CSG
 
Galveston Island Geohazards
James C. Gibeaut, principal investigator; Thomas A. Tremblay, Rachel Waldinger, Edward W. Collins, Rebecca C. Smyth, William A. White, Tiffany L. Hepner, John R. Andrews, and Roberto Gutierrez.

Introduction
Galveston Island formed through the interaction of processes such as waves, tides, storms, plant colonization, and sea-level change which concentrated sediment making up the island as we know it today. These same processes, plus human activities, continue to cause changes on the island. The overall goal of this project is to provide information to improve how we live with these ongoing processes. Specifically, this work is providing the City of Galveston with a geohazards map of Galveston Island. It is an outgrowth from the document, “Living with Geohazards on Galveston Island: A Preliminary Report with Recommendations,” presented to the City by an expert panel of geologists in July 2004.

Through other projects, the Coastal Studies Group acquired or developed several data sets pertinent to delineating geohazards on Galveston Island. These data sets, which include topography from lidar, recent and historical aerial photography, wetlands maps, geoenvironment maps, historical shoreline positions, and model projections of future shoreline and wetland changes are combined and analyzed to create a hybrid map of geological hazards. The resulting Galveston Island Geohazards Map shows areas on the island that vary in their susceptibility to, and function for, mitigating the effects of geological processes. These processes include sea-level rise, land subsidence, erosion, and storm-surge flooding and washover.

Explanation of Map Units

The Galveston Island Geohazards Map depicts four levels of hazard areas plus special hazard overwash delineations that may overlap the four levels. Areas mapped as having imminent geohazard potential include the presently existing critical environments of bayside estuarine wetlands, freshwater wetlands in the interior of the island, and the beach and foredune system on the Gulf side. Wetlands are essential to the ecology of the barrier-island system and provide a physical buffer to protect upland areas against storm surge, waves, and currents. The Gulf side beach/foredune area is also an important habitat and, if allowed to develop, offers a natural barrier to washover. It also stores sand for rapid beach recovery following erosion by storms. Areas of future critical environments are designated as having a high geohazard potential and include areas of uplands that are projected to become critical environments in 60 years; these areas should thus receive special consideration today. The 60-year projection is based on modeling of expected wetland transition caused by relative sea-level rise and historical shoreline-change rates. Moderate geohazard potential areas are upland areas that are neither currently, nor are expected to become, critical environments during the next 60 years. However, these areas may be inundated during a category-one hurricane or even a tropical storm. The island core unit has a low geohazard potential because it is relatively high in elevation and interior to the island, making it overall less susceptible to geohazards than other parts of the island.

A beach/dune ridge generally more than 6.5 feet high and set in a seaward position extends along the island from the end of the Galveston Seawall to about 6.5 miles from San Luis Pass. This ridge forms a natural barrier to cross-island washover, making it an important feature to preserve. Other ridges behind East Beach also offer protection and should be preserved. The Gulf seawall is effective in lessening the danger of washover and shoreline retreat as well. The western 6.5 miles of the island, however, is not well protected by natural or manufactured structures, making it a special hazard zone with enhanced potential for washover. Within this washover area, potential washover pathways are marked where washover was concentrated during Hurricane Carla in 1961 and may be again during future storms.

Map Legend

  Open Water: Bay, ocean, natural or excavated ponds and swales that are always inundated.
  Imminent Geohazard Potential—Present Critical Environments: Salt and freshwater wetlands, including beaches, tidal flats, and marshes. Along Gulf of Mexico shoreline, including beaches and foredunes.
  High Geohazard Potential—Future Critical Environments: Areas expected to become critical environments (see above) in 60 years’ time (2062) if historical rates of relative sea-level rise and shoreline change continue and if development or restoration projects do not affect natural processes.
  Moderate Geohazard Potential—Upland: Upland areas generally less than 5 feet above sea level that are not expected to become critical environments during the next 60 years (2062) (see above) but may be affected by storm surge caused by typical tropical storms or categ­­­ory-one hurricanes.
  Low Geohazard Potential—Island Core Upland: Centrally located upland areas generally more than 5 feet above sea level and not expected to become critical environments in 60 years’ time (2062).
Special Hazard Zone Washover Area—Area with Enhanced Potential for Washover: Particularly susceptible to storm surge completely washing over the island during a hurricane.
Special Hazard Zone Washover Path—Past and Potential Future Washover Pathways: Places where Hurricane Carla (1961) cross-island storm-surge flow was concentrated and where it may be concentrated during future hurricanes. 
Natural Protective Beach/Dune Ridge—Relatively high beach/dune ridge complex set in a seaward location, forming a natural barrier to storm surge from the Gulf. Generally greater than 6.5 feet above sea level but may encompass lower areas, including marsh and open water, as part of the complex.
Galveston Seawall: Seawall with top approximately 14 feet above sea level, forming an effective barrier to shoreline retreat and storm-surge flow from the Gulf.
The map is currently under review; please send comments to Jeff Paine at jeff.paine@beg.utexas.edu.
   
BEG