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Factors accelerate erosion along Honoapiilani Highway

By Staff | Feb 9, 2012

WEST MAUI – Honoapiilani Highway (30) is the major artery into and out of West Maui and a vital connection in the island transportation system, providing visitor, residential, commercial and emergency access to the West Side.

Unfortunately, the eight-mile stretch of scenic byway between Puamana and the Pali is a victim of natural and manmade forces.

“It is highly vulnerable to sea level rise, and coastal erosion is taking away the road,” Dr. Chip Fletcher, professor of geology and geophysics at the University of Hawaii at Manoa, told the Lahaina News.

Tara Owens, coastal processes and hazards specialist with the University of Hawaii Sea Grant College Program, is a consultant to the County of Maui Department of Planning.

She added to Fletcher’s analysis, attributing the erosion to human impacts, waves and currents and sea level rise.

“Globally, the sea level is rising, so that means the water level is going up. That has to do with water getting warmer; glaciers are melting. Warmer water expands. When water expands, it takes more volume; and, therefore, it rises,” she said.

Manmade factors threatening the two-lane asphalt throughfare include the construction of revetments and seawalls that have ironically been built to protect it.

“If you build a seawall, especially if you have a sandy beach and build a seawall, the adjacent properties begin to experience accelerated erosion on the ends of those seawalls. This is called flank erosion,” Owens explained.

“In front of the seawall,” she continued, “the waves get reflected off of the seawall, and it causes accelerated erosion. Basically, it transports the sand away from the shoreline with the wave.”

A previous plan to stabilize the shoreline with a revetment at Punahoa Beach, south of Lahaina, backfired.

“A formerly wide and healthy beach has been lost in the process,” Fletcher wrote in 1992.

Owens explained the difference between a revetment and a seawall.

“Seawalls are typically vertical and concrete. Revetments are usually made out of rock, and they are sloped,” she said.

Erosion rates on Maui between 1912 and 1997 have been tracked and mapped.

The story of the Ukumehame and Papalaua shoreline is most telling.

According to U.H. maps prepared for the county, “The area, as a whole, has experienced moderate to high erosion since 1912, with an average Annual Erosion Hazard Rate (AEHR) of -1.0 feet per year

“Between Ukumehame Beach Park and Papalaua State Wayside Park, abutting the highway, is a focus of significant shoreline erosion with an average AEHR rate of -1.8 feet per year.

Further, “As a whole, average beach width has decreased eight percent between 1949 and 1997. Where revetments have been installed, beach width change and erosion have resulted in the loss of approximately 590 feet of beach.”

The Jersey Barriers lining the seaside edge were not designed to protect the battered highway.

“The road in certain places is being undermined from the bottom. That’s because of erosion; the shoreline is retreating. The Jersey Barriers are just put on top, so that when there are high waves, it helps prevent the water itself from overtopping and spilling over into the road,” Owens said.

The shoreline hazard specialist cautioned the Lahaina News not to jump to conclusions.

“You can look at the history of shoreline changes, and that’s really important; but you can’t always translate that into exactly what the future is going to hold,” she said.

Other ideas have been on the drawing board for years, like relocating the highway further inland.

Whether it’s revetments, seawalls or relocation, Owens urged the community to avoid “short-term solutions that have long-term impacts on the shoreline.”

Fletcher was more direct: “Sea level is rising now. It’s going to accelerate in the future. It’s a very vulnerable stretch of highway, and to ignore it and not make plans for either raising it or moving it is a serious mistake,” he said.

In any case, Owens advised, “The community should be completely educated before it makes up its mind.”

Next week: Views on relocating the highway.