Restoring wetlands in Hawai‘i can help reduce residential flooding from climate change.
In 2018, a rainbomb dropped 50 inches of rain onto the North Shore of Kaua’i. Houses were torn from their foundations; residents ended up in shelters; and a massive landslide cut off northern communities for weeks. The phenomenon repeated in 2021, causing collective trauma – this time, a landslide isolated the town of Hanalei. At the same time, news footage captured terrifying flood scenes in Hale‘iwa, O‘ahu.
Climate change has arrived in Hawai‘i. While nature might leave us feeling helpless in the face of her fury, she has also generously provided us with the solution. It might seem counterintuitive to use water to prevent flooding, but wetlands can do just that. And the people of Hawai‘i have a long tradition of creatively managing and maintaining them.
The Roots of Our Flooding Problem
Intense periods of rainfall are not new for the Hawaiian Islands; it’s no coincidence that Hawaiians have more than 200 words for rain. The forests here evolved perfectly to capture this water. In an ua lani pili (sudden downpour) the forest acts like a giant sponge. However, dramatic changes to upland areas have reduced the ability of forests to soak up the rain. Native trees have been cleared, allowing invasive species to get established.
The fast-growing invasive trees like Albizia outcompete our native species but cannot weather the cycles of drought and flood; their roots do not hold the soil to absorb water; instead, it cascades out of the watershed. Many local people have watched benign streams flash flood into raging torrents within minutes; and major rivers like the Wailua on Kaua‘i or the Waimea on O‘ahu become unrecognizable as they jump their banks and race across the land. Since much of our natural floodplain is now covered by homes and hotels, communities are in the firing line.
How wetlands fight floods
That’s where wetlands come in; studies show that these habitats are naturally efficient at slowing and storing flood waters when they are well managed, especially when combined with floodplain restoration.
In a completely natural setting, the native forest would provide the first layer of defense by sponging up most of the rain; lowland wetlands would help mop up the remainder. These natural systems have been altered but even so, a relatively small wetland might hold millions of gallons of floodwater and slow its release, buying precious time. Wetlands also filter pollutants and sediment before they reach the ocean, where they can make our ohana sick and kill the reef.
And wetlands are home to our four endangered native Hawaiian waterbirds. Wetland restoration supports the culturally and ecologically important koloa maoli (Koloa duck), ‘alae ‘ula (Hawaiian common gallinule), ‘alae ke‘oke‘o (Hawaiian coot) and a‘eo (Hawaiian stilt) and takes them one step further away from extinction.
Managing for the future
Some mainland and international communities are exploring the idea of connecting rivers back to their floodplains and planting native vegetation to restore wetland functions such as floodwater control.
These ‘nature-based options’ can be cheaper and more effective over the long term at managing riverine flooding events. They are certainly more attractive than traditional cement options. In contrast, ‘grey’ infrastructure (such as levees and bulkheads) may work in the short term but can fail catastrophically in massive flood events, causing millions of dollars in damage and potentially loss of life.
Elsewhere, nature-based flood control options have been shown to work. A study of wetlands in New Jersey estimated that they saved the state $9.4 billion per year in flood control costs. An Environmental Protection Agency report summarizing 17 “green infrastructure” case studies concluded that the capital cost savings ranged from 15% to 80% when compared to “grey” infrastructure. These projects have additional benefits including improved mental health for residents able to access nature, and a more buoyant local economy, with investment and businesses attracted by the quality of life improvements when employees live near natural features like wetlands, walks and wildlife.
Inspired by these ideas, the Hanalei Watershed Hui in Kaua‘i is embarking on a study to investigate different flood mitigation measures and provide the community with a cost-benefit analysis so that they can make informed decisions on how to protect their town and homes. Several other watersheds statewide will be watching the project with hopes of following their lead.
Learning from our History
The Hawaiians are masters at managing water for kalo (taro), and the lo‘i (pondfields) fulfil some of the same functions as wetlands, providing excess water with a place to go. When floodwaters meet taro fields rather than cement, flooding is reduced.
However, the fields themselves can be severely damaged in flood events, which is economically challenging for farmers and may tip the scales away from this traditional crop. A study by the University of Hawai‘i found that taro farming has reduced from 20,000 acres at its peak (when the now endangered Hawaiian waterbirds were also thriving) to 300 acres today. Projects adjacent to and within the He‘eia National Estuarine Research Reserve on O‘ahu and Kaupapalo‘i o Ka‘amola on Molokai are working to expand this indigenous agro-ecology.
Wetland magic wand
So would a magic wetland-restoration wand solve Hawai‘i’s flooding problems? Sadly not … it will take a combination of wetland, floodplain, and watershed restoration, along with other interventions such as effective early warning systems, insurance, key infrastructural defenses and managed retreat of people away from flood zones. However, doing nothing is not an option.
The flooding crises that we face here in Hawai‘i may be an opportunity to think differently about our relationship with nature, to help human communities as well as wildlife. The oral histories of Native Hawaiians identify thousands of deities of the forest, ocean, winds and rains as ancestors; working with those elements to combat climate change requires all of us to understand that we are keiki o ka ‘aina – children of the land. Instead of spending more on hard infrastructure to withstand natural events, it’s time to pivot towards using natural capital to look after the infrastructure we have already built.
•••
Helen Raine is the conservation coordinator of Pacific Birds Habitat Joint Venture.
