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.

Planners Can’t Afford to Overlook Groundwater-Driven Sea Level Rise Flooding — University of Hawaii Study

Researchers at the University of Hawaii at Manoa studied three different sources of sea level rise flooding and found groundwater inundation — a rising of the water table to the surface due to the pressure created by sea level rise — is the major threat to urban Honolulu.

To arrive at this conclusion, the researchers studied the likely causes of sea level rise flooding over the next few decades in Honolulu. They did this by producing flood maps that considered the threats posed by sea level rise-driven groundwater inundation, reverse municipal drainage — where ocean water flows backward through the drainage system and rises up through stormwater drainage grates — and direct marine inundation — where ocean water flows in from the sea. Among their surprising findings, they determined that direct marine inundation represented only 3 percent of the overall near-term threat.

The researchers said their findings clearly indicate that seawalls, one of the most popular approaches to holding back sea level rise flooding from damaging real estate and critical infrastructure, such as roads and water and sewer pipes, won’t be able to stop groundwater inundation. They said the only options in areas impacted by groundwater inundation will be to adapt to the flood waters or abandon the affected area.

“What this means for the future of Honolulu is that we need to consider each different type of flooding individually and really think about how we’re adapting to each one,” said Shellie Habel, a coastal geologist with the University of Hawaii Sea Grant College Program.

The University of Hawaii report should act as a wake-up call to coastal areas everywhere that have high water tables and porous bedrock, like South Florida. The groundwater inundation threat must be considered before enormous amounts of money are invested in sea walls that ultimately won’t be able to protect real estate and critical infrastructure from sea level rise flooding. (Photo courtesy of School of Ocean and Earth Science Technology, University of Hawaii at Manoa)

Hawaii’s Trying to Decide Where to Allow New Real Estate Developments in Areas Threatened by Sea Level Rise

Lawmakers in Hawaii are taking on an issue few coastal states are ready to address: Where do you allow new real estate developments when scientists are predicting up to three-to-six feet of sea level rise by the end of this century?

The answer could have a huge impact on real estate developers, builders and people seeking affordable housing.

According to a report on Honolulu Civil Beat, some legislators in Hawaii are promoting a bill that would prevent new construction in areas below 6 1/2 feet of the current sea level. With land elevations varying greatly, that would still allow some construction right on the shore while developers wouldn’t be allowed within a half mile of beaches in other areas. Builders in Honolulu say that limit would prevent them from constructing many new developments, including projects that would be sold at a more affordable price point.

The stakes are high for Hawaii. Scientists predict that even 3.2 feet of sea level rise by 2100 would displace more than 13,000 people and lead to $12.9 billion in economic losses in Oahu alone.

As in other states, there are areas in Hawaii where coastal real estate is already experiencing flooding due to rising seas. Finding solutions that serve the needs of current generations while looking out for the future is a difficult political tightrope to walk. Further complicating the issue is the fact that the burning of fossil fuels, global warming, and rising sea levels are continuing at an accelerating pace, which makes it difficult to issue the accurate predictions coastal communities need for planning purposes.

That Hawaii is discussing this difficult issue is commendable. Other states need to step up to the plate to protect people and property.

Sea Level Rise Reality: No Roads, No Real Estate

Communities from Hawaii to the Florida Keys are already confronting a harsh reality of sea level rise flooding. When flood waters inundate or undermine roads, they have a choice: spend millions or even billions of dollars to save the roads, or abandon them and the real estate that relies on them.

According to a recent report by Mahealani Richardson for HawaiiNewsNow, sea level rise-driven erosion recently caused 1,500 feet of highway to collapse in Haaula, a town on O’ahu. The state is spending $600,000 on emergency repairs, but a permanent solution to save the coastal highway from rising seas could cost up to $1.5 billion for a dozen miles.

Ed Sniffen, a highways administrator, told NewsNow, “It’s a huge but complex situation that we have to consider. Not only are we affecting who can drive through that area in the future, but access to that area in the future.”

Monroe County officials in the Florida Keys are facing the same challenge. According to an article by Theresa Java posted on KeysNews.com, county commissioners there are considering whether to elevate a road in Stillwright Point that flooded 91 days between September and December or abandon it altogether. The road’s fate — and the property owners who rely on it to get around — will depend on how much it will cost to save the road and, considering that seas continue to rise, how much time the repair will buy.

The county’s resiliency officer said a billion dollars probably isn’t enough to save all of the county’s 314 miles of roads. Mayor Heather Carruthers said, “This is the very beginning of very difficult decisions that governments around the world will be forced to make.”

If you search “sea level rise road” on Google, you’ll find dozens of cities and town are confronting the same sea level rise problem. Finding a solution isn’t just a cost-benefit question. Officials also have to consider the decision’s impact on local residents. In some cases, residents have threatened to sue if the government abandons their lifeline roads.

Buyers taking a look at real estate in coastal areas need to consider not only whether or not a property of interest is experiencing sea level rise flooding, they also have to consider how sea level rise flooding is impacting critical infrastructure, such as roads and water and sewer service. The floodwaters could not only prevent them from getting around and receiving critical services, they could also result in a huge tax hike if a community has to initiate projects to save the infrastructure. In a worst case scenario, flooding could force them to move.

Infrastructure issues are discussed in detail in “7 Sea Level Rise Real Estate Questions.”