Maps of Cape Cod drawn over the last 150 years record major changes in the shoreline caused when storms pile up protective sand barriers or sweep them away. Without the sand spits (narrow barrier beaches) to absorb the energy of breaking waves, winter storms batter and erode the shoreline, sometimes carrying away buildings.
Focusing on the Cape Cod town of Chatham, Mass., Ole Madsen, the Donald and Martha Harleman Professor of Civil and Environmental Engineering, addressed this topic in a Jan. 7 IAP seminar titled "Seawalls: Are They Sons of Beaches or Not?" In January 1987, a fierce storm punched a hole in the sand spit protecting the town from the Atlantic Ocean. Waves that previously had pounded the sand barrier now traveled straight to the Chatham shoreline, eroding beaches and front yards. Within a year the beach had visibly diminished, and it continued to shrink for years until the northern end of the barrier island formed by the breach attached itself to the shore and the beach began to rebuild.
"As the beach retreats from erosion and your house threatens to collapse into the water, you may want a seawall, for example, constructed of large stones," said Madsen. "However, if you just protect your house, erosion will continue unabated or even increase on the neighbors' unprotected beach. For seawalls to offer good protection, everyone along the threatened coastline must agree to build a seawall, not just a few."
The intense nor'easter now commonly known as "The Perfect Storm," from the best-selling book and popular movie, hit Chatham hard on Halloween 1991. By the time it dispersed, 10 houses without seawalls had toppled into the ocean. The Massachusetts Department of Environmental Protection allows seawalls to be constructed to protect glacial moraine, but not sand dunes. Madsen suggests that Chatham would have suffered less damage if a seawall had been built along the entire coastline that was exposed to direct wave attack, rather than having unprotected stretches where waves could gouge out sand and destroy houses.
"Good seawalls can protect the shore and decrease the loss of sand for the whole beach," said Madsen. "But the wall must be continuous. If you interrupt the flow of sand along a beach with a jetty or seawall, increased erosion will occur downstream on someone else's beach or property."