EM 1110-2-1100 (Part V)
31 Jul 2003
(3) Wave reflection and transmission through and over sloping structures and beneath vertical wave
screens is also found in Part VI-5-2.
c. Interaction with adjacent beaches.
There is a common perception that "... seawalls increase erosion and destroy the beach." The limited
available evidence is examined in this section. The term seawall herein means any type of coastal armoring
that hardens the shoreline to a fixed position, hence, also applies to bulkheads and revetments.
(1) Background. Concern with how seawalls interact with adjacent beaches can be traced to events
in the 1960s and coastal geology studies on the origins and movements of barrier islands (Hoyt 1967).
Barrier islands are one of the 11 types of land/water interfaces on earth (Shepard 1976). Barrier beach
systems make up about 35 percent of the United States coast stretching from Maine to Texas. They protect
the bays and estuaries that lie behind them from direct wave attack, but are dynamic systems with sand
volumes that depend on changing ocean conditions, sand supplies, and control boundaries that define the
volume.
(a) As depicted schematically in Figure V-3-9a (adapted from Dolan and Lins 1987), barrier islands are
commonly perceived to migrate landward with constant volume as sea level rise continues. Storm surge with
high waves produce sand overwash into the back bay. The barrier is said to roll over itself, shoreline
movement is termed recession, and no volume change means no coastal erosion. Some scientific evidence
disputes the rollover model. Leatherman (1988) used shoreline position data to show that tidal inlet formation
processes dominate and move far greater sediment quantities over the long term. The migration model also
requires the moving sand volume to overlay continuous, basal peat layer from the muds and plants in the
lagoon. Stratigraphic evidence contradicts this important aspect along the East Coast of the United States
(Oertel et al. 1992). Using the Bruun (1962) rule, a 1-2 mm/year rise in sea level translates to about 0.05-
0.2m/year shoreline retreat rate. These are relatively small changes in shoreline position and herein labeled
as those at geologic time scales. See Part IV-2-9 for a full discussion of marine depositional coasts and
barriers.
(b) When man enters the picture by constructing a road on the shore, he establishes a fixed reference line.
The shoreline position relative to the road decreases in time as depicted in Figure V-3-9b. Once development
has been permitted, continued erosion may threaten man's artifacts (roads, buildings, bridges, etc.) and some
type of shore protection may be undertaken such as seawall construction. These structures are not intended
to protect the beach, but areas landward from the beach. Armoring provides a nonmoving reference point
on the beach to make the existing, historic erosion more noticeable. Few argue that the road alone is
"...destroying the beach", but this same logic is applied by some when a revetment or seawall is present on
an eroding shoreline and the dry beach width is reduced each year in front of the hardened shoreline (Pilkey
and Wright 1988). Eventually, the ocean will reach the seawall (and road) and the dry beach will be gone.
(c) As also depicted in Figure V-3-9b, a seawall traps sediment behind the structure, reduces overwash
and fixes the shoreline position. Continued erosional stress over time acts to deepen the water depth at the
structure that is of concern for structural design. The trapped sediment formerly in the dune, bluff or cliff)
is removed from that available to contribute to subaqueous bar building during storms. This trapped material
is also prevented from contributing to the longshore sediment transport processes along the coast and may
alter the sediment budget. The volume trapped relative to that naturally active in the cross-shore profile will
be discussed further in the following paragraphs.
V-3-28
Shore Protection Projects
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