As the Earth continues to warm, sea levels are rising at an accelerating rate, from 1.4 millimeters per year to 3.6 millimeters per year between 2000 and 2015. This increase in sea levels will inevitably lead to more flooding, particularly in low-lying coastal regions where over a billion people reside. Solutions are needed to protect homes, property, and groundwater from flooding and the intrusion of saltwater. Seawalls and similar infrastructure are obvious options to protect against flooding, with cities like New York and San Francisco already working on potential plans with the Army Corps of Engineers that heavily rely on seawalls. However, implementing these plans could come with a hefty price tag, estimated to be tens of billions of dollars.
A new paper published in Scientific Reports, part of the Nature portfolio, raises concerns about the efficacy of seawalls and other shoreline barriers in protecting against flooding. The paper, titled “Shoreline barriers may amplify coast groundwater hazards with sea-level rise,” was authored by Xin Su, Kevin Befus, and Michelle Hummel. The paper highlights how seawalls and other barriers extending below the surface might lead to increased groundwater flooding, less protection against saltwater intrusion, and potential issues with water retention within the area the barriers were supposed to protect. This could result in significant challenges with managing excess groundwater levels.
The researchers explained that rising sea levels cause salty groundwater to move inland and replace fresh groundwater, a phenomenon known as saltwater intrusion. As both fresh and salty groundwater rise toward the ground surface, groundwater emergence, or flooding from below, can occur. Building walls underground can help reduce saltwater intrusion into groundwater, but it can also trap groundwater behind the walls, acting as an underground dam. This may lead to further rising groundwater levels and subsequent infiltration into sewer systems and water mains, posing risks to infrastructure and water quality.
According to the researchers, the traditional approach of building seawalls as a means of flood protection may not be as effective as previously thought. Their simulations showed that seawalls could lead to water seeping in from both the ocean and the landward side, requiring continuous pumping to keep the area dry. The study emphasizes the importance of considering potential inland flooding risks from rising groundwater levels when constructing protective barriers. Failure to account for these risks could exacerbate the very issues the barriers were intended to address.
The findings underscore the need for careful planning when building flood-related or underground walls, particularly in densely populated coastal communities. To address potential issues, coastal cities must develop comprehensive strategies that account for saltwater intrusion, groundwater flooding, and the management of excess water behind barriers. Incorporating pumps or French drains, which redirect water away from foundations, may be necessary to mitigate the risks associated with building barriers. City planners worldwide should take heed of these recommendations as they devise strategies to mitigate the impact of rising sea levels on coastal communities.