Seawalls pose greater risks with higher ocean levels

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Seawalls are meant to protect coastal properties, but they can also cause erosion and hurt shoreline ecosystems. That's likely to worsen with sea level rise, according to new a study co-authored by researchers at UC Santa Barbara

UCSB research biologist Jenifer Dugan said to understand the ecological impact of seawalls, she looked at the birds on the beach.

"The birds would not sit in front of the seawalls — their food was gone," she told KPCC. 

The study, which included teams nationwide funded by the National Science Foundation's Long Term Ecological Research (LTER) project, shows the harmful ecological impacts of seawalls in environments like coastal beaches, estuaries and salt marshes. 

It's the first study to look at the impacts of seawalls across a diversity of coastal settings.

Dugan's team focused on coastal beaches and found that seawalls wiped out ecosystems behind them by preventing the natural wash of waves necessary for marine critters and plants to live.

​The study concluded that sea level rise will cause seawalls to "experience greater hydrodynamic energy" or stronger tides and waves that will smother biodiversity in front of the seawall. 

This seawall in Summerland, California reflects waves at high tide.
This seawall in Summerland, California reflects waves at high tide. Jenifer Dugan/UCSB

A report this year from the California Ocean Science Trust shows the Pacific is rising twice as fast as it did in 1990. The report estimates that the shoreline could rise anywhere from three to 10 feet by the end of the century. 

Scientists in the seawall study looked at 88 existing studies on seawalls to evaluate their conceptual model of how seawalls affect existing ecosystems. 

The study catalogued 207 different ecological responses to seawalls and found that 71 percent were "significantly negative."

Dugan said in some coastal environments she's surveyed, up to 50 percent of the biodiversity was gone in front of the seawalls. 

While the types and functions of the seawalls studied varied, Dugan said the most negative effects to existing biodiversity came from structures built to stop water from entering an area versus simply slowing the water. 

A rock wall in Carpinteria, California may prevent existing shoreline communities from adapting to sea level rise.
A rock wall in Carpinteria, California may prevent existing shoreline communities from adapting to sea level rise. Jenifer Dugan/UCSB

It's no secret that seawalls are controversial in Southern California. They've been shown to disrupt the natural pattern of beach replenishment along the coast. Seawalls also speed up erosion on bluffs, putting adjacent ocean-front property at risk.

According to the California Coastal Commission, about a third of Southern California beaches are armored with seawalls. And it's already caused problems for existing ecosystems. 

"When you put armoring, you really have reduced the options for a beach to respond to sea level rise," she said. 

A Supreme Court case recently upheld permit regulations on a seawall in Encinitas that placed an expiration date on a seawall that coastal property owners built to protect their property. 

As more coastal property owners look to protect their property against sea level rise, Dugan hopes that the results of this study will help people design and install less harmful seawall structures.  

"It's sort of a call to push people to think about how you can minimize the ecological effects of what you need to do," she said. 

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