Groundwater Pushed Up By Sea Level Rise Poses a Threat To Coastal Real Estate

When we think of sea level rise flooding, we think of salt water spilling across beaches, wetlands or sea walls and onto the land, but that’s not the whole story.

In many coastal areas, salty seawater sits in porous soil layers beneath the fresh water aquifer. As sea level rises, the salty seawater, which is denser than fresh water is forced inland where it pushes the fresh water table up toward the surface. This type of groundwater flooding is already creating a challenge for many coastal communities battling classic sea level rise flooding.

In Miami, for instance, some neighborhoods far from the coast are flooding because soils saturated from higher groundwater can no longer absorb heavy rains. The saturated soils are also rendering septic systems inoperable as wastewater that’s carried out into leaching fields cannot be absorbed by already saturated soils.

According to an article by Kendra Pierre-Louis published in MIT Technology Review (“How rising groundwater caused by climate change could devastate coastal communities”) rising groundwater presents a “potentially catastrophic” threat to homes and infrastructure. “Roadways will be eroded from below,” she writes, “septic systems won’t drain, seawalls will keep the ocean out but trap the water seeping up, leading to more flooding. Home foundations will crack; sewers will backflow and potentially leak toxic gases into people’s homes.” Pierre-Louis explores the challenges in great detail in her excellent piece.

Experts say Miami (and all of South Florida, for that matter) isn’t alone in confronting this threat. Kristina Hill, an associate professor at the University of California, Berkeley, told the Pierre-Louis that flat coastal areas with a type of geology that allows water to move easily through the ground are at risk. The list also includes Oakland, California, Brooklyn, New York, Mountain View, California and Washington, DC.

Of great concern here is that cities might spend billions of dollars on seawalls, elevating streets and properties, and other efforts to combat classic sea level rise, but if they don’t consider the threat posed by the rising water table that can defeat those measures from below the investment could be a colossal waste of time and money.

Buyers of real estate in coastal communities that are vulnerable to this type of flooding need to understand that flooding from beneath poses as great a threat to their investment as flooding on the surface. They need to make sure that properties of interest — and nearby properties and roads — have not experienced groundwater flooding and are not at risk of experiencing it during the period they intend to own a property. They also need to know if the septic system (if they’re not hooked up to municipal service) is operational, if pipes providing water and sewer service are in good shape, what government officials are doing to address the problem, and how much any efforts to mitigate the problem will contribute to their water and sewer bills and taxes.

Ultimately, with the enormity of the land area at-risk, combatting groundwater rise will likely prove as — if not more — difficult than fighting classic sea level rise. The best choice, therefore, is to prevent the sea level/groundwater rise itself by adopting renewable energy sources that slow and stop the global warming that’s driving it.

The Threat of Flooding in Coastal Communities Rises as Sea Level Rise Lifts Water Tables

When Tropical Storm Eta soaked South Florida with torrential rains in November, many property owners far inland were shocked to see streets and homes flood in their neighborhoods. Experts say that the extreme flooding was due to the enormous amount of rain that fell on land already saturated by heavy rains that fell in October. They also said that the nearly 75-year-old canal system built to drain what had for been Everglades swamplands was unable to cope with the volume of water.

Sea level rise was part of the problem, too. The drainage canals rely on gravity to transport water from land to sea. As sea levels rise, the difference in height between water on and under the land and the ocean is becoming narrower. As a result, floodwaters don’t flow as quickly downslope to the sea, and, during extremely high tides, sea water actually tries to rush inland through the canals.

Another reality of the canal system is that if the region is experiencing higher than normal “king tides” during a storm, authorities who oversee the drainage system have to close gates to keep seawater from rushing up the canals. During heavy downpours, floodwaters can get caught behind the gates and, with no place to go, they accumulate and flood the land.

Sea level rise poses another less obvious threat that’s right under our feet. As the sea rises, water pressure causes it to migrate inland underground through porous rock and/or soil. The pressure from the salt water, which is heavier than fresh water, forces the fresh water upward, effectively raising the water table.

This can have several negative effects. When the water table rises, it saturates the land. When it rains, the water that falls cannot be absorbed by the soil and flooding results. Another negative effect is that the groundwater itself can rise up to the surface and create flooding.

An even nastier effect of rising water tables is that floodwaters can, as was experienced in South Florida, flow into the wastewater treatment system through manhole covers and broken pipes greatly increasing the flow to wastewater treatment facilities. This influx of water can cause the facilities to lose efficiency or fail all-together. The higher water tables can also cause on-site septic systems to fail. Both problems can result in the release of stinky, and potentially infectious sewage into floodwaters and onto the land.

The flooding Eta brought to South Florida isn’t unique to the region, and it illustrates a problem that many coastal communities and real estate owners are coping with now or will confront soon as seas continue to rise.

Many coastal communities from Florida to Oahu are racing to cope with the problem of sea level rise-induced rises in water tables. A superb article by Grace Mitchell Tada titled “The Rising Tide Underfoot” recently published in Hakai Magazine discusses in detail how rising seas are threatening Oahu, Hawaii. As Dolan Eversole, a management coordinator with the University of Hawaii Sea Grant College Program, told the reporter: “Sea level rise does not look like the ocean coming at us. It looks like the groundwater coming up.”

In South Florida, seawater is migrating inland through porous limestone. In Oahu, it moves through basalt rock. The end result is the same. According to the article, higher water tables are wreaking havoc, flooding residential neighborhoods and commercial and industrial areas. It’s also threatening critical infrastructure, such as roads, pipes that carry fresh water, wastewater, and gas, and underground wires that carry electricity and information.

As the groundwater rises, it also has the potential to release and spread toxic substances, such as oil and chemicals, deposited in the soil, which could lead to environmental catastrophe.

As sea levels continue to rise, groundwater issues will pose an even greater threat to at-risk communities.

Owners and buyers of residential and commercial real estate in coastal areas can’t ignore the threat posed by sea level rise-heightened water tables. The flooding can not only damage their property, it can make driving and communicating difficult, it can cause a spike in maintenance costs and in tax and insurance rates, it can discourage buyers from entering the market, which will drive down prices, it can discourage tourism and other business activity, and it could ultimately lead to lenders and insurers pulling out of the local market altogether, which would be the death knell for a healthy real estate economy.

Unfortunately, there is no easy way to combat rising water tables. For example, if you construct sea walls or natural berms, the seawater can easily migrate under and behind them through the porous rock and soil. With this in mind, owners and buyers of real estate in areas at-risk of rising water tables, need to perform due diligence and determine the level of the threat — has it happened in the past, is it happening in the present, and/or how far in the future will it happen. This information is critical when you decide if you can handle the risk and whether it’s worth taking on to begin with.

%d bloggers like this: