Walling off Bay to reduce floods would cause deluge of other woes
Model finds that barrier would result in more algae and dead zones and fewer aquatic creatures
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The slowly-sinking Norfolk region faces a growing threat of catastrophic flooding if a hurricane hits the mouth of the Bay. Such a storm could send a massive surge of water through the Chesapeake’s second largest, metropolitan area and some of the nation’s most important military installations.
The flooding risk will only grow in coming decades as sea levels rise and, with warming temperatures, the potential for intense hurricanes increases.
But there is a relatively simple way to reduce the risk of a flood: Build a barrier across the mouth of the Chesapeake Bay.
Of course, such a change would dramatically alter the nation’s largest estuary. And no one would consider such a radical action. Right?
“I wish that were true,” said Carl Hershner, director of the Center for Coastal Resources Management at the Virginia Institute of Marine Science.
While there are no serious proposals to build a storm barrier across most of the Bay’s 20-mile-wide mouth, huge barriers do defend many major cities around the world, including New Orleans, London, Rotterdam, St. Petersburg and Venice.
In the aftermath of widespread New York City flooding from Hurricane Sandy, officials openly contemplated whether walls should be built in surrounding waters to protect against future storms.
So, in the last couple of years, scientists have been contemplating how the Bay would change if such a barrier were built. To address that question, scientists developed hydrologic models to determine how a barrier would affect circulation and water quality in the Bay. Those results will eventually be linked to habitat and fisheries models to examine how the change in circulation and water quality would affect Bay resources.
In the model, the impacts of two barriers constructed along the 20-mile Chesapeake Bay Bridge-Tunnel — with openings left to allow for shipping — were tested. In one scenario, 85 percent of the mouth would be closed off, in the other, 60 percent.
Such barriers could significantly reduce the storm surge flooding associated with hurricanes that move into the Bay’s mouth from the ocean, though not from storms that blow wind — and water — down the Chesapeake.
“In the model, the storm surge barrier works in terms of reducing risk.” said Donna Marie Bilkovic, a VIMS researcher working on the project. “But then there are these ecological impacts that offset that.”
The modeling shows that the barriers would reduce tidal range in the Bay by as much as a vertical foot in places, something that could result in an extensive loss of salt marshes, which are important habitats that have already declined substantially in the region.
Through changes such as reducing the influence of tides, the barriers effectively reduce the overall amount of energy to move water in the Bay.
“More importantly, we found that when you reduce the energy, you increase the stratification and you really increase the hypoxic area,” said Jian Shen, a VIMS hydrodynamic modeler working on the project.
In the Chesapeake, freshwater flows along the surface of water toward its mouth, while salty ocean water flows up the Bay along its bottom. Even under normal conditions, that makes it harder for the top and bottom layers to mix, but the barriers make that stratification more pronounced.
As a result, the model showed that when the bottom layer is unable to mix, it actually pushes its way farther up the Bay, covering more area, and ends up increasing salinities in many areas.
It would also make the bottom water more hypoxic, or oxygen-starved, because stronger stratification reduces the ability of those layers to mix. While surface water can have oxygen replenished from the air, bottom water needs to be stirred with surface water to maintain oxygen levels, otherwise bacteria and bottom-dwelling organisms will use it up.
The increased area of saltwater along the bottom would therefore become more prone to oxygen depletion, expanding the potential areas that could be affected by hypoxia, and making those conditions more severe.
Exacerbating that problem, the barrier would increase residence time — the amount of time water stays in the Chesapeake — especially in the Lower Bay. That could trap nutrients, which could grow more algae that ultimately would die, sink to the bottom and be decomposed by bacteria, which would further drain oxygen from the water.
That suite of problems was created by either barrier, but conditions tended to be more severe with the larger barrier. With either barrier, model results show the impacts were not uniform across the Bay, with things like salinities increasing in some places but decreasing in other.
The exact implications that a barrier would have on resources are still being estimated, but scientists say they would result in substantial ecological impacts throughout the system.
Tidal marshes in general would decrease, and so would underwater grass beds. Algae would increase, while many fish populations would likely decrease.
Salinity changes could have profound impacts, such as altering spawning areas for fish and changing the types of phytoplankton that are available for their young to eat.
A greater area of saltwater would reduce spawning areas for anadromous fish, and the increased bottom salinities could promote the growth of the parasites that have plagued oysters for decades, the scientists said.
Increased water residence time near the mouth of the Bay could affect the movement of fish and blue crab larvae — the Bay’s most economically valuable species — in and out of the Chesapeake.
The scientists working on the project acknowledge that it’s unlikely anyone will seriously propose a barrier at the mouth of the Bay anytime soon, but the models being developed will eventually have other uses, such as helping with local planning. For instance, if local officials were trying to protect roads near an eroding shoreline, the habitat model being developed could help them weigh the ecological impacts of a bulkhead versus living shorelines.
“This was our test case,” Bilkovic said. “But down the road we want this to be a tool that can be used to assess different types of management actions and the implications on both the built human environment and the natural system.”
While the scientists consider a proposal for a barrier at the mouth of the Bay a remote possibility, they do anticipate requests for barriers on particular tributaries or creeks. The research done for this project, they say, will give an idea of what types of local impacts should be examined in considering such proposals.
“They are talking about small creeks in Norfolk, they are looking at creeks in some of the other low-lying communities here,” Hershner said. “We would like to have some basis for formulating advice about those projects. This helps us design and develop the analytical protocols that we might use for looking at those kinds of projects.”
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