EM 1110-2-1100 (Part V)
31 Jul 2003
Kraus (1988) reviewed over 100 references (laboratory, field, theory, and conceptual studies) to make a
thorough examination of the literature. This review and seven companion papers are presented in Kraus and
Pilkey (eds. 1988). An updated literature review is found in Kraus and McDougal (1996) who examined 40
additional papers. In general, these extensive literature reviews agreed with Dean (1987) regarding which
concerns were probably false and which many are true. The interested reader should consult these references
for all the details.
(b) Definitions. The natural, background shoreline erosion rate, PN and the rate after human
activities PA can define a coastal erosion ratio, RP
PA
Rp(x,t) '
(V-3-3)
PN
where the subscript Rp means shoreline position is used to define R. If profile data are available, then actual,
coastal erosion volume could be employed to find a volume ratio, RV as
VA
Rv(x,t) '
(V-3-4)
VN
where VN is the natural erosion (volume loss) rate and VA is the volume loss rate after construction
of roads, seawalls, etc. at a given location. Clearly, if Rv (or Rp) is proven greater than unity under
similar climatological conditions, then we may conclude that armoring has increased the natural,
historical conditions at the site. The level of impact (if any) on the frontal and laterally adjacent
beaches (1 percent, 5 percent, 10 percent, 50 percent, etc.) needs quantification. Pilkey and Wright
(1988) use the terms passive and active erosion of the beach to distinguish between the perceived
versus real natural and manmade causes, respectively.
The volume of sediment trapped behind a seawall depends upon its position on the beach, crest
elevation and length. Weggel (1988) defined six types of seawalls depending on their location on
the beach and water depth at the toe. At one extreme (type 1) the wall is located landward of the
limit of storm wave runup to have zero impact. At the other extreme (type 6) walls are located
seaward of the normal breaker line. Types 2-5 lie in between and are said to have increasing effects
on coastal sediment processes as the type number increases. Storm surges can create all six type
conditions during a single storm event. Coastal erosion may also gradually alter the types.
Dean (1987) postulated that the sediment trapped behind the wall resulted in an excess erosional
stress to produce toe scour and excess erosion on unprotected adjacent property.
(c) Frontal impacts. Beach profile change, toe scour during storms and nearshore bar differences have
been attributed to seawalls. Conventional wisdom has been that these impacts were due to wave reflection.
Kraus and McDougal (1996) studied the field results by Griggs et al. (1997); laboratory work by Barnett and
Wang (1998) and Moody and Madsen (1995) and their own research in the SUPERTANK (large scale)
seawall tests (McDougal, Kraus, and Ajiwibowo 1996) to conclude that reflection is not a significant factor
in profile change or toe scour. In the field, toe scour is more dependent on local, sediment transport gradients
and the return of overtopping water (through permeable revetments or beneath walls) than a result of direct,
cross-section wave action. Their conclusions also negate the common perception that sloping and permeable
V-3-32
Shore Protection Projects
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