A seawall is a large, expensive concrete wall designed to protect buildings or other man-made structures from beach erosion. A revetment is a cheaper option constructed with "rip rap" such as large boulders, concrete rubble, or even old tires and other junk. Although these structures may serve to protect beachfront property for a while, the resulting disruption of the natural coastal processes has serious consequences for all the beaches in the area. Coastal communities around the world have lost their recreational beaches to seawalls and revetments.
Why do seawalls destroy beaches? A seawall inhibits the natural ability of the beach to adjust its slope to the ever changing ocean wave conditions. Large waves wash up against the seawall and rebound back out to sea carrying precious beach sand with them. With each storm the beach narrows, sand is lost to deeper water, and the longshore current scours the base of the wall. Eventually large waves impact the seawall with such force that a bigger structure becomes necessary to continue to resist the forces of the ocean.
A hardened shoreline is very different from a sandy beach. With the beach gone, access to the waterís edge becomes hindered by the rock and concrete structures. There is no safe area to walk the shoreline (except along the top of the seawall), and recreational activities such as swimming and surfing become hazardous.
Awareness of the long term effects of "hard" structures on the coast has made "soft" solutions like sand replenishment more popular. After all, seawalls do not protect beaches, they protect property... and what is the point of beachfront property if there is no beach?
Reference: The Corps and the Shore, Orrin Pilkey and Katharine Dixon, Island Press, 1996.
An example of this situation occurs along the shoreline known as the Rincon Parkway between Rincon point and Emma Wood State Park. The eroding coastal cliffs in this area once served as one of the primary sources of sand for Ventura area beaches (the other source was the Ventura river). Coastal development and the construction of Highway 101 prompted coastal hardening, and today most of the Rincon coast consists of seawalls or rock revetments. This 12.5 mile stretch of coast that once supplied sand to downdrift beaches in Ventura is now sealed in concrete.
To make matters worse, reflected waves in front of the seawalls move sand offshore and out of reach of longshore currents. Along the Rincon Parkway over 20% of the longshore transport is lost in this manner, and this shortfall also contributes to the shrinking beaches in Ventura.
Manís attempts to interrupt the natural erosion of the shoreline has had many unforeseen consequences. Armed with a better understanding of coastal processes from past mistakes policy makers will hopefully look to other solutions to our currrent beach erosion problems. Some alternatives to hard structures are "soft" solutions such as sand replenishment, or relocating the threatened structures. Perhaps the best approach is to have the foresight to build far enough away from the influence of shoreline processes that erosion does not become a threat.