How Seawalls Just Make Beach Erosion Worse

That ancient advice hasn’t stopped the California Coastal Commission from signing off on a massive $175-million seawall project at the south end of San Francisco’s Ocean Beach as we reported here.

Dean’s thesis has been proven over and over again. Seawalls harden the coast and reduce its ability to adjust naturally. The seawalls reflect and concentrate wave energy which leads to further erosion. Meanwhile, groynes starve downdrift the coast of sand sediment which also leads to further erosion. It’s a vicious cycle. And always ends with surfing’s version of kryptonite;  backwash.

At Ocean Beach, the commission said the planned 3,200-foot-long buried seawall that will reach 55 feet deep is needed to protect a local wastewater treatment plant and 14-foot diameter sewage tunnel from erosion and sea level rise. 

It’s an old but increasingly frequent problem seen all around the world, most recently at the base of the Uluwatu Cliffs. Existing infrastructure or private property, often built before the threat of erosion and water level rise was known about, or understood, are now under threat. Seawalls are often the first response to protect the property. 

“It’s a 19th-century response to a 21st-century problem. A step back in time to the non-environmental brutalist engineering solutions of the 1900s,” the esteemed Australian coastal engineer, Angus Gordon, told The Guardian. That was in response to erosion in Collaroy-Narrabeen in Sydney and the government building a huge concrete vertical seawall – 7.5 meters high and pinned 10 meters below the sand. 

On that famous surfing stretch storms have often resulted in beachfront properties and pools tumbling into the ocean, landing not far from where surfers get drained on the shorebreaks of South Narrabeen.

Now beach systems, and sand movement, on the East Coast of Australia are some of the best understood in the world. Many of the groundbreaking studies have been done by the University of Sydney coastal geomorphologist Prof. Andrew Short. He's been studying the Collaroy erosion issues since 1976.

“What we have now with this seawall is a total disaster,” Short told a protest meeting after the latest seawall construction. “The only people who benefit are the private landowners.” Short said there are other options; all expensive, and complicated. The government could buy the land back from residents, or nourish the beaches. The cheapest alternative, to do nothing, has been the option taken so far.

The buyback plan is a tough political sell as it costs money upfront. However, it would see the existing houses turned to parkland and the beach return to a more natural state that would buy decades of time for other property further inland. Short said purchasing the homes at market value would be far cheaper than the cost of construction involved in the endless seawall cycle. The sand nourishment is more expensive and involves filling the Narrabeen basin with sand dredged from offshore. It however might give a century of protection. 

That was the response on the Gold Coast. In the 1960s, extending walls on the Tweed River to make it safer for navigation had stopped the longshore flow of sand from NSW into Queensland. This added to the erosion on the Gold Coast beaches. In 1998, The Tweed River Sand Bypass Project pumped sand from Letitia Spit to Snapper Rocks ‘bypassing’ the Tweed River entrance. 

By 2015, 8,105,141 million tonnes of sand had been pumped at a cost of around $A7 million per year. It has significantly contributed to reducing the vulnerability of southern Gold Coast beaches to erosion and gave us the Superbank. On the flip side, it sandstormed Kirra to death. The bypass giveth...

At Ocean Beach, the Surfrider Foundation also believes there are better, cheaper options that don’t involve dredging. They have asked the commission to reduce the project scope by building a smaller wall away from the coast, restoring dune habitat and conducting studies on moving the plant’s tunnel.

All the studies have shown that seawalls are not only not the solution, but tend to make the original problem even worse. By thinking long term, and using a range of shoreline stabilization techniques made of vegetation or other living, natural ‘soft’ elements, everyone wins. King Canute knew that the sea always obeys eternal laws. It’s time we listened to him.