The shore protection qualities of marshes are particularly important when they provide a buffer
zone in front of a sandy beach or other area vulnerable to erosion. While not providing full protection,
they effectively diminish wave energy and allow for less massive and costly back protection.
The Causes of Erosion
Wave Action. This is the most obvious cause of erosion.
Littoral Material Supply. Waves keep the littoral materials constantly moving downdrift. As long
as equal quantities of material are transported from the updrift direction, the shoreline remains stable.
when the updrift supply exceeds the amount moving downdrift, the shoreline accretes (material
accumulates). However, when the updrift supply is deficient, the shoreline retreats.
Much of the littoral material supplied to shorelines results from updrift erosion. Therefore, if large
amounts of updrift shoreline are suddenly protected, material is lost to the littoral system. This decreases
the supply to the downdrift shores, resulting in erosion problems unless they are also protected.
Determining the transport direction is necessary in some cases but usually difficult because of
variation in wave directions throughout the year. Summer winds (and waves) may be primarily from one
direction, while winter storm winds may come from an entirely different quadrant. When winds and
waves change direction, the transport direction also changes (transport reversal).
The gross transport rate is the total amount of sand that annually moves past a point in either
direction. The net transport rate is the quantity moved in one direction minus that moved in the opposite
direction. For example, if the amount of sand moved in one direction in one year was equal to the
amount moved in the other direction, the net transport rate would be zero.
Wind. Wind is a problem where large volumes of sand may be transported by prevailing breezes
to form dunes. This seldom occurs along sheltered shorelines.
The Effects of Erosion
The most obvious and noticeable effect of erosion is the loss of shorefront property. Less apparent
are the increases in sedimentation caused by erosion in adjoining areas since all materials eroded from
the shoreline at one point are eventually deposited elsewhere. If this were an individual's beach, the
additional material would be welcomed. However, it is more likely that deposition would occur in
deeper water, such as a navigation channel crossing or closely paralleling the shoreline. You should
carefully consider, therefore, all possible effects of increasing or decreasing sediment movement by your
actions. Significant effects of either kind will probably make it impossible to obtain required federal and
state permits.
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