Posted on October 30, 2024
Centuries ago, estuaries around the world were teeming with birds and turbulent with fish, their marshlands and endless tracts of channels melting into the gray-blue horizon.
Fast forward to today, and in estuaries such as New York Harbor, San Francisco Bay and Miami’s Biscayne Bay – areas where rivers meet the sea – 80 to 90 percent of these habitats have been built over.
The result has been the environmental collapse of estuary habitats and the loss of buffer zones that helped protect cities from storm surge and sea-level rise. But the damage isn’t just what’s visible on land.
Below the surface of many of the remaining waterways, another form of urbanization has been slowly increasing the vulnerability of coastlines: vast dredging and engineering projects have more than doubled the depths of shipping channels since the 19th century.
Some of these oceanic highways enable huge container ships, with drafts of 15 meters below the waterline and lengths of 400 meters, to glide into formerly shallow areas. An example is New Jersey’s Newark Bay, which was as little as three meters deep in the 1840s but is 15 meters deep today.
A consequence of dredging deep channels is that water enters and exits estuaries more easily. The natural resistance to flow created by a rough and shallow channel bottom is reduced. With less friction comes larger high tides and storm surge.
As coastal engineers and oceanographers, we study coastal ocean physics and storm surge. There are solutions to the problems “estuary urbanization” is causing, if people are willing to accept some trade-offs.
The effects of dredging are most visible in daily tides, which have grown larger over the past century in many estuaries and aggravated nuisance flooding in many cities, as our research shows.
Tide range – the average variation between high and low tide – has doubled in multiple estuaries and changed significantly in others. As a result, high-tide levels are often rising faster than sea-level rise, worsening its consequences.
The most common culprit for these larger tides is estuary urbanization.
In Miami, where the tide range has almost doubled, a major contributor is the construction and dredging of a 15-meter-deep, 150-meter-wide harbor entrance channel beginning in the early 20th century.
In New York City, some neighborhoods in Queens see 15 minor tidal floods per year today. Computer modeling shows these floods are caused in equal measure by sea-level rise and landscape alterations, including dredging and reclamation projects that fill in wetlands to build industrial sites, airports and neighborhoods.
Evidence and modeling show any hurricane storm surge hitting parts of New York, Jacksonville, Wilmington, Philadelphia, Florida and Louisiana will likely produce higher water levels due to estuary urbanization and potentially more damage.
These costs have gone largely unnoticed, since changes have occurred gradually over 150 years. But as sea-level rise and turbo-charged storms increase flooding frequency and severity, the problem is getting more visible. In response, a different form of urbanization is attracting new attention as a possible solution.
Gated storm-surge barriers or tide gates are being built across estuaries or their inlets so they can be closed off. Examples include barriers for New Orleans, London, Venice and the Netherlands. Such barriers are increasingly being proposed alongside levee systems for coastal protection of urbanized estuary shorelines.
The US Army Corps of Engineers has recommended surge barriers for 11 more estuaries, including near Miami, Jamaica Bay in Queens and Galveston, Texas.
Surge barriers are not long-term solutions to flooding driven by sea-level rise, and their impacts remain poorly understood.
Wetlands and mangroves have also emerged as a popular nature-based solution.
Communities and government funding have focused on attempts to restore or create wetlands as buffers along shorelines. But this is ineffective for flood protection in most harbor cities, such as New York, due to lack of available space.
A storm surge crossing over 1.6 kilometer of marsh can be reduced by inches. But typical urban estuary waterfronts have only a few meters of open space to work with.
In a narrow space, the best that vegetation can do is reduce wave action, which often isn’t the most pressing problem for cities on estuaries that are typically sheltered from wind-driven storm waves.
As a result, engineered wetlands may be ineffective, especially if trends in ship sizes and estuarine urbanization continues.
Our research reveals an opportunity for scientists, engineers and broader society to think bigger – to consider a more comprehensive reshaping and restoration of the natural features of estuaries that once mitigated or absorbed flooding.
Possible solutions include halting maintenance dredging of underutilized shipping channels, gradually retreating from vulnerable – and now often waterlogged – landfill industrial sites and neighborhoods, and restoring these larger expanses to wetlands.
These approaches can sharply reduce flooding and provide years of protection against sea-level rise.
Restoration to historical channel and wetland configurations is rarely given serious thought in risk management studies because of economic cost, but also because the cumulative effect of deeper channel depths is often unrecognized.
Renaturing urbanized estuaries in these ways could be paired with buyout programs to also reclaim floodplains. Or it could be paired with seawalls to protect neighborhoods in an ecologically beneficial way.
These approaches should be considered as alternatives to further urbanizing our cities’ few remaining natural areas – estuaries.
