Erosion of Shorelines
The erosion of shorelines is a natural process that can have beneficial or adverse impacts on the creation and maintenance of habitats. Sands and gravels eroded from the shores of coastal bays maintain the beach as a natural barrier between the open water and coastal wetlands. Beaches move back and forth onshore, offshore and along shore with changing wave conditions. The finer-grained silts and clays derived from the erosion of shorelines are sorted and carried as far as the waters of wetlands or tidal flats, where benefits are derived from addition of the new material. However, excessively high sediment loads can smother submerged aquatic vegetation beds, cover shellfish beds and tidal flats, fill in riffle pools, and contribute to increased levels of turbidity and nutrients (http://www.epa.gov/OWOW/NPS/MMGI/Chapter6/ch6-4.html).
Longshore drift is the movement of sand parallel to the shoreline, in the “along-the-shore” direction (H. Nepf). Longshore drift is caused by the water waves breaking against the shore. As the waves break, they thrust water forward, creating movement of water in the direction the wave is traveling. If a wave approaches a beach at an angle, the forward rush of water is directed partially parallel and partially perpendicular to the shore. The parallel component of motion creates the longshore current, a steady movement of water parallel to the shoreline, that carries sand and contributes to the longshore drift. The longshore current is confined to the region where the waves break, called the surf zone (the frothy, white water created by the breaking waves).
In regions of strong wave activity the longshore drift steadily carries sand away, eroding the coast. In regions of weak wave activity the longshore drift is halted and its sediment load deposited, so the sand settles to the ocean floor and remains there. The longshore drift shapes the coastline by carrying sand from sites of high wave activity to those of low wave activity. The volume of sand carried away from or delivered to different points along the coast can be as much as 2,000 cubic meters per day (71,000 cubic feet), enough sand to fill an Olympic-sized swimming pool (Nepf).
The seepage of ground water and the overland flow of surface water runoff also contribute to the erosion of shorelines (http://www.epa.gov/OWOW/NPS/MMGI/Chapter6/ch6-4.html). The role of ground water is most important wherever permeable subsurface layers of sand are exposed in high bluffs along coastal bays. In these areas, the seepage of ground water into the waterway can cause erosion at the point of exit. The surface flow of upland runoff can also dislodge sediments through the creation of rills and gullies on the shoreline banks and bluffs (http://www.epa.gov/OWOW/NPS/MMGI/Chapter6/ch6-4.html).
Some amount of natural erosion is necessary to provide the sediment for beaches in estuaries and coastal bays. However, excessive erosion has...