Measuring the Continuum of Geomorphic Variability along the Texas Coast Using Lidar

Airborne topographic lidar can acquire detailed and accurate data over a broad region allowing geomorphic analysis across a continuum of spatial scale. Subtle changes and variations in topography is both a result of acting processes and a control on the distribution of habitats such as beaches, dunes, and wetlands. Detailed and accurate topographic data, therefore, is a fundamental requirement for understanding the coastal zone.

Using an Optech scanning lidar instrument installed in a single-engine aircraft, the Bureau of Economic Geology at The University of Texas at Austin has acquired surveys of the barrier islands along the Texas coast since 1997. A network of GPS reference points, some of which are located at tide gauges, and GPS ground surveys of roads provide control for research-quality surveys. Absolute vertical accuracies of 5 to 10 cm on non vegetated substrates, such as beaches, are achievable. Vegetation surveys show that typical dune and marsh vegetation 30- to 100-cm high cause vertical upward biases of 10 to 20 cm and increase lidar point scatter by about 10 cm. Where detailed change analysis of elevations of beach surfaces is required, stable structures, such as building roofs, are used to adjust the lidar digital elevation models (DEM).

Shorelines for change analysis are mapped using lidar DEM's. So that we can compare historical shorelines derived from mapping the wet beach/dry beach line on aerial photography, the lidar elevation and intensity data for several surveys were compared to determine the typical elevation of the wet/dry line. Along the Gulf of Mexico, this elevation is 60 cm above mean sea level; a +60 cm contour line extracted from the lidar data represents the shoreline. Lidar DEM's are also used to determine the elevation dependence of coastal habitats and as a basis for modeling habitat transition during relative sea-level rise.