Walling one coastal area can flood another

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As rising waters encroach on coastal communities, governments and landowners often attempt to block the water by installing barriers such as dikes or dikes. But a new study has found this conventional solution can seriously backfire on bays and estuaries, potentially causing more severe flooding and massive economic damage to neighboring unprotected communities. The research, published in July in Proceedings of the National Academy of Sciences USA, focused on San Francisco Bay, the largest estuary on the West Coast, and the results could also apply to other major cities, from London to Hong Kong.

According to the newspaper, bays and estuaries make up about a fifth of the world’s coastal shores, but these areas are home to nearly 500 million people. More than half of the world’s population would be at risk of flooding from rising sea levels. Other major cities on the bays and estuaries include New York, Calcutta and Buenos Aires.

The partially enclosed nature of estuaries and bays means that floodwaters have limited space to propagate when they hit a barrier. “If you protect an open part of the coast, water can seep into many places,” says Ming Li, physical oceanographer at the University of Maryland’s Horn Point Laboratory on Chesapeake Bay, the largest estuary. the United States. “But the estuaries are closed, with a connection to the ocean. If you are protecting part of an estuary with a dike, the water must go elsewhere in the estuary.

The San Francisco Bay coastline stretches for about 1,000 miles, and lead author of the new study, Michelle Hummel, and her colleagues have divided it into 30 sections based on its geography. Sections ranged from steep rocky headlands and cliffs to wide, low alluvial valleys where rivers flow into the bay. The researchers used computer models to examine how a dike along each section would affect future flooding in the rest of the bay, taking into account the rise in sea level during the highest tides of the year. .

Hummel and the other researchers found that dikes in one section could increase flooding in another, even at considerable distances from the barriers themselves. Notably, a seawall along the southern tip of the bay near San Jose would increase flooding in the northern tip near Napa, Calif., A town about 60 miles away. “It really surprised us that the impacts could extend across the bay and from south to north,” says Hummel, a civil engineer at the University of Texas at Arlington.

This type of unintended consequence is most pronounced in large alluvial valleys, which, when left in their natural state, protect against flooding by slowing potentially damaging waves as well as storing flood water. Dikes along alluvial valleys can negate the natural advantages that these low lying areas give to unprotected sections.

Sea level in California is expected to rise up to seven feet by 2100, according to the state legislative analyst’s office. Hummel and his colleagues discovered that at about 6.5 feet of sea level rise and during a single high tide, a dike along the San Jose Alluvial Valley would increase flooding by 36 million cubic meters (1.27 billion cubic feet) and would increase damage costs by $ 723 million. in the rest of the bay.

An alternative to dikes is to allow flooding in lower parts of the bay, a tactic researchers call “strategic flooding.” The team evaluated this natural solution next to Highway 37, a key traffic artery that connects the north shore of the bay and the intertidal marshes, and which is already inundated during intense thunderstorms and very high tides. The researchers compared the scenario of elevating the highway onto a causeway, which would allow incoming tides to pass under the road and flood the marshes, with the approach of construction on a dike, which would effectively act as a dike. They discovered that “the construction of the sea wall would flood communities all around the bay and increase [overall] damage for a single high tide of $ 293 million, ”Hummel said. In this case, the damage to other areas would outweigh that avoided in the protected section.

Documenting the economic impact of individual seawalls could help promote the work of Bay Adapt, an initiative of the San Francisco Bay Conservation and Development Commission (BCDC) to build regional consensus on how to respond. to sea level rise. “It’s critical to put the damage in monetary terms because, at the end of the day, that’s what’s going to drive action,” said Jessica Fain, head of Bay Adapt , who is leading planning at BCDC and was not involved in the new study. “Everyone understands dollars. “

Reach a broad consensus, instead of asking each community to ask, “How can I protect my coastline?” As Hummel puts it, could be an antidote to the fairness issues raised by unilateral protections. Wealthier communities are more likely to have the funds to pay for dikes and other flood protection infrastructure, to the potential detriment of low-income areas, which often overlap with communities of color. In the San Francisco Bay Area, many people of color and low income live along the shores of the bay.

Li, who was not involved in Hummel’s research, hopes it will grab the attention of policymakers and the vulnerable communities they are tasked with protecting. “Most people don’t think about it,” he says. “In many big cities in many parts of the world, the first instinct of people is to build a dike or a dike. But for the good of the whole community, it is better to think collectively.


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