As the sea level rises and erodes coastal communities, the government and owners often try to block the water by setting up barriers such as seawalls or dams.But a new study found this traditional repair method It can be severely counterproductive in bays and estuaries, May cause more serious floods and huge economic losses in nearby unprotected communities.The research was published in July Proceedings of the National Academy of Sciences, Focusing on the San Francisco Bay, the largest estuary on the west coast, the findings are also applicable to other major cities, from London to Hong Kong.
According to the paper, bays and estuaries account for about one-fifth of the world’s coastal coastlines, but these areas are home to nearly 500 million people. It is believed that more than half of the world’s population is at risk of flooding caused by rising sea levels. Other big cities on the bay and estuary include New York, Kolkata and Buenos Aires.
The partially enclosed nature of estuaries and bays means that floodwaters have limited space to spread when they encounter obstacles. “If you protect the open part of the coast, water can flow to many places,” said Li Ming, a physical oceanographer at the University of Maryland’s Corner Laboratory, Chesapeake Bay, the largest estuary in the United States. A connection. If you protect part of the estuary with a seawall, the water must flow to other parts of the estuary.”
The coastline of the San Francisco Bay is about 1,000 miles long. The lead author of the new study, Michelle Hummel and her colleagues, divided it into 30 parts based on its geographic location. These parts range from steep rocky headlands and cliffs to wide, low-lying alluvial valleys where rivers flow into the bay. Researchers use computer models to study how each part of the seawall will affect future flooding in the rest of the bay, taking into account sea level rise during the highest tides of the year.
Hummel and other researchers found that one part of the seawall may increase flooding in another part, even if it is far from the barrier itself. It is worth noting that the seawall at the southern end of the bay near San Jose will increase flooding at the northern end near Napa, California, which is about 60 miles away. Hummel, a civil engineer at the University of Texas at Arlington, said: “The impact can extend all the way to the entire bay, from the southern end to the northern end, which really surprised us.”
This unintended consequence is most obvious in the wide alluvial valleys. When these alluvial valleys are in their natural state, they can withstand floods by slowing potentially destructive waves and storing floods. Seawalls along alluvial valleys can offset the natural benefits that these low-lying areas confer on unprotected areas.
According to the California Legislative Analysis Office, by 2100, California’s sea level is expected to rise by 7 feet. Hummel and her colleagues found that during a sea level rise of about 6.5 feet and a high tide, seawalls along the San Jose alluvial valley would increase flooding by 36 million cubic meters (1.27 billion cubic feet) and an increase of $723 million. The cost of damage is in the rest of the bay.
Another option for seawalls is to allow inundation of selected low-lying parts of the bay. Researchers call this strategy “strategic flooding.” The team evaluated this natural solution for Highway 37, an important arterial road that runs through the northern shore of the bay and tidal marshes, which has been flooded during heavy rains and super high tides. The researchers compared the scene of raising the highway on the causeway, which would allow the tide to pass under the road and flood the swamp, and build it on the embankment, which would effectively act as a seawall.They found that “building a dyke would flood the communities around the bay and increase [overall] The damage caused by a high tide is as high as $293 million,” Hummel said. In this case, the damage to other areas will exceed the damage avoided in the protected part.
Documenting the economic impact of a single seawall helps advance the work of Bay Adapt, an initiative of the San Francisco Bay Conservation and Development Commission (BCDC) to reach a regional consensus on how to respond to sea level rise. “Calculating damages in monetary terms is critical, because ultimately it will drive action,” said Jessica Fain, head of Bay Adapt, who is responsible for planning at BCDC but was not involved in the new research. “Everyone understands the dollar.”
Reach a broad consensus-instead of letting every community focus on “How do I protect my coastline?” As Hummel said, this may be a good medicine to solve the fairness problem caused by unilateral protection. Wealthy communities are more likely to have funds to pay for seawalls and other flood protection infrastructure, which can harm low-income areas, which often overlap with communities of color. In the San Francisco Bay Area, many people of color and low-income people live on the Gulf Coast.
Li, who was not involved in the Hummel study, hopes to attract the attention of policymakers and the disadvantaged communities they are responsible for protecting. “Most people don’t think about this,” he said. “In many big cities in many parts of the world, people’s first reaction is to build dams or seawalls. But for the benefit of the entire community, it’s best to think collectively.”