EM 1110-2-1100 (Part V)
31 Jul 2003
surfaces produce less effects than vertical, impermeable walls. Scour and scour protection is covered in detail
in Part VI-5-6.
(d) Impacts on laterally adjacent beach. Perhaps the key environmental concern is how a seawall affects
a neighbor beach with no armoring. Does the wall create end-of-wall or flanking effects, i.e., R (x,t) greater
than unity? Two studies are often cited to demonstrate flanking effects. Walton and Sensabaugh (1979)
provide posthurricane Eloise field observations (14 data points) of additional bluff (contour) recession
adjacent to seawalls in Florida. McDougal, Sturtevant, and Komar (1987) and Komar and McDougal (1988)
present small scale, equilibrium beach, laboratory measurements (nine data points) for 7-14-cm waves at
1.1-sec periods normal to a median grain-size, sandy beach. The 23 data points are then combined to
demonstrate the excess flanking erosion. The extent and length of the excess erosion is related to seawall
length and is explained in terms of the seawall denying sand to the littoral system (e.g., Dean 1987).
However, other mechanisms may be responsible. If the seawall extends seaward, it may act like a
groin to cause downdrift impacts. Tait and Griggs (1991) measured an area of lowered beach profile
extending 150 m downcoast at Site No. 4 in California. They proved that the upcoast end of the wall
produced sand impoundment or a groin effect. Toue and Wang (1990) conducted laboratory
experiments with waves attacking walled and nonwalled beaches at angles and concluded that
downdrift impacts were a groin effect.
Plant (1990) and Plant and Griggs (1992) observed rip currents at interior sections and at the ends
of armored sections. These rip currents were attributed to wave overtopping, return flows and
elevated, beach water tables during storms. McDougal, Sturtevant, and Komar (1987) also observed
rip currents in their model tests previously described and from field evidence in Oregon. They
concluded that this mechanism may be more responsible for end-of-wall, flanking effects than the
sand trapping theory of Dean (1987).
Griggs et al. (1997) discuss eight full years of field monitoring including the intense winter storm of
January 1995. This storm did not produce end scour on the control beach at Site No. 4. They
concluded from a comparison of summer and winter beach profiles at beaches with seawalls and on
adjacent, control beaches, that no significant long-term effects were revealed.
Basco et al. (1997) summarize the results of 15 years of profile survey data with 8-9 years taken
before seawall construction at Sandbridge, Virginia, on the Atlantic Ocean. The shoreline has been
eroding on average 2m/year (Everts, Battley, and Gibson 1983) long before wall construction began.
One part of the study used five years of monthly and poststorm profile data at 28 locations
(62 percent walled; 38 percent nonwalled) of the 7,670 m study reach. They concluded that the
volume erosion rate was not higher in front of seawalls. However, seasonal variability of sand
volume was slightly greater in front of the walled locations. Winter waves drag more sand offshore
in front of walls, but summer swell waves pile more sand up against walls in beach rebuilding.
Walled sections recovered about the same time as nonwalled beaches for both seasonal transitions
(winter to summer) and following erosional storm events. These results were for a weighted average
of total sand volume (subaerial) in front of the walled section and seaward of a partition for the
nonwalled beach sections.
At individual profile locations adjacent to walls, using the full 15 years of data, Rv values varied
considerably. The evidence for any long-term, end-of-wall effects were considered inclusive for
Sandbridge beach. There was never evidence of flanking effects after storms on adjacent beaches
(Basco et al. 1997). This study continues. In general, Basco et al. (1997) have confirmed all the
conclusions of Dean (1987), Kraus (1988) and Kraus and McDougal (1996) except the end-wall,
flanking effect.
Shore Protection Projects
V-3-33
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