Analysis of Shoreline Erosion at Sargent Beach, Texas and Projection of Shoreline Positions in the Years 2000 and 2050

Abstract

The greatest losses of coastal land in Texas occur along highly erosional deltaic headlands having relatively narrow and steep beaches. This type of beach occurs south of Sargent, Texas, a small community located on the western flank of the Holocene Brazos-Colorado delta (figs. 1 and 2). At Sargent Beach, the transgressive beach deposits are composed of sand and gravel (shell and rock fragments) derived from relict sediments exposed on the inner shelf. Whole and broken surf-zone, shelf, and bay species constitute the biogenic detritus; bay species (Crassostrea virginica, Rangia cuneata, and Mercenaria spp.) are the most abundant shell material concentrated on the beach and in the adjacent washover terrace. This mixed bay-estuarine shell assemblage is eroded from the Holocene delta-plain muds as the shoreline retreats. Sargent Beach is far removed from major sediment sources (river mouths) and human activities that directly influence shoreline changes. For that reason, it is significant that the area has experienced the highest long-term and short-term rates of erosion along the Texas coast (Seelig and Sorensen, 1973; Morton and Pieper, 1975; Sealy and Ahr, 1975). Accelerated erosion at Sargent Beach since 1930 (fig. 3) is probably related to significant reductions in sediment supplied by the Brazos River (Morton and Pieper, 1975; Mathewson and Minter, 1976) rather than to the relative rise in sea level. The shoreline of the Holocene Brazos-Colorado delta has been retreating for the past few thousand years, but construction of dams and relocation of the Brazos River channel have recently caused substantial reductions in the amount of sand delivered to downdrift beaches (Morton, 1979).

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