COASTAL CHALLENGESCoastal Cities Have a Hidden Vulnerability to Storm-Surge and Tidal Flooding − Entirely Caused by Humans
Today, 80% to 90% of estuaries – areas where rivers meet the sea – such as New York Harbor, San Francisco Bay and Miami’s Biscayne Bay has been built over. The result has been the loss of buffer zones that helped protect cities from storm surge and sea-level rise. And below the surface of many of the remaining waterways, another form of urbanization has been slowly increasing the vulnerability of coastlines to extreme storms and sea-level rise.
Centuries ago, estuaries around the world were teeming with birds and turbulent with schools of 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% of this habitat has 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 to extreme storms and sea-level rise: 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 50 feet below the waterline and lengths of nearly a quarter mile, to glide into formerly shallow areas. An example is New Jersey’s Newark Bay, which was as little as 10 feet (3 meters) deep in the 1840s but is 50 feet (15 meters) deep today.
A consequence of dredging deep channels is that water also enters and exits the estuaries more easily with each tide or storm. In these dredged channels, the natural resistance to flow created by a rough and shallow channel bottom is reduced. With less friction, that can lead to 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.
An Unintended Side Effect of Dredging
The effects of dredging are most visible in the 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.
For example, in Miami, where the tide range has almost doubled, a major contributor is the construction and dredging of a nearly 50-foot-deep (15 meter), 500-foot-wide (150 meter) harbor entrance channel beginning in the early 20th century.
