EM 1110-2-1100 (Part V)
31 Jul 2003
Figure V-4-23 illustrates an example of this type of hot spot at Folly Beach, South Carolina. A beach-fill was
constructed along much of the island, and high loss rates were observed locally at the place where the
shoreline noticeably changes orientation and creates a convex curve relative to the incident waves (Ebersole,
Neilans, and Dowd 1996). Prior to construction of the nourishment project, this area was known as the wash-
out area, suggesting an area that previously experienced high erosion rates. Local knowledge or an analysis
of historical shoreline change rates may help identify such a zone. However, areas that have historically
suffered high sand loss rates may be presently armored with seawalls or revetments, or controlled by some
other structures such as groins, thereby pinning the shoreline position and masking the presence of a future
hot spot. Any area in which the shoreline has a bulge or convex shape, relative to the incident waves, is a
potential candidate for this type of hot spot.
(b) The presence of a submerged offshore shoal, or some other highly irregular bathymetric feature such
as an underwater canyon, can also be a cause of erosion hot spots. Morphologic features can alter the
propagation of incident waves in a persistent manner, creating strong gradients in net longshore sand
transport. Stauble (1994) documented the occurrence of multiple erosional hot spots in a beach nourishment
project constructed at Ocean City, Maryland, and hypothesized that the cause of the hot spots was the
presence of shore-attached finger shoals that characterize the nearshore bathymetry off Ocean City (see Figure
V-4-24). The finger shoals appear as lighter-shaded areas. Stauble also identified corollary cold spots, or
areas of unusual sand accumulation, which were formed by the sand that was transported out of the hot spots.
Smith and Ebersole (1997) showed that the positions of the hot spots/cold spots were well-correlated to zones
of divergent/convergent potential longshore sand transport caused by persistent changes to nearshore wave
patterns induced by the shoals. The shoals act to alternately focus and spread wave energy due to the process
of wave refraction. Results from the application of a wave transformation model (see Part II-3) were used
to compute potential longshore transport rates. Figure V-4-25 shows the variation in transport rates for Ocean
City, Maryland. Positive rates are directed to the south, negative to the north. Hot spots correspond to strong
uphill gradients (e.g., changes from low southerly transport rates to high southerly transport rates). Cold spots
occur at downhill gradients.
(c) Just as natural bathymetric irregularities can lead to the development of a hot spot, an excavated
borrow area or a submerged mound constructed out of dredged material can produce the same result. If the
manmade feature results in a significant change in bottom relief and is located in water depths where the
waves feel the change in depth, persistent longshore sand transport gradients may develop. Hot spots can also
form in areas where the incident wave directions are constrained to a narrow window and the area is blocked
or sheltered by some land feature or coastal structure, creating a gradient in wave energy and longshore sand
transport.
(d) A third type of hot spot can result as a consequence of how a project is designed and built to protect
existing structures and infrastructure. Typically beach nourishment projects are built in areas where structures
and infrastructure are vulnerable to storm-induced damage. Oftentimes, the project site already has
revetments or seawalls that have been built by private landowners. Both protected and unprotected structures
may protrude beyond the natural prevailing shoreline position, thereby creating a very irregular, and
unnatural, shoreline. It may be difficult to estimate where the prevailing natural shoreline position would be
if the structures were not present.
(e) If a project is designed and constructed to "wrap around" protruding structures, in an attempt to create
a uniform width of beach in front of each structure, the beach will most likely readjust following
construction. The readjustment process will lead to a more stable arrangement of sand in which the design
width may not be retained in front of the seaward most structures, whereas the beach width may exceed the
design width in other areas. Figure V-4-26 illustrates this process. An offshore depth contour equal to the
V-4-62
Beach Fill Design
Previous Page
