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Environmental Issues - Shoreline Change and Shoreline Change Rates Contributor: Kindinger
Diverse and complex natural processes within the coastal zone serve to continually change the physical, chemical, and biological features of fragile coastlines. Regional and local shoreline characteristics control the differing interactions and relative consequences of these natural processes. Small vertical changes in sea level can impact coastlines dramatically, especially on gently sloping coasts such as those found along the Gulf of Mexico and in the lakes of Pontchartrain Basin. Natural processes that influence shoreline changes include waves, tides, littoral currents, water depth, sea-level rise, subsidence, severe storm events (such as hurricanes), and sediment transport.
Human activities in the coastal zone add yet another dimension affecting changes to our coastlines. Activities that modify and disturb the natural processes in the coastal environment can directly and indirectly impact rates of shoreline erosion and wetland loss. Sediment starvation, sediment trapping, water level changes, ship-generated waves, pollution, dredging, and overall coastal development influence the rates of coastal erosion.
Loss of the Pontchartrain Basin's coastal wetlands and barrier shorelines is well recognized by government agencies, industry, universities, and the public. Between 1930 and 1990 Pontchartrain Basin lost over 188,000 acres of land due to a complex suite of causes. Land loss is typically the result of complex interactions among natural and human activities upon the landscape. Therefore, it is difficult to isolate an activity as the singular cause of a specific area of land loss. However, general assumptions can be made for most areas regarding the primary process responsible for erosion, as well as the actions and catalysts that initiated the process. To interpret the process results and determine the major causes of coastal land loss requires mapping the location of land loss, quantifying the spatial and temporal magnitude of land loss, and identifying significant historical trends in land loss rates
Shorelines compiled in these maps were derived from either topographic or near-vertical aerial surveys conducted between 1899 and 1995 (Figure 4, a, b and c). The high-water line is used as the official shoreline on cartographic data and is interpreted and determined on near-vertical aerial photographs according to the location of the wet and dry beach contact or the high-water line. To evaluate change in shoreline position, shore-normal transects were constructed at approximately two minute intervals of longitude or latitude and used to measure the change in location of two corresponding shorelines in different years (Figure 5). Average rates of movement were calculated by dividing absolute measurement by elapsed time. Tables 1 and 2 present the transect measurements and shoreline change rates for Lake Maurepas. A negative (-) sign signifies landward shoreline movement or erosion and no sign (positive) signifies a lakeward movement or progradation.
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