CETN III-46
Coastal Engineering
Technical Note
SUMMARY OF SEAWALL AND BEACH INTERACTION
AT NORTHERN MONTEREY BAY, CALIFORNIA
PURPOSE:
To summarize results of recent field studies on the effects of
coastal armoring on beaches along northern Monterey Bay, California.
BACKGROUND;
Engineers and scientists are studying seawalls, revetments and
bulkheads to define the effects of coastal armoring on beaches. Seawalls,
revetments and bulkheads are structures built to protect a coastal community
against wave attack and flooding during severe storms, since there are few
other structural alternatives available to provide the same degree of
p r o t e c t i o n . Throughout the remainder of this document seawalls, revetments
and bulkheads will be grouped together and referred to simply as seawalls.
The failure of some seawalls and rapid erosion of the foreshore in the
vicinity of some seawalls has prompted controversy as to whether seawalls
protect or damage natural beaches. Coastal planners and managers become
increasingly reluctant to consider these structures for their coastal defense
applications.
Both North Carolina and Maine have legislation prohibiting the
construction of seawalls..
In other states such as Texas, Florida, and
M a s s a c h u s e t t s , construction of seawalls or other types of hard structures
along the coastline is severely restricted.
Kraus (1987,1988) conducted extensive literature reviews and concluded
that "beaches with and without seawalls exhibit similar behavior and variation
with regard to short-term erosion and recovery associated with storms and
post-storm wave conditions." He further pointed out that "seawalls are
relatively innocuous with regard to cross-shore sediment transport processes
and only have potential to damage neighboring beaches if longshore processes
are interrupted." Accordingly, a "properly" designed and engineered seawall
should not cause or accelerate beach erosion provided a sediment supply
exists.
Structures improperly designed and sited can adversely impact beaches
adjacent to and/or in front of structures. These potential impacts are (1)
accelerating or enhancing beach erosion in the form of either lowering the
beach profile or causing toe scour during wave attack and (2) causing
downdrift flanking and updrift accretion. On a receding beach, hard
structures interfere with nearshore sediment processes if the shoreline
retreats to the proximity of the structures. In this case, corrective
measures such as beach nourishment can be effective to mitigate the
structure/beach
interactions.
Structural impacts on beaches can be minimized or avoided through
comprehensive analyses of site-specific wave climate and coastal processes
along with sound engineering judgement and design practice. To develop
effective functional design guidance, the research Work Unit, "Engineering
1
US Army Engineer Waterways Experiment Station, Coastal Engineering Research Center
3909 Halls Ferry Road, Vicksburg, Mississippi 39180-6199
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