That sinking feeling: US coastal port cities sinking

Many cities along the U.S. Atlantic and Gulf coasts that are also port cities are sinking. The effect is caused by coastal lands subsiding at the same time as sea levels are rising, according to a study published in the scientific magazine Nature.

The study says the effects of rising seas and subsiding land will be felt most prominently in Gulf coast states including Biloxi, Mississippi, Corpus Christi, and Galveston in Texas.

On the Atlantic coast, Charleston, South Carolina and Savannah, Georgia will be impacted.

Cities on the U.S. Pacific coast will also be impacted but not as severely.

Many of the 32 cities cited in the study are also port cities. The 32 coastal cities evaluated in this study are impacted to a greater and lesser degree and include:

US Atlantic coast:

1. Boston, MA; 2. New York City, NY; 3. Jersey City, NJ; 4. Atlantic City, NJ; 5. Virginia Beach, VA; 6. Wilmington, NC; 7. Myrtle Beach, SC; 8. Charleston, SC; 9. Savannah, GA; 10. Jacksonville, FL; 11. Miami, FL.

US Gulf coast:

12. Naples, FL; 13. Mobile, AL; 14. Biloxi, MS; 15. New Orleans, LA; 16. Slidell, LA; 17. Lake Charles, LA; 18. Port Arthur, TX; 19. Texas City, TX; 20. Galveston, TX; 21. Freeport, TX; 22. Corpus Christi, TX.

US Pacific coast:

23. Richmond, CA; 24. Oakland, CA; 25. San Francisco, CA; 26. South San Francisco, CA; 27. Foster City, CA; 28. Santa Cruz, CA; 29. Long Beach, CA; 30. Huntington Beach, CA; 31. Newport Beach, CA; 32. San Diego, CA.

The study “Disappearing cities on US coasts” concluded that: ““The sea level along the US coastlines is projected to rise by 0.25–0.3 m (between 9.8 inches to 11.8 inches) by 2050, increasing the probability of more destructive flooding and inundation in major cities. However, these impacts may be exacerbated by coastal subsidence— the sinking of coastal land areas —a factor that is often underrepresented in coastal-management policies and long-term urban planning… In this study, we combine high-resolution vertical land motion (that is, raising or lowering of land) and elevation datasets with projections of sea-level rise to quantify the potential inundated areas in 32 major US coastal cities. Here we show that, even when considering the current coastal-defense structures, further land area of between 1,006 and 1,389 km2 (about 536 sq/mi) is threatened by relative sea-level rise by 2050, posing a threat to a population of 55,000–273,000 people and 31,000–171,000 properties …These potential consequences show the scale of the adaptation challenge, which is not appreciated in most US coastal cities.”

Main Culprit Extraction of Water

In an interview with AJOT, Leonard Ohenhen, a study co-author and a doctoral candidate in the Department of Geosciences and National Security Institute at Virginia Tech, said: “the primary driver of the sinking or the land subsidence is … due to the extraction of water from the aquifer. And so, as you keep extracting groundwater, the land basically collapses on itself because of the increase in poor pressure in the aquifer. But if we take proactive measures to reduce or limit how much water is being extracted from the aquifer, or we look for alternative sources of water, so surface water to … balance out the budget of water being used by different communities, then we can slow how much water is being extracted from the aquifer, which then plays an important part in how much the land sinks.”

Ohenhen noted that there are ways to mitigate the impact to cities by pumping water back into water tables such as systems used for wastewater treatment and by desalination of sea water into fresh water. However, these mitigations will require a national initiative by the federal government because the problem is so widespread and getting worse. There is also an urgency for a solution as saltwater caused by sea level rise is starting to contaminate fresh water in states such as Delaware and Florida.

Wanted: A National Initiative

Ohenhen urges a national approach because so many cities and communities are impacted: “And we can see far worse scenarios because right now we use a medium case of sea level rise, but we know sea level continues to rise every day as a result of emissions. And so, if there are no reductions, the worst-case scenario could be even greater sea level rise, which would impact even more people. And right now, we are measuring land science based on the measurement that we have taken over the last 13 years, but we do not know if there will be an increase in groundwater extraction in different cities, which would increase how much the land is sinking ...”

In 2023, drinking water from saltwater creeping up the drought-stricken Mississippi River threatened New Orleans. It is expected to be a growing threat to coastal cities around the United States: “It is accelerating,” said Soni Pradhanang, a hydrologist with the University of Rhode Island who told the Guardian. “In the next five to ten years we really need to figure out how to tackle this situation.”

Ohenhen says that as sea levels rise and land subsides, sea water starts to enter into the fresh water. This process of saltwater contaminating freshwater table is happening in Delaware and Florida: “In places like Delaware, some people who withdraw a lot for agricultural use, already … seen a contamination of their fresh water becoming salty … And so, the only way that you can restore this balance is if there is more influx of freshwater that would push this boundary further backwards … Florida … would be another example of where that is happening as well.”

Ohenhen said that “where water is being pumped into the aquifer, and that pumping of water into the aquifer had a direct (impact), even in the short term in a few months, you can see the reversal from subsiding to uplift in this region. And that is what the Japanese were able to do…”

Solutions to Replendish Aquifers

Ohenhen said another option to slow subsidence and replenish groundwater is utilizing wastewater treatment plants: “So, if there are places that are sinking significantly, those could be potential areas that could be used for such treatment. I know in California such (facilities) already exist…”

One example is run by the Los Angeles County Sanitation Districts who operate wastewater treatment facilities across Los Angeles County: “The Sanitation Districts clean the wastewater of over 5 million people and turn it into resources 24 hours a day, 7 days a week, 365 days a year. Through the operation of 11 wastewater treatment facilities, approximately 510 million gallons per day (MGD) of wastewater are treated at these facilities and 165 MGD are available for reuse.

Los Angeles County says: “Our facilities significantly reduce dependence on costly imported water by replenishing the groundwater in the Los Angeles Basin.”

Another approach would be to build desalination plants which convert sea water to fresh water. The process has been criticized as expensive but new technologies are bringing the costs down: “I … can't speak directly to policy. So, from a researcher point of view, we can only show what the problem suggests … It takes everyday citizen(s) and the media to keep talking about the problem in order for policy makers to make such grandiose plans of desalination plants … to move the needle of policy to make people really, really care about the problem. Right now, people tend to think of climate change and sea level rise as a future realistic problem.”

Ohenhen says there is an urgent need to address the problem now: “Which was why this study was really important that we did, because we showed a short term frame, 2050 was basically our projection year to show all of these consequences are things that would unfold in about 26 or so years, which is a significant consequence that we would face in most communities.”

Sea Wall Limitations

Ohenhen says, “We can keep building sea walls and keep building levies to protect different sections, but all you are doing is moving the risk from you to another location … or in some cases delaying what's the … inevitable would be, which is you are still going to face impact in the future. We have seen the failure of the New Orleans levy system during that heavy hurricane Katrina, even in California during the heavy atmospheric downfall that we had precipitation and flooding … We also saw in different sections of California levy failure that occurred in … some places and such things … that will keep occurring. That is just because most protections are not built with climate change in [mind] …”