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 shorelines.
Digitally rectified historical aerial photography, airborne topographic
lidar surveys, and select ground surveys of beach profiles are
key components of the program.
The rate of
shoreline change is determined by comparison of shoreline positions
through time. Vertical aerial photography since the 1930's is
scanned and rectified using 1995 digital orthophotos produced
by the Texas Orthophotography Program as the base map. Shorelines
and vegetation lines are then interpreted and digitized on the
computer screen. The latest shorelines are being mapped using
airborne topographic lidar techniques.
the Bureau conducted lidar surveys along the Gulf of Mexico shoreline
and the lower Galveston Bay system 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. The height of the water level,
as displayed in the DEM, was then compared with water levels recorded
by open-coast tide gauges during the survey. This allowed the
correlation of geoid heights to heights above a local tidal datum.
Comparison of beach profiles and the wet/dry line as shown by
lidar intensity data was used to pick levels to represent the
shorelines (+0.6 on Gulf beaches). These shorelines are comparable
to those mapped using aerial photography but are much more accurate.