Changes in the sediment supply from updrift will also cause movement of the profile. A decrease
in th6 supply will cause the beach to erode, and the profile will retreat landward, while still maintaining
the same shape. Conversely, an increase in the supply will cause accretion and the profile will advance
toward the water.
Wetlands and Marshes
Although they are treated separately in this section, wetlands and marshes usually occur in
combination with sand beaches or low erodible plains. For federal regulatory purposes, wetlands are
defined as:
Those areas that are inundated or saturated by surface or groundwater at a frequency and duration
sufficient to support, and under normal circumstances do support, a prevalence of vegetation
typically adapted to life in saturated soil conditions. Wetlands generally include swamps, marshes,
bogs, and similar areas. [U. S. Army Corps of Engineers (1977b)]
Marsh plants are primarily herbaceous (lack woody stems) and include grasses, sedges and rushes.
The species present depend on location and whether the marsh is low (regularly flooded) or high
(irregularly flooded).
Until recently, marshes were considered undesirable and regularly drained and filled for new
development or agriculture. Their value has now been recognized as an important environmental
resource, but they also protect the shore by absorbing the energy of approaching waves and trapping
sediment that is being carried along by currents. These shore protection qualities are particularly
important when the marsh fronts a sandy beach or other area where erosion is to be prevented. In that
case, the marsh provides a front line of defense for the shore. While it may not provide full protection, it
may, at least, partially dampen wave action and allow for less massive and costly backup protection.
THE CAUSES OF EROSION
Wave Action
Wave action is the most obvious cause of shoreline erosion.
Littoral Material Supply
Stable shorelines are in a state of dynamic equilibrium. Waves keep the littoral materials in
constant motion in the downdrift direction, and the shoreline remains stable provided there is an equal
supply of material from updrift. When the updrift supply is deficient, the shoreline erodes.
A substantial portion of the littoral material supplied to shorelines is the result of updrift erosion.
If large amounts of the updrift shoreline are suddenly protected, material is lost to the littoral system.
This decreases the supply to the downdrift shore, resulting in erosion problems unless that land is also
protected.
Determining the longshore transport direction is sometimes necessary. This is usually a difficult
task because it depends on wave directions that can vary considerably with the seasons. Summer winds
(and waves) may be from one predominant direction, while winter storm winds may be from an entirely
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