Hanalei Watershed Flood Study and Solution Evaluation
The Hanalei Watershed Flood Study and Solution Evaluation project is designed to provide information to the Hanalei community to determine the best actions to reduce flooding impacts in the Hanalei Basin. The Hanalei Watershed Hui, along with our project partner Sustainable Resources Group Intn’l, Inc., is conducting a flood study, using hydraulic and hydrologic modeling. Our study will create highly accurate topographic maps that show all waterways (i.e. ditches, ‘auwai, streams, rivers, and ponds). Furthermore, data will be collected in this study to show areas and depths of water that cover the land after various flow levels in the Hanalei and Wai'oli Rivers. 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. From these various models a prioritized list of recommendations to minimize flooding will be created so that the community may decide how they wish to move forward flood mitigation projects they wish to pursue.
The goal of this project is to evaluate solutions to reduce the impacts of flooding in the Hanalei basin. Approximately 1,800 acres of the Hanalei Bay Watershed will be studied, including the documentation of water flows 4.5 miles upstream from Hanalei River mouth and 1.7 miles up Wai'oli Stream. The evaluation will be informed by science-based modeling of the movement of flood waters in Hanalei and community engagement to identify issues and assess potential solutions. A forensic analysis of the 2018 April flood will be included to better our mitigation recommendations with the inclusion of the community's greatest concerns and risks. The outcome will be a prioritized list of recommended mitigation measures to reduce damage and economic impacts caused by flooding.
Aerial and ground-based topographic and bathymetric surveys will be used to develop spatially accurate, high-resolution Digital Elevation Models (DEM).
Unmanned aerial vehicle (UAV) fitted with LiDAR sensor will acquire accurate topography data.
Ground-based surveys using survey grade GPS will be conducted in areas of dense vegetation and for features of special importance.
Bathymetric data will be 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.
LiDAR Air Targets,
aid in collecting topography and high-resolution air images.
Once the topographic and bathymetric data is collected, innovative, scenario-based hydrologic and hydraulic modeling will evaluate effects of mitigation strategies on reducing the impacts of flooding in the Hanalei Basin. Modeling will assess 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 will be employed, solo and in combination, to demonstrate flooding behavior.
The study will consider implementation options including permitting requirements, cost estimates, and maintenance needs. Land ownership along each water body will be identified, and rules, regulations, and responsibilities for maintaining water bodies will be provided to 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 will provide 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:
Phone: (808) 826-1985
Updates and results of our research will be communicated to the community on our blog.