New tool developed by University of Hawaii researchers predicts coastal flooding in West Maui

With the beaches having a limited sand supply and seasonal waves striking shores from both north and south, West Maui is probably Hawaii’s most vulnerable coast when it comes to the effects of sea level rise.

Add in the fact that condos, hotels, neighborhoods and roads hug much of the eroding shoreline, and the situation only promises to get worse as the climate changes.

How bad is it going to get?

A new interactive mapping tool will be able to give you a good idea. Developed by researchers at the Pacific Islands Ocean Observing System, or PacIOOS, at the University of Hawaii at Manoa, it offers predictions of coastal flooding at various locations in West Maui under different scenarios of sea level rise and a range of wave events.

Created using next-generation computer modeling, the West Maui Wave-Driven Flooding With Sea Level Rise tool (bit.ly/3pzXhQ1) is the prototype for additional tools expected to be developed in the next few years for vulnerable coasts across the islands.

Scientists warn that the combination of increasing sea levels and large swells is likely to generate substantial coastal erosion, damage to infrastructure and property as well as land-based sedimentation that impairs water quality.

The islands already have experienced a growing amount of wave- and tide-driven flooding in recent years, and these events are expected to only grow in number and duration and unleash more powerful wave action.

West Maui, more than most coasts, has been experiencing the serious effects of sea level rise. Dozens of West Maui hotel and condo resorts — built next to ample beaches a half-century ago — are now too close to the shoreline because of the erosion.

“They’re in real trouble,” University of Hawaii oceanography professor Doug Luther said.

Condos and resorts from Kaanapali northward have been forced into armoring their beaches with such things as temporary seawalls, sandbags and burritos. At least one condominium building — a cluster of about a dozen units — has been undermined by the ocean and rendered unsafe to inhabit.

Honoapiilani Highway, the major access corridor in West Maui, finds itself regularly overtopped by big-wave events in some areas, and three pricey beach restoration projects — at Kaanapali, Kahana and Napili Bay — are in various stages of planning.

Meanwhile, the county is now considering strengthening its development setback rules. Under a new proposal expected to be brought to the Maui Planning Commission before the end of the year, the setback would be tied to sea level rise as the state’s more stringent coastal planning requirement.

The reasons for why this coast of West Maui is so afflicted are varied, but one reason in particular sticks out.

“Scientifically, it’s real interesting,” Luther said. “West Maui is supposed to be nicely shadowed by Lanai and Molokai. But the bathymetry (shape and depth of the ocean floor) is configured in such a way as to focus the waves both from the north and the south toward West Maui. It’s a lose-lose situation.”

Tara Owens, co-investigator on the grant that funded the West Maui tool and extension specialist with UH Sea Grant College Program, said one of the models shows a finger of wave energy from the south reaching the northwest coast and causing sizable waves there.

During a recent south swell, Owens drove north to see whether that finger of wave energy was real.

“You would have assumed that the south swell would have affected just the southwest coast, but there were huge waves at Napili Bay. It’s cool that the model matched up,” Owens said.

Several factors are built into the new flooding tool: daily tidal cycles, long-term sea level rise, moderate to large wave events and the slowly oscillating ocean sea level height around Maui, which is caused in part by the El Nino weather pattern.

The tool examines 12 different West Maui shoreline sections and allows the user to adjust wave heights, sea level and wave direction.

The tool and the related six-day Wave Run-Up Forecast for West Maui (bit.ly/3chu6hz) were developed by PacIOOS along with the Coastal Hazards Group in the UH Manoa Department of Oceanography. The project was funded in part by a $500,000 grant from National Oceanic and Atmospheric Administration.

The new tool is a localized version of the State of Hawaii Sea Level Rise Viewer, a tool developed in 2017 that provides mapping of projected coastal hazard exposure and an assessment of economic and other vulnerabilities caused by rising sea levels.

A new state law this year requires oceanfront homeowners to disclose potential hazards identified by the Hawaii viewer (bit.ly/3T0rQMp).

The West Maui tool offers even more information to coastal planners, real estate professionals and potential oceanfront property buyers.

“It’s a breakthrough technology,” Maui County coastal planner Jim Buika said of the new tool. “It’s giving us an order-of-magnitude understanding of what the impacts will be on specific shorelines. It’s going to aid our shoreline work in many ways.”

Buika said a large storm in 1980 prompted many West Maui property owners to build seawalls to protect their property. Over those years many other owners installed seawalls, leaving a large percentage of West Maui’s coast hardened. Today, research has shown that seawalls upend the natural coastal sand migration system and lead to requests for additional seawalls.

“So we have to fight to keep the beaches that we have,” he said.

And some beaches may be rising from the grave.

At Kaanapali the state Department of Land and Natural Resources and the Kaanapali Beach Resort are teaming up on an $11 million- plus beach nourishment effort that officials hope will turn back the clock to 1988 and keep climate change at bay there for up to 20 years.

Officials hoped to get the project started in October, using sand located just offshore, but the effort keeps getting delayed.

At Kahana a draft environmental impact statement calls for restoring a three-quarter-mile stretch of beach with sand dredged from offshore and using seven rock formations known as T-groins in a project similar to the one at Iroquois Point.

The project has the support of the condominiums there, and residents and property owners are considering asking the Maui County Council to create a community facilities district to help pay for the multimillion-dollar project.

At Napili Bay the community is trying to raise funds to pay for permit applications, shoreline certification and a reef study in hopes of conducting a smaller beach restoration.

LEARN MORE AT MEETING

>> The public is invited to learn about the new West Maui flooding tool during a virtual meeting Aug. 30 from 4 to 5 p.m. Register online to receive log-in information at bit.ly/3wmViCp.