Change along the Texas shoreline
The Texas Gulf of Mexico shoreline is 600 kilometers long and is generally
undergoing long-term (tens of years) retreat. Rates of change in specific areas,
however, vary greatly from seaward advance of 1 meter per year to retreat of
more than 3 meters per year. Bay shorelines, which total more than 9,000 kilometers
in length, are also generally retreating.
In 1999, largely in response to ongoing shoreline retreat intersecting new and old development along the southeast Texas Gulf of Mexico coast, the Texas Legislature passed the Coastal Erosion Planning and Response Act authorizing the Texas General Land Office to implement a statewide coastal erosion response program.
The Bureau of Economic Geology (Bureau) has been helping to identify critical coastal erosion areas in support of implementing the act. For decades, the Bureau, which is part of the John A. and Katherine G. Jackson School of Geosciences at the University of Texas at Austin, has had a strong program in coastal research, studying coastal processes, evaluating the distribution of different habitats along the coast, and mapping and analyzing the changing shoreline.
Because accurate topographic information is vital to our understanding of coastal environments, in 2000, the Bureau purchased a LIDAR system manufactured by Optech Inc., and embarked on the Texas Shoreline Change Project (TSCP). The goal of the 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.
Pictured here is a perspective view of the digital elevation model for a portion of Matagorda Island in Texas, produced with LIDAR.
Comparing shoreline positions through time yields the rate of shoreline change. To arrive at an annual rate of change, it is best to compare multiple shoreline positions over a long period. The central concept is that past long-term shoreline movement is the best indicator of what is likely to happen in the long-term future. This approach works provided there is no fundamental change in the future system, such as construction of large engineering structures, including seawalls or jetties, which can change the movement of sand along the coast. The construction of jetties extending well seaward of the surf zone at bay entrances, for example, occurred during the early 1900s and has greatly affected shoreline change patterns along the Gulf coast.
Shorelines mapped in the 19th century by the former U.S. Coast Survey are the earliest source of shoreline position accurate enough for shoreline change analysis, but the bulk of our shoreline data in Texas comes from vertical aerial photography taken since the 1930s. We scan the photos and digitally match them to 1990s digital orthophotos produced by the Texas Strategic Mapping Program. Orthophotos are photographs that were scanned and processed to remove distortion. They are geo-referenced using surveyed locations of features or targets visible in the photographs.
Unfortunately, the old photographs were rarely taken at any specific time with regard to the tides, so documentation regarding the time of day is usually not available. Therefore, the waterline is not at a consistent or known elevation in the photographs, and even if it were, the level of the waterline is not the best shoreline indicator to map for change analysis. Therefore, a shoreline feature must be interpreted from 2-D photography and then digitized. This boundary between wet and dry sand on the beach is displayed as a tonal contrast on the photographs.
The position of the wet/dry line is more consistent in time than the waterline and approximates the landward extent of the waves rushing up the beach during the last high tide. It is also high enough on the beach so that short-term erosion and deposition that occur in the swash zone on a daily or monthly basis do not skew the long-term analysis.
Although not affected as greatly as the waterline, the wet/dry line is still shifted by short-term changes in water level, wave activity and recent rainfall, which have nothing to do with the shoreline changing. Furthermore, the wet/dry line is not always a distinct and easy line to draw on the photographs. Error is also involved when attempting to take the distortion out of a photograph by digitally warping it to match an existing map (or orthophoto), or by going into the field and surveying positions of features or targets visible in the photograph and then warping the image to match those features. This procedure can be difficult in remote areas with few distinct features to use as reference points or where access is difficult. These problems have been overcome by applying LIDAR to map shorelines; however, LIDAR also poses its own unique set of challenges.
Airborne LIDAR mapping requires the integration of three basic measurement
sources: laser ranges and associated scan angle information; information on
the roll, pitch and yaw of the aircraft from an Inertial Measurement Unit; and
absolute aircraft positions (trajectory), derived from a differential, geodetic
GPS network. The slant distance from the aircraft to the ground is determined
by measuring time elapsed from when the laser is fired to when energy reflected
by the target returns to the sensor, then multiplying this time by the speed
of light. We divide this distance by two to account for the two-way travel time
and combine it with data from the other measurement systems to yield the horizontal
and vertical position of the reflectors. In our Optech LIDAR system, the laser
shots are directed from side to side across the flight path by a mirror oscillating
in a full range of up to 40 degrees.
