Coastal Cities Have a Hidden Vulnerability to Storm-Surge and Tidal Flooding − Entirely Caused by Humans

In New York City, some neighborhoods in southern Queens see 15 minor tidal floods per year today. Computer modeling shows that these floods are caused in about equal measure by sea-level rise and landscape alterations, including dredging and wetland reclamation projects that fill in wetlands to build industrial sites, airports and neighborhoods.

Evidence and computer modeling show that any hurricane storm surge affecting parts of New York City, Jacksonville, Wilmington, Philadelphia, Southeast Florida and Southwest Louisiana, among other locations, will likely produce higher water levels due to estuary urbanization, potentially causing more damage in unprotected regions.

These costs have gone largely unnoticed, since changes have occurred gradually over the past 150 years. But as sea-level rise and turbo-charged storms increase flooding frequency and severity, the problem is becoming more visible.

Building Solutions to the Flooding Problem
In response to rising sea levels, a different form of estuary 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 during storm-surge events. Some examples include barriers for New Orleans; London; Venice, Italy; and the Netherlands. Such barriers are increasingly being proposed alongside levee systems for coastal protection of urbanized estuary shorelines.

The U.S. Army Corps of Engineers recently recommended surge barriers for 11 additional 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 negative impacts remain poorly understood.

Natural Solutions
Wetlands and mangroves have also emerged as a popular nature-based solution.

Communities and government funding have focused on attempts to restore or create new wetlands as buffers in shoreline areas. But this solution is ineffective for flood protection in most harbor cities, such as New York, due to the lack of available space.

A storm surge crossing over a mile of marsh can be reduced by several inches, depending on the site’s characteristics. But typical urban estuary waterfronts have only tens of feet of open space to work with, if that much. In a narrow space, the best that vegetation can do is reduce wave action, which often isn’t the the most pressing problem for cities on estuaries that are typically sheltered from wind-driven storm waves.

As a result, engineered wetlands, while attractive, may be ineffective, especially if trends in ship sizes and estuarine urbanization continues.

Better Ways to Put Nature Back to Work
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 the 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, however, is rarely given serious consideration in coastal storm risk management studies because of the perceived 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 the floodplain, reducing risk in more sustainable ways. Or it could be paired with seawalls to protect existing neighborhoods in a more ecologically beneficial way. These approaches should be considered as alternatives to further urbanizing our cities’ few remaining natural areas – their estuaries.

Philip M. Orton is Research Associate Professor in Ocean Engineering, Stevens Institute of Technology. Stefan Talke is Professor of Water Resources, California Polytechnic State University. This article is published courtesy of The Conversation.