Paper No. 0
Presentation Time: 1:00 PM-4:00 PM
BEACH WIDTH AND PROFILE COMPARISON BETWEEN NATURAL BEACHES AND BEACHES FRONTING SEAWALLS: PUERTO RICO
Controversy exists over the impact of seawalls on beaches. Few studies have addressed the problem. One study was a master's thesis (Wright, 1989) in which 230 profiles were taken along 48 seawalls and adjacent unwalled beaches around Puerto Rico using the Emery (1961) stake and horizon method. In Summer, 2000, approximately one-half of Wright's sites were reoccupied to test if his results could be replicated, and to evaluate trends of erosion/accretion for these beaches over a decade time span. Although only 45% of Wright's profile sites were reoccupied, preliminary results replicate his overall conclusions. Dry beach widths are narrower in front of seawalls than for unstabilized beaches in 97% of the locations. No dry beach exists in front of 55% of the shoreline stabilization structures, and many lack any beach. Many of the beaches are steepening and narrowing. The three types of beach loss in front of seawalls include loss due to placement of the wall on the beach, active loss due to wave interaction with the wall, and passive loss where a wall is placed at the back of the beach, but becomes active as the shoreline retreats into the wall. For many of Puerto Rico's seawalls, no historical data exists for the conditions at the time of seawall construction, but this study supports the conclusion that seawalls contribute to beach loss. Owing to the increased beach steepness and dry beach narrowing, many properties are at higher risk for damage today than 12 years ago. This trend likely will continue. Shoreline management in the 21st Century will need to consider that seawalls may actually increase risk of property damage and that alternatives such as retreating from the shoreline must be considered. Critical erosion areas delineated based on the new data set will aid in coastal-zone planning and shoreline management.