Geologic Migration of Barrier Islands
Sandy barrier islands are the most conspicuous coastal feature of Texas.
Barrier islands, such as Mustang Island shown here, are migrating landward as the mainland shore retreats due to relative sea level rise.
The development and maintenance of the barrier islands involves a balance between sediment supply, longshore currents, bottom topography, and wave and tidal activity. Texas barrier islands will likely continue to migrate landward as the mainland shore retreats due to relative sea level rise.
The southeasterly offshore winds also contribute to this migration as gulf-facing sand is picked up and deposited into back bays and on the mainland.
Relative Sea Level Rise
Geologic studies indicate the Texas coast is being affected by land subsidence and global sea level rise.
A dune walkover on the Bolivar Peninsula ends in Gulf waters instead of the beach it once gave access to.
Land subsidence is caused by the extraction of water and petroleum from the subsurface along the Texas coast. The land "shrinks" downward, much like a sponge that has its water removed. A more regional form of land subsidence ― caused by natural compaction, faulting, and bending of the continental shelf ― is occurring within the earth's crust along the northwestern Gulf of Mexico.
Historical global monitoring indicates a trend of sea level rising in many parts of the world, including Texas. The causes are debated in the scientific community, but sea level is rising in relation to the land surface along the Gulf Coast. Small increases in sea level can have profound storm surge impacts in low-lying coastal areas of Texas.
Alteration of Natural Sediment Transport Processes
The removal, transport, and deposit of sediment by offshore currents can cause shorelines to expand or recede.
The white ribbon seen tracing the western, or Gulf side of Bolivar Roads above, is sand, which fans out into the Gulf due to sediment transport.
One key factor is the reduction of sediment carried by rivers to the coast due to dams and the removal of water for urban water supply and irrigation. This reduction in fresh water to the coast reduces the amount of transported sand available for natural renourishment of gulf beaches.
Another factor is the influence of hard structures called jetties and groins, which are built perpendicular to the coast. Though jetties and groins are effective at reducing erosion in their immediate area, they also impede the flow of sediment along the coast, much like a dam.
Bulkheads and seawalls ― built parallel to the shoreline along the water’s edge ― are designed to protect property from erosion by wave action. They prevent shorelines down the coast from receiving sediment trapped behind them. Seawalls tend to erode beaches in front of them due to increased wave energy reflected off the wall.
Coastal Wetland Habitat Loss
In addition to supporting complex food webs and fisheries, coastal wetlands serve as buffers against storm surge by reducing wave energy.
Coastal wetlands, like these on Mustang Island, provide essential habitat for many fish and bird species, as well as buffers against storm surges.
The shallow root systems of wetlands vegetation help to bind sediment as it gets deposited by rivers. This helps to create additional land, further enhancing the protection of adjacent uplands. Recognizing wetlands as a vital natural resource, Texas implements policies to help avoid, minimize, and/or compensate for the historical trend in wetland loss.
Natural Wind and Wave Processes
Wind and waves caused by storms can rapidly erode beaches and dunes.
The Texas coastline is exposed to the vast expanse of the Gulf of Mexico. It’s at the convergence of two major currents circulating within the gulf basin in opposing north and south directions. This dynamic zone of current convergence, combined with predominately southeasterly winds, help shape Texas beaches.
Between spring and autumn, the reduced energy of offshore waves allows sediment accretion at some areas of the shoreline, as sand gets deposited from offshore sand bars. When wave energy increases during storms, large quantities of sand may be temporarily removed from the shoreline and deposited offshore. Over time, normal wave and wind action may cause this sediment to be redeposited on beaches.
Large storms can push huge volumes of water inland in a phenomenon called storm surge, eroding vast areas of land and destroying property.