EM 1110-2-1100 (Part V)
31 Jul 2003
V-3-3. Beach Stabilization Structures
a. Naturally stable shorelines. Part IV-2 classifies coasts and their morphology. Marine depositional
coasts with barriers and beaches are one of the most widely distributed geomorphic forms around the world.
(Part IV-2-9, 10). They form a flexible buffer zone between land and sea. The subaerial beach has two major
zones. The foreshore extends from the low-water line to the limit of wave uprush at high water. At this point,
the backshore extends to the normal landward limit of storm wave effects. This landward limit is usually
marked by a foredune, cliff, structure, or seaward extent of permanent vegetation. The backshore is only
affected during storms when surges and high waves transport backshore sediments. This exposed, subaerial
beach definition is accepted by the general public. Some authors include the surf zone and bars out to closure
depth, i.e., the subaqueous part of the beach, because both parts, subaerial and subaqueous, exchange
sediment. Beaches may stretch for hundreds of kilometers or others, called pocket beaches, are restricted by
headlands and are only tens of meters in length. Figure IV-3-31 schematically summarizes factors controlling
morphodynamics along a range of coastal environments extending from rocky, to noncohesive sediments to
cohesive shorelines.
(1) Many beaches are naturally stable. In general, wide beaches are exposed to more severe wave
conditions at that location, but the relationship between beach width (or section volume) and storm energy
for naturally stable shorelines has yet to be determined. Figure V-3-10a displays a stable, pocket beach on
Bruny Island, Tasmania, Australia, where beach width increases along the more exposed section of coast
(from Silvester and Hsu 1993). The protected reach behind the headland is much narrower than that receiving
a direct attack by large waves during storms. The dark area is vegetation and landward limit of the backshore,
subject to normal storm wave effects. If this photo were taken at high tide, a minimum beach width for a
stable shoreline could be determined.
(2) This concept of a minimum beach width (or volume) is schematically illustrated in Figure V-3-10b.
The volume of sediment present protects the uplands (foredune, cliff, structure, or vegetation) from damage
under normal or average storm conditions. The landward boundary of the backshore is a reference baseline
for shore protection. On eroding beaches, the backshore may be missing, and the high-water uprush may
impinge directly on cliffs or structures. Both natural and anthropogenic agents may cause the erosion. But
a minimum, beach width is still necessary for natural shore protection at the eroding site.
b. Minimum dry beach width. Professor Richard Silvester in an article on the stabilization of
sedimentary coastlines (Silvester 1960) wrote:
"...to allow for storm-cycles and the short-term reversals of drift, a sufficient width of beach
should be allowed as working capital on which the sea can operate. Once the coast has been
stabilized, by preventing the net movement of sediment, no long-term erosion need be
anticipated and the `active' beach width can be minimized." (p. 469)
(1) As illustrated in Figure V-3-11a,b, for both naturally open beaches and pocket beaches between
headlands, the minimum, dry beach width, Ymin is defined as the horizontal distance between the mean
highwater (mhw) shoreline and the landward boundary or base (reference) line. The mhw shoreline is
employed because it is the common, land/water boundary shoreline on maps; it is more readily identified from
aerial photos; and it is a more conservative, minimum width (and volume) for shore protection. It is the
minimum, dry beach width required to protect the foredune, cliff, structure, or vegetation behind the baseline
from normal storm conditions. The beach does the work, and it's resilience and recovery are critical for long-
term shore protection.
Shore Protection Projects
V-3-35
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