ANTHROPOGENIC AND NATURAL CONTROLS ON SHOREFACE EVOLUTIONALONG GALVESTON ISLAND, TEXAS

摘要

Tightly spaced (50-100 m) sidescan sonar and CHIRP subbottom profiling lines,rnsurface grabs, and sediment coring were conducted on two, 5-km-long areas offshore ofrnGalveston Island, Texas to examine the geologic evolution of the shoreface. Multibeam (swath)rnbathymetry was also conducted in the study areas in January 2001 and July 2001 to examinernseasonal changes in the shoreface profile and to facilitate bathymetric comparison with historicalrndata. One study grid was located on the east end of the island adjacent to the Galveston Seawallrnthat has undergone several phases of sand renourishment since 1995, while the second wasrnfurther west along a natural beach opposite the community of Pirate’s Beach. Both survey gridsrnshow a tripartite division of shore-parallel surface sediment facies. The innermost zone is thernseaward-thinning sand wedge of the barrier island. Seaward of the island toe in both grids is anrnoutcropping area of relict island shoreface that formed during progradation of the island prior torn1,200 yBP. Further offshore, the relict units are buried below a burrowed mud that thickensrnoffshore. 137Cs geochronology of sediment cores from this unit indicate most of the mud layerrnhas accumulated in the last 40-50 years. Bathymetric change maps show little or no significantrnseasonal alteration in the shoreface profile between winter and summer conditions. However,rncomparison with historical bathymetric data indicates significant ravinement of the shoreface hasrnoccurred in the last century in both grids.