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
variations 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 longshore
transport rate is the total amount of sand that annually moves past a point regardless of direction. The net
transport rate is the quantity moved in one direction minus that moved in the other 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 mechanism seldom occurs along sheltered shorelines.
The Effects of Erosion
The most obvious and noticeable effect of erosion is the loss of shor6front property. Less apparent
are the increases in sedimentation caused by erosion in adjoining areas since all materials eroded from a
shoreline at one point are eventually deposited elsewhere. It is likely this will occur in deeper water such
as a navigation channel crossing or closely paralleling the shore. This can be as serious a problem, in
terms of total utilization of the shoreline, as the eroding property. All possible effects of increasing or
decreasing sediment movement by any actions should be carefully considered. Significant effects of
either kind, will probably make it impossible to obtain required federal and state permits.
THE SHORELINE EROSION CONTROL DEMONSTRATION PROGRAM
From 1975 to 1980, the U. S. Army Corps of Engineers conducts a program to develop and
demonstrate methods of low cost shore protection. This program was mandated by Section 54 of Public
Law 93-251, the Shoreline Erosion Control Demonstration Act of 1974. Working with the Soil
Conservation Service, the Corps designated 16 demonstration sites throughout the Atlantic, Gulf, an
Pacific coasts, Alaska and the Great Lakes. The sites were chose because they represented a broad cross
section of possible shoreform and climatic conditions. At each of these sites, various structures and
kinds of vegetation were established to demonstrate their effectiveness (or ineffectiveness) in the local
environment. Twenty-one additional sites were chosen where shore protection devices existed that had
previously been established by others. All 37 sites were intensively monitored over a period of months.
The results obtained by observations at these sites are generally applicable to other sites located
throughout the country.
The program demonstrated that commonly available material (e.g., timber, concrete, stone, etc.),
when integrated in structures that adhere to sound design principles, can produce successful protective
devices that meet the aforementioned criteria for low cost. Violation of proper design principles, or use
of no durable materials, led to inferior performance in almost all cases The program also showed that