LONG-TERM BEACH RESPONSE TO THE PRESENCE OF A SEAWALL ON A NON-ERODING BEACH

The impact of seawalls on the beach remains an important research issue after more than a decade of controversy. Some investigators have suggested that the interaction of seawalls with beach sediment-dynamic and hydrodynamic processes can induce beach erosion in the vicinity of the seawall. A recent field experiment supported this notion by comparing in situ measurements of suspended sediment concentration and longshore current velocities in front of a seawall and on an adjacent unaltered beach.

Eight years of biweekly to monthly beach profile surveys were conducted in front of a seawall and on two adjacent control beaches along a non-eroding beach in Monterey Bay on the central California coast. Average littoral drift rate near the study site was estimated to be between 200,000 and 250,000 m³/yr. The seawall is situated well-seaward (75 m) on the beach profile and interacts with storm waves every winter. Robust statistical analysis of the profile data indicates that any cumulative impacts due to the presence of the seawall were smaller than the natural alongshore variability of beach width and volume during the study period. This suggests that under the conditions of a stable beach and a high rate of littoral drift, seasonal impacts of a seawall on the beach may have limited or no significant long-term influence.

This information is important to coastal managers who may be faced with difficult and costly choices between allowing protection of coastal property and preserving public resources. Decisions involving seawall permitting should optimally be informed by geological data concerning long-term beach stability, littoral drift rates, and natural profile variability as well as by traditional engineering parameters such as run-up elevation.