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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 , and – areas where rivers meet the sea – 80% to 90% of this habitat has been built over.

The result has been the of estuary habitats and the 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 since the 19th century.

Some of these oceanic highways , 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 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 created by a rough and shallow channel bottom is reduced. With less friction, that can lead to .

As , we 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 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 . 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 of a nearly 50-foot-deep (15 meter), 500-foot-wide (150 meter) harbor entrance channel beginning in the early 20th century.

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 and landscape alterations, including projects that fill in wetlands to build industrial sites, airports and neighborhoods.

Evidence and computer modeling show that any hurricane storm surge affecting , , , , and , 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 ; ; ; and . Such barriers are increasingly being proposed alongside levee systems for coastal protection of urbanized estuary shorelines.

The U.S. Army Corps of Engineers recently , including near Miami, Jamaica Bay in Queens and .

Surge barriers 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, , due to the lack of available space.

A storm surge crossing over a mile of marsh can be reduced by several inches, . 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 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 and estuarine urbanization .

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 of underutilized shipping channels, gradually retreating from vulnerable – and now often waterlogged – landfill industrial sites and neighborhoods, and .

These approaches can 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 to also reclaim the floodplain, 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.

This article is republished from The Conversation under a Creative Commons license. Read the original article here: .

Photo: Sarasota Bay (Courtesy of Sarasota County)

Topics Windstorm