Can seawalls prevent beaches from eroding?

The short answer is maybe — but only if they’re part of a bigger strategic plan.

By Peter Dunn

Living next to the ocean is both romantic (salt air, sunsets, beach walks) and complicated (hurricanes, oil spills, beach traffic). One of the most challenging aspects of beach living is the changeable nature of shorelines. Ocean and land interact in complex and often-counterintuitive ways to erode, extend, and reshape the contours of terra firma, sometimes with disastrous consequences for waterfront homes and communities.

For centuries, seaside dwellers have sought greater control over the line where the ocean meets their earth. One common method has been the seawall — a masonry wall or piles of stones designed to rebuff waves and maintain a consistent shoreline. Their efficacy has been a source of heated debate, and of intense study by people like Ole Madsen, the Donald and Martha Harleman Professor of Civil and Environmental Engineering at MIT.

Using wave tanks, historical study, and coastal monitoring, Madsen and other researchers have found that relatively gentle summer waves tend to pile up sand along the shore, while winter storm waves tend to remove it. Depending on local factors, this can cause a wide range of outcomes, including cyclical creation and destruction of islands, inlets, spits, and other coastal features, sometimes over periods of decades or centuries. Long-term erosion can result when winter storms remove the tops of dunes—the damage is irreparable because the summer waves cannot reach high enough to redeposit sand.

Madsen’s finding is that seawalls do prevent sand removal and dune erosion along their lengths, but with an important caveat — erosion increases in areas beyond the walls’ ends. So unless they are built according to a coordinated plan, seawalls simply pass along the effects of the waves, which end up eating more violently at the first unprotected spot. Moreover, walls are effective only on the mainland, not on fragile barrier beaches.

“We need something like city planning: coastal planning,” notes Madsen. “You can’t solve it on a lot-by-lot basis, only as a whole. We need more and better information, but we can crack the nut if we approach it in an organized manner.”

Posted: July 7, 2009