Seawalls Mean Death to the Beach

Don Barber, Kevin DuBois, and Gordon Labedz

In the middle of a surfless summer day, houses in San Diego are crashing into the sea. The houses were built on a coastal bluff to take advantage of a beautiful ocean view, but now the bluffs are doing what coastal bluffs do, they are crumbling to the beach.

From the point of view of the beach, this is wonderful. It means more sand for the beach and more sand bars offshore. The sand bars will make better surf for surfers and the sand on the beach will help protect the bluffs from the ocean waves, slowing the bluff erosion. From the point of view of a homeowner who didn't do any investigation before buying the house, this is a "disaster." Beaches and bluffs don't need protection from the ocean; on the contrary, sand beaches are the natural result of interactions between bluffs and ocean waves. Buildings and roads positioned too close to the beach do require protection from our ocean, because in most cases they are not designed to interact with the ocean.

Along much of the West Coast the shoreline is retreating. This means that the shoreline is moving inland. Anything built in the way of this retreat will eventually be swallowed up by Mother Ocean in the process known as beach erosion. On the East Coast, it is not just the shoreline that is moving. The entire system of barrier beaches, islands, wetlands, inlets, and bays are constantly shifting and migrating landward. On both coasts, the natural movement of our beaches means that roads and buildings built near the shoreline must be seen as temporary. Because previous generations thought they could stop Mother Ocean's forces, billions of taxpayer dollars have been spent for the Army Corps Of Engineers to "harden our shoreline" to protect coastal roads and buildings that never should have been built.

Many beaches, dunes and coastal bluffs have been hardened by various rock, concrete, wooden and steel walls designed to prevent coastal erosion. These seawalls, as they are called, are sometimes temporarily successful at protecting buildings or roads. Most seawalls fail sooner than their designers expect, and the disintegrating walls often litter the beach with debris and place buildings at greater risk than before. Although they may sometimes achieve marginal success as building protectors, research has shown that seawalls are almost always detrimental to beaches. For example, Tait and Griggs (1990) found that seawalls have an adverse (i.e., increased erosion) on beaches undergoing retreat, but the effect is more pronounced on beaches undergoing long-term, rapid shoreline retreat, as is common on the Atlantic and Gulf coasts. On the other hand, the deleterious effect of seawalls is more seasonal and less adverse on the more "slowly retreating west coast beaches." (Plant and Griggs, 1992, pg. 185).

Seawalls influence the beach in front of them in various ways. Observations of a seawalled beach at Aptos, south of Santa Cruz in Monterey Bay documented that "swash that is reflected by a seawall is directed seaward several seconds earlier than swash on an adjacent natural beach, increasing backwash duration and velocity" (Plant and Griggs, 1992, pg. 183). This means that at high tide, when waves wash up against the walls, waves reflect back towards the ocean with much more energy than if the wall wasn't there. These reflected waves often cause the sand beach in front of a seawall to erode twice as fast as an adjacent beach without a seawall. As the beach continues to erode, the seawall may also block natural replenishment of sand from the dunes or cliffs behind the wall. Reflected waves and the diminished sand supply may also degrade the sand bars and destroy the surf. The underwater part of the beach profile will become steep and deep, sometimes allowing waves to reach the seawall without even breaking. The surf may never return.

Federal and state agencies pay lip service to the destructive nature of seawalls, but without local grass roots activism, more and more of these beach destroyers are being built. If buildings are in danger of falling into the sea, the buildings should be moved landward or abandoned. If the building's owners choose to maintain the building in its threatened position, the owners should pay to artificially replenish the beach with sand. Although beach replenishment is costly and supplies only temporary protection for buildings, replenishment is the only engineering solution that does not endanger the beach and surf. We must not accept a walled coastline of buildings without beaches and surf. Our Mother Ocean's waves are relentless in their reshaping of the coast; no human will win this battle against her. We must learn to see both seawalls and shoreline buildings as yet more human follies.

References:

Another useful reference is: