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GPS-based analysis of shoreline change, 1995-2005, Mad Island Marsh Preserve, Matagorda County, Texas.

Article Excerpt
Abstract. -- A combination of a global positioning system (GPS), high-resolution aerial photographs and a geographic information system (GIS) was used to measure shoreline change at Mad Island Marsh Preserve during the period 1995-2005. The magnitude and direction of shoreline change was found to vary between five contrasting shoreline types included in the study. Compared to earlier studies that suggested erosion occurred along all sections of shoreline in the study area between 1941 and 1993, this study found sections of shoreline that had essentially stabilized and others where previously documented erosion had slowed considerably. The reason(s) for these changes is uncertain--two possibilities are that a progressively-widening wave-cut platform is dissipating barge-generated wave energy or that the exposure of dense root mats along some receding shorelines is increasing resistance to wave erosion. A concrete erosion control mat emplaced along one section of shoreline in 1996 was found to have been unsuccessful, because soil had been washed out from under the concrete.

The Texas Nature Conservancy's Mad Island Marsh Preserve is a 29-[km.sup.2] tract containing the east arm of Mad Island Lake, its associated freshwater and brackish marshes, and surrounding upland prairie and shrub land habitats. The marshes on the preserve provide an important habitat for many aquatic organisms and an important wintering ground for millions of migratory birds that use the Central Flyway each year. Since 1993, the preserve has ranked among the top-five areas in the nation in number of species counted during the annual Audubon Society's Christmas Bird Count (L. Halsted, Texas Nature Conservancy, Pers. Comm., 2003). The shoreline of the preserve borders the Gulf Intracoastal Waterway (GIWW), a man-made canal constructed in 1941 (Figure 1a).

Barge traffic in the GIWW has caused considerable wave erosion of preserve shorelines over the last 65 years. Williams (1993a) calculated erosion rates along the shores of the preserve based on sequential aerial photographs covering the period 1943-1991. In 1993 Williams conducted a second study of shoreline erosion based on repeated measurements of the separation between the shoreline and a series of survey stakes set in the ground every 30.48 m along the shoreline. The surveys were conducted in June of 1992 and in May of 1993, and were used to derive 1-year erosion rates (Williams 1993b). The results of those studies showed long-term shoreline erosion approached 2 m/year on some parts of the preserve. The studies also suggested that erosion may threaten freshwater marsh habitats bordering Mad Island Lake, since erosive shortening of Mad Island Bayou--a tidal inlet connecting the lake to Matagorda Bay--is likely to increase salt water intrusion into the lake. In response to these findings, the Texas Nature Conservancy in 1996 constructed a protective concrete barrier along the shoreline bordering Mad Island Bayou in the hopes of preventing further erosion...

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