Up and down the New Jersey coast, you can find stark examples of the law of unintended consequences. Sea walls, jetties and other hard structures that were built to protect towns from the ocean have had an unforeseen effect: Where these structures were built, beaches were washed away.

Now, as towns rebuild from Sandy, officials are understandably looking for ways to hold back future storms. Some of them are proposing steel sea walls, but the security offered by sea walls is often a false one.

The evidence is clear that sea walls — especially the most common vertical steel walls — do more harm than good.

As coastal engineers explain it, vertical walls do not dissipate the force of waves. They merely reflect that force, causing churn, and redirect it upward and downward. That downward energy eventually scours a hole at the base of the wall. The scouring not only undermines the sea wall, but reduces the elevation of the beach.

This brings larger waves closer to the wall — and the structures it was intended to protect — and the cycle accelerates. Storm surges increase. Beaches south of sea walls also experience more erosion because there isn’t as much suspended sand drifting their way. To avoid these problems, some states prohibit sea wall construction altogether.

In New Jersey, towns such as Monmouth Beach and Sea Bright, with large rock-and-concrete sea walls, are the most dependent on constant replenishment projects to keep sand on their beaches.

The image of structures sitting behind sea walls where once there were beaches is so common in our state that, as Associated Press writer Wayne Parry noted recently, it was given a name in a Duke University study of East Coast shoreline protection: New Jersey-ization.

Since Sandy, many people, including Gov. Chris Christie, have noted that the places that best survived the storm’s fury were the ones with strong dune systems. And much of the past opposition to dunes — because they block ocean views — has drifted away. Christie has said he envisions a wall of dunes protecting the New Jersey shoreline from Sandy Hook to Cape May.

Dunes won’t be the answer everywhere, at least not without more drastic approaches, such as pulling back some existing development. Some of the state’s beach areas may never be wide enough to support a healthy dune system. But dunes still offer the best defense against erosion, as long as they are maintained, nourished with fresh sand and strengthened with vegetation.

The idea of building a wall to hold back damaging waves has obvious appeal. The reality is that learning to work with and live with natural systems offers us a far better chance of surviving the next Sandy.

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