Hanalei Watershed Flood Study | HanaleiWatershed Hui

Hanalei Watershed Flood Mitigation Study

Hanalei River and Wai‘oli Stream are vital watercourses on the north shore of Kaua‘i, flowing from upper watersheds into a flat, wide floodplain that supports the abundant life and community of Hanalei. Across the floodplain is a mix of agricultural taro plots, wetland habitats for endangered waterbirds, pasture lands, parks, local shops, and residential areas. The lower reaches of both rivers are prone to flooding and can quickly inundate the surrounding areas, creating significant disruptions and risks to the community.

Despite the recognized hazards, no comprehensive analysis had been conducted to see how changes to the Hanalei River floodway might be increasing flood risks or affecting water flow through key areas like Kūhiō Highway. This led to a patchwork response, making it hard for the community to create lasting solutions for the protection of people, infrastructure, and wildlife.

In response, the Hanalei Watershed Flood Mitigation Study was launched to evaluate potential strategies for reducing flooding in the Hanalei and Wai‘oli Watersheds. The study focused on assessing whether these mitigation strategies could help reduce the frequency of Kūhiō Highway closures between Hanalei Bridge and Hanalei Town. A hydraulic model was used to simulate river conditions and evaluate the effectiveness of flood mitigation strategies in reducing flooding under current and future scenarios.

The study found that none of the potential strategies had a significant effect on reducing flooding in Hanalei or Wai‘oli. The primary reason is the strong tidal influence on the lower reaches of both rivers, which limits the effectiveness of traditional flood mitigation methods.

Climate change, rising sea levels, and increasingly variable rainfall are expected to exacerbate existing flood risks. As a result, the emphasis should shift toward climate adaptation rather than mitigation.

FINAL REPORT
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MITIGATION STRATEGIES

Flow conveyance improvements

intercept and temporarily store flood waters to reduce the rate and or volume of water impacting lower areas

Dredging
Vegetation removal
Ditch and ‘auwai maintenance
Berm and rock groin structure removal

Flood water storage solutions

slow, impound, or redirect flood waters from flowing freely through the river channel to the ocean

Construction of flood water detention basins
Enhancement of existing or historic wetlands and taro lo‘i

Dredging

Dredging deepens the river mouth, allowing easier passage for large vessels. Dredging does not reduce the height of floodwaters or the extent of floodplain inundation.

Vegetation Removal

Maintaining Hau bush enhances habitat and improves public access to the river channel.

Ditch and ‘Auwai Maintenance

Regular maintenance of drainage systems improves water conveyance during non-flooding periods. During high floods maintenance reduces the chance of debris jams that redirect water, causing channel erosion and localized flooding.

Berm Removal

Berm removal was assessed at two locations in the floodplain. While berms alter flow conveyance across the floodplain, removal would result in minimal flood relief.

Rock Groin Removal

Three rock groin structures can be seen on the eastern bank of the Hanalei River mouth.

Flood Water Detention Basin

Detention basins are not recommended due to their limited flood water storage capacity and high cost, with expenses primarily driven by the need to haul away excavated soil.

Hanalei Taro Lo'i

Taro lo‘i are abundant across the floodplains. They function as detention basins. Making the lo‘i deeper to increase flood water storage was not considered since the depth is directly related to the productive capacity for growing taro.

PRIORITIZING ACTION AND ADAPTATION

Although none of the evaluated strategies significantly reduce flooding, the study highlights actions that can address existing vulnerabilities and help prepare for the ongoing impacts of rising sea levels caused by climate change.

Raising Kūhiō Highway

Sea level rise is expected to continue affecting the region, with mid-range projections suggesting an increase of 1 foot by 2050 and 3 to 4 feet by 2100. This study used a 2-foot sea level rise estimate to model river water levels during regular and high flow events.

Future sea level rise will worsen flooding in Hanalei River and Wai‘oli Stream, leading to more frequent highway closures. Flood waters that currently stop short of the road will eventually overtop it, and the time it takes to flood Kūhiō Highway will decrease.

Raising Kūhiō Highway to a height that will remain above future flood water levels is recommended as an adaptation to impending sea level rise.

Kūhiō Highway Flooding

Minor flood events consistently cause the channel banks to overflow first at these two locations.

Historic Hanalei Bridge

Kūhiō Highway, the main access to Kaua‘i’s North Shore, crosses the Hanalei River on the Hanalei Bridge.The bridge closes when the road floods.

Hanalei Bridge During Typical Flow Conditions

Typical flow of Hanalei River underneath Hanalei Bridge.

Hanalei Bridge During Storm Event

Road closures are caused by highway flooding. Debris blockages can cause structural damage to the historic bridge.

Upstream Frequent Flood Zone

Flood prone area along Kūhiō Highway. Picture from Kauai Police Department patrolling a road closure following February 2021 flooding.

Problem Areas

Map shows areas vulnerable to channel erosion and debris blockages.

Channel Erosion Downstream of Hanalei Bridge

Erosion destabilizes river banks.

Albizia Trees Upstream of Hanalei Bridge

Albizia trees can grow up to 15 feet per year and reach heights of 100 ft. Their shallow roots and large size make them a significant hazard during flood events.

Albizia Trees Blocking Wailua Stream

Debris jams can compromise the structural integrity of bridges.

Hau Bush Removal

Although hau bush removal was not found to result in a significant reduction in flooding, it will:

Improve river navigation
Stabilize riverbanks
Reduce floodwater redirection

Enhance habitat for waterbirds
Open up scenic views

Channel Erosion

Riverbank erosion along the Hanalei River poses a risk to Kūhiō Highway and Ohiki Road. Road failures would have significant impacts.

Some vulnerable sections were identified during fieldwork.
Further evaluation should be conducted to identify all hazard areas.
Bank stabilization measures should be implemented to prevent erosion-related damage

Watersheds of Halele‘a

Project Area

Flood Simulation

Watersheds of Halele‘a

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Funding for this project came from two sources: Act 12, Session Laws of Hawai‘i 2018, which provided appropriations for funding disaster relief for Kaua‘i after significant flooding in April 2018; and a grant from the National Fish and Wildlife Foundation under the National Coastal Resilience Fund.

For more information on the Hanalei Watershed Flood Mitigation Study and its findings, please refer to the full report.