Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Coastal GeoTools Conference, Charleston, South Carolina, January 8, 2003
Texas Shoreline Change Project: Combining Lidar,
James C. Gibeaut, William A. White, Roberto Gutierrez, Rachel Waldinger, John R. Andrews, Tiffany L. Hepner, Rebecca C. Smyth, and Thomas A. Tremblay
In 1999, the Texas State Legislature passed the Coastal Erosion Planning and Response Act. This act authorized the Texas General Land Office (GLO) to implement a comprehensive coastal erosion response program. The Bureau of Economic Geology (Bureau) is working with the GLO to identify critical coastal erosion areas. The goal of the Texas Shoreline Change Project is to establish a state-of-the-art, shoreline-monitoring and shoreline-change analysis program that will help guide coastal-erosion and storm-hazard-mitigation projects along bay and Gulf of Mexico shorelines. All data and reports are presented on the project's web page that includes web-based GIS (ArcIMS) pages.
The rate of shoreline change is determined by comparison of historical shoreline positions. Vertical aerial photography since the 1930's is scanned and co-registered with 1995 digital orthophotos produced by the Texas Orthophotography Program. Shorelines and vegetation lines are then interpreted and digitized on the computer screen.
During 2000/01, the Bureau conducted LIDAR surveys along the Gulf of Mexico shoreline. A 1-m digital elevation model (DEM) was constructed from the laser points. A grid of the G99SSS gravimetric geoid model was subtracted from the DEM to obtain heights above the geoid. Using tide gauges the geoid heights are then vertically adjusted to correspond to local mean sea level. Comparison of beach profiles and the wet/dry line as shown by LIDAR intensity data was used to pick elevations to represent the shorelines. These shorelines correspond to those mapped by using aerial photography along the Gulf of Mexico shoreline.
The wet/dry line may also be used as the shoreline feature along bay shorelines, but the variety of shoreline types in the bays requires other features such as marsh vegetation and bluffs to be used. An airborne lidar survey in the Galveston Bay system during 2002 will extend the application of this technology to mapping a variety of shoreline types.
After shorelines are mapped and compiled in a GIS, rates of change are computed using the Shoreline Change and Projection Program (SCAPP) developed at the Bureau. SCAPP operates within the ArcView GIS. The program automatically constructs a segmented baseline that follows the trend of historical shorelines and constructs transects perpendicular to the baseline segments. Shoreline change is determined along these transects and various methods, such as linear regression, are used to determine the rate of change. SCAPP then projects future shoreline positions along the transects. Undulating shorelines can be successfully processed by SCAPP by varying the length of baseline segments. Typically, complicated bay shorelines are processed using 100-m baseline segment lengths and transects spaced 25 m alongshore.