ERDC/CHL CHETN-II-44
September 2001
(1989) demonstrated that these segments of the shoreline erode at much higher rates and in some
instances can be classified as EHSs. Further, Moody (1964) and Kraft et al. (1975) established
that these sections of the shoreline erode at significantly higher rates and are frequently breached
by tidal inlets. Leatherman et al. (1989) confirmed similar behavior at Ocean City, MD. In this
case wave refraction induced by a large linear sand ridge caused higher rates of erosion in the
vicinity of the 1933 Ocean City Inlet breach site. Galgano (1998) demonstrated that the area of
the inlet breach experienced rates of erosion double that of the surrounding beaches. Mohr,
Pope, and McClung (1999) showed that variable erosion rates along a beach nourishment project
on Presque Isle, Pennsylvania (Lake Erie) were related to irregularities in offshore bathymetry.
Borrow Pit Located within Active Profile Zone: A nearshore borrow site, such as created
by mining the fillet at an updrift jetty, may be located in the active profile, shallower than the
average annual depth of closure, h*. In this situation, sediment moving offshore during storms
will tend to fill the borrow pit until the equilibrium profile is achieved. Filling of the pit will
translate the profile landward.
In Figure 3, the solid line denotes the equilibrium profile that would result in the absence of the
borrow pit, and the dashed line denotes the profile after adjustment to fill the borrow pit by
cross-shore transport. Suppose a volume V per unit length alongshore is removed from a borrow
site. It can be shown by equating volumes that the profile will translate landward a distance ∆x ≅
V/h* under the assumption that the distance translated is small compared to the distance from the
shoreline to the location of the depth of active movement. For example, if V = 30 m3/m and h*. =
6 m, then ∆x = 5 m. The adjustment is expected to take several months to years, depending on
where the pit is located and possible infilling by longshore processes.
∆x
V
h*
V
Borrow Pit
Figure 3. Definition sketch for borrow
site located on active profile
Updrift Barrier: Littoral processes at tidal inlets are the most dynamic and complex element of
the barrier island system, and beaches located directly downdrift of inlets have long been viewed
as high-erosion areas (i.e., EHSs). Both natural and stabilized inlets exert influence along
adjacent shorelines and within large-scale coastal compartments. At stabilized inlets, jetties
extend offshore for hundreds of meters and block the transport of sediment, inducing a notable
response on the adjacent beaches, and potentially altering the configuration of a natural barrier
island (e.g., Assateague Island, MD). Likewise, the tidal jet produced by an unstabilized inlet
can disrupt the flow of littoral sand and modify the coastal configuration for tens of kilometers
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