Identification of Shoreline Erosion Features and 60 Year Projection of the Gulf Shoreline Position, Galveston and Brazoria Counties, Texas

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1996

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Changes in shoreline position along the southeastern Texas Gulf coast between 1974 and 1996 were documented by conducting a kinematic real-time differential global positioning system (GPS) field survey in 1996 and comparing that shoreline with other shorelines archived in a geographic information system. Results of the investigation show that (1) beach morphology, shoreline movement, and the regional geologic framework are closely interrelated, (2) Gulf beaches are generally retreating, and (3) the rates of retreat have accelerated locally.

Gulf beaches between High Island and Rollover Pass have been retreating for centuries, and they continue to retreat. On Bolivar Peninsula, beaches between Rollover Pass and Caplen are retreating rapidly, whereas those farther southwest are either slowly retreating or are relatively stable; at the southern end of the Peninsula, beaches are stable or advancing from the sand supplied by updrift erosion.

The shoreline on East Beach of Galveston Island undergoes minor fluctuations, but the beach position is relatively stable. West Beach of Galveston Island continues to retreat, but rates of retreat vary depending on location. The beach segment between the Indian Beach and Sea Isle subdivisions has the lowest long-term average retreat rate, whereas retreat rates generally increase to the northeast toward the seawall and to the southwest toward San Luis Pass. These three segments with different beach morphologies and retreat rates were also identified in prior studies. Beaches from San Luis Pass to the mouth of the Brazos River also are generally retreating, and rates of retreat near Surfside, Quintana, and Bryan Beach are influenced by the jetties at Freeport Harbor and the diversion of the mouth of the Brazos River.

The long-term average annual erosion rate of beaches in Galveston and Brazoria Counties was determined using the database of digital shoreline positions and a framework of shore-normal transects spaced 150 ft apart along the shore. The linear regression statistical function of the Shoreline Shape and Projection Program (SSAPP) was used to calculate the average annual erosion rate and to estimate the position of the shoreline erosion feature in 60 years. The possible long-term effects of engineering projects such as shoreline protection structures and beach nourishment projects also were analyzed to assist in evaluating the validity of the projected shoreline position.

The field surveys and statistical analyses demonstrated that the high water line mapped on aerial photographs undergoes large seasonal fluctuations and therefore is a less reliable indicator of shoreline position than the vegetation line, berm crest, or backbeach erosional scarp. The study also showed that GPS field surveys are a rapid, relatively low-cost method of acquiring accurate shoreline positions, and they have many advantages compared to aerial photographs.

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