The concept of how the survey positions are obtained is rather simple; however, considering the aircrafts height above the ground, the aircrafts speed and turbulent movement, and the fact that the laser is firing upwards of 25,000 times per second, actual implementation of the concept is quite a feat! We must take extreme care in conducting the surveys to get the best results. Many things can go wrong.
Successful airborne surveys begin with preparation on the ground. The Bureau uses a network of tidal stations, survey monuments and temporary reference points along the Gulf of Mexico coast as GPS base stations during LIDAR surveys. The farther the aircraft is from the ground reference station, the more potential error there is in computing the aircraft trajectory. Therefore, we choose base stations, which are occupied during the survey flight, along the flight path no farther than 100 kilometers apart so that the airplane is always within 50 kilometers. We also conduct a high-accuracy ground survey of a stretch of road, runway or some other stable surface with no vegetation on it in the survey area. The ground survey is used to calibrate the LIDAR system, and we pass over it during each flight.
Other factors involved in planning a coastal LIDAR survey include determining the period when the GPS satellites form the best constellation for the most accurate results, when the tides, waves and weather conditions are most favorable, and when survey operations wont interfere with other aircraft in the area. Our LIDAR system uses a near-infrared frequency for the laser, which is optimum for terrain but cannot penetrate water, so the survey target must be above the waterline.
Another timing restriction is daylight. Surveys are conducted in daylight for safety reasons if using a single-engine aircraft or when navigating the shoreline by sight, which we have found to be most efficient for relatively straight, open-ocean shorelines. The laser energy cannot penetrate fog, rain or clouds, so it either has to be clear or we need to fly below the clouds. We cant fly too low, however; the laser is too strong and can damage peoples eyes on the ground. The swath width also becomes narrower the lower we fly. The lowest we prefer to fly is 450 meters above the ground, but for beach surveys, the ideal flight altitude is about 800 meters with an aircraft speed of about 100 knots. This height and speed combination gives us a 550-meter wide swath, 1-meter data-point spacing and potential accuracy of better than 10 centimeters vertically.
We fly the shoreline using a video camera that has the same field of view as the laser. The pilot maneuvers the aircraft to keep the waterline in the middle of the view. This gives us data to about 275 meters landward, or one-half the swath width, more than enough to cover the first two rows of dunes and structures. We make at least two passes along the shoreline to increase the density of the data and to fill in any holes from prior cloud cover.
Extracting a shoreline
Having collected these LIDAR data, we must couple them to the older data to
see the shorelines history. We must extract a shoreline from the LIDAR
data that is comparable to shorelines mapped from historical photography. We
therefore need to determine the typical elevation of the wet/dry line. Going
back to our ground-surveyed beach profiles, we noted the location of the wet/dry
line, and we determined that it averages around +0.6 meters above mean sea level
along the Texas Gulf coast. This also generally coincides with being just below
the major berm crest on the beach.
To get the shoreline from the LIDAR data, we process the millions of LIDAR data points into a 1-meter grid, contour the grid, and extract the +0.6-meter contour line. After cleaning the contour line to eliminate effects of cars, people and small closed contours, we have a shoreline that is comparable to our historic shorelines but one that is much more accurate and rigorously selected.
In addition to measuring the two-way travel time of the laser pulse, our LIDAR system also measures the intensity of each returning pulse. Wet sand causes a lower-intensity reflection than dry sand, which allows us to map the wet/dry line in the LIDAR data. We can then measure the horizontal and vertical variation of the wet/dry line. As we collect more LIDAR data we can determine the variability of the wet/dry line alongshore and through time, giving us a better idea of the reliability of the shorelines mapped from photography.
While the legacy database of historical photography is invaluable in understanding the process of shoreline change, LIDAR is the future for addressing this problem. Annual LIDAR surveys can be used for short- and long-term shoreline change studies. The data can provide a baseline for studying the effects of storms that may occur during the year and for delineating areas prone to damage caused by washover or episodic retreat during subsequent storms.
There is much to be done with regard to developing and applying LIDAR technology, and with our research and development partners, we are applying new instrument components and developing surveying, processing and analysis techniques for coastal surveys. LIDAR will allow major advances in our understanding of coastal environments, just as the advent of vertical aerial photography did in the 1930s.
go on a virtual field trip
Policy-makers, who have varied and generally nonscientific backgrounds,
often find themselves trying to digest the tables and graphs of data technical
staff may present to them as background for making decisions. And it can
be difficult for an environmental policy-making panel, such as the 12-member
Texas Coastal Coordination Council, to make field trips together to observe
the issues they are facing.