology
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Aerial and ground-based topographic and bathymetric surveys were used to develop spatially accurate, high-resolution Digital Terrain Model (DTM).
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Unmanned aerial vehicle (UAV) fitted with LiDAR sensor acquired accurate topography data.
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Ground-based surveys using survey grade GPS were conducted in areas of dense vegetation and for features of special importance.
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Bathymetric data was collected using narrow beam acoustic Doppler sonar and survey grade GPS mounted to a remotely operated unmanned water vehicle.
Remotely operated Acoustic Doppler Sonar system
used to document bathymetric and flow rate data.
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LiDAR Air Targets
aid in collecting topography and high-resolution air images.
Once the topographic and bathymetric data was collected, innovative, scenario-based hydrologic and hydraulic modeling was used to evaluate effects of mitigation strategies on reducing the impacts of flooding in the Hanalei Basin. Modeling assessed whether changes (e.g. topographical, invasive vegetation, wetland alteration) adversely affect flow conveyance and flood water hydrodynamics and increase duration of flood waters in the Hanalei and Wai'oli Rivers. Various scenarios were assessed, solo and in combination, to demonstrate flooding behavior.
The study considered implementation options including permitting requirements, cost estimates, and maintenance needs. Land ownership along each water body was identified, and rules, regulations, and responsibilities for maintaining water bodies are outlined for land owners.
Significance of Study
Flooding along the lowest reaches of the Hanalei River causes significant impacts to the livelihoods of residents, from large disruptions to commerce in Hanalei Town, to the threats of loss of life or property. The frequency of flood events is expected to increase as current climate projections predict increased weather variability.
At times flooding has completely disrupted vehicle travel into and out of Hanalei Town, causing isolation of North Shore communities. During these flood events emergency personnel and evacuation efforts are extremely limited by road access. Developing effective community response during these events is imperative. We aim to facilitate planning and aid response by providing accurate information regarding the flow and direction of flood waters in Hanalei.
Although flooding has been identified as a hazard, to date no analysis has been done to evaluate community and government concerns. Collected data and modeling will exhibit if alterations across the floodway of the Hanalei River have adversely impacted the flood regime, flow conveyance, flood water hydrodynamics; and or increased the onset and duration of flood waters. There has not been a robust and system-wide analysis of the flooding regime of the Hanalei River, which has resulted in an incomplete approach to dealing with problem areas of the river, and a reactionary approach to dealing with the results of flooding on the local community and wetland habitat.
This project is providing information for the community and their leaders to make informed and inclusive decisions about sustainable solutions to the flooding problems as they pertain to people, property, infrastructure, and wildlife habitat in the Hanalei area.
There are funds set aside by Act 35 to be used towards implementation of flood mitigation projects that the community wishes to pursue. Our study will help to inform and guide the community's decision on the best actions to take.
This project is a community effort; community engagement is a priority. All experiences and perspectives are valuable. If you have questions or concerns about this work please share them with our team.
Get in contact:
Email: hanaleiflood@srgii.com
Phone: (808) 826-1985
Updates and results of our research will be communicated to the community on our blog.
Hanalei Flood Study Community Advisory Team (10/11/23)
Hanalei Flood Study Community Advisory Team (8/15/23)
Hanalei Flood Study - Agencies (3/2/22)
Hanalei Flood Study - Businesses (3/10/22)
