CETN-III-11
3/81
swells, which originate from distance storms, tend to rebuild the beach.
On most beaches, there is a constant erosion process of beach material by
local storm waves followed by gradual rebuilding by swells. A series of
violent local storms in a short time can result in severe erosion of the
shore if there is not enough time between storms for swells to rebuild,the
beach. Alternate erosion and accretion may be seasonal on some beaches;
the winter storms erode the beach, and the summer swells rebuild it.
There
are stretches of shoreline (e.g. California) though, where littoral drift
moving offshore and along the coast falls into submarine canyons and thus
does not return to build back the beach.
STRUCTURAL ASPECTS: The following design parameters must be chosen to suit
the beach's function and site.
1 . Berm Elevation. The berm elevation is chosen to reduce the occurrence
of overtopping by waves and storm tides.
A dune may be added landward of
the protective beach to provide additional protection against hurricane
wave attack. Information on tide elevations and wave data are needed to
evaluate the limitations on overtopping imposed by a proposed beach berm
elevation.
Berm Width. Berm width is chosen to provide protection for a selected
2.
The predicted erosion rates
number of years before refill is necessary.
of fill material over the long term and when first placed must be esti-
mated to determine the desired width.
The long term rate can be calcu-
lated by applying the "Renourishment Factor," discussed in Chapter 5 of the
Shore Protection Manual (SPM), to the erosion rate of the native beach
sand. When new fill is first placed, much of the fine-grained material
will be washed out immediately.
To account for this early loss, extra fill
must be added during placement.
The additional berm width is calculated
from the required volume, which in turn is calculated using the "Adjusted
Fill Factor" described in Chapter 5 of the SPM. Both the "Renourishment
Factor" and "Adjusted Fill Factor" are determined by comparing the grain
size distributions of the native and potential fill sands.
Site condition,
such as a steep offshore beach slope, may limit the location of the seaward
edge of the fill and consequently dictate use of a narrower berm than de-
sired. On the other hand, the berm may be widened to meet greater recrea-
tional or other demands.
Previous Page
