 Three days after Hurricane Isabel thrashed the North Carolina coast, Wayne Wright took his twin-engine Cessna 310 to the skies. Flying just 300 meters above the islands of the Outer Banks, he and his team bounced a green laser across the landscape, tracing the contours of the beaches to build a picture of how the land had changed. Sand dunes were missing and a new inlet was gouged across Hatteras Island. They took millions of measurements and over 30,000 pictures that will help geologists understand how extreme events, such as hurricanes, shape our coast.
Water washes through a newly formed breach on Hatteras Island, North Carolina, a few days after
Hurricane Isabel swept through the area.
Wright is the lead investigator for NASA's Experimental Advanced Airborne Research Lidar (EAARL) system at Wallops Flight Facility on Virginia's eastern shore. He is collaborating with Asbury Sallenger, Jr., a research oceanographer at the US Geological Survey in St. Petersburg, Florida, to determine how much damage Hurricane Isabel inflicted on US beaches.
"If you fly along the barrier island coast, you'll see cuts through the islands. Almost all of them are formed the same way," Sallenger
explains -- by hurricanes. To understand how severe storms shape the coast, Sallenger needed accurate before and after maps. But
beaches shift daily with the tides, waves, and winds, so no map would ever provide a perfectly accurate before-after comparison. He needed to make a map of the beach immediately before a major event and compare it to a map made immediately after to know what changes were caused by the storm and what occurred as a result of other natural processes.
To quickly map out beaches, Sallenger turned to lidar, which is short for Light Detection and Ranging. Lidar works like radar, but uses light waves instead of radio waves to measure the distance to objects. Typical mapping lidar systems bounce a laser light pulse off a surface and record the time it takes for the light to return to determine the distance it traveled. Plants, water, bare exposed earth, and the bottom of a shallow sea each absorb and reflect different colors (wavelengths) of light in different ways. Most lidars are optimized to map one or two types of surface terrain because vastly different lasers and laser receivers are required for, say, bare earth compared to plant cover. NASA's Experimental Advanced Airborne Research Lidar (EAARL) is distinct in that it can trace out the topography of bare ground under vegetation, map out the vegetation itself, and, where the water is clear enough, measure the depth and clarity of the water as well as the shape of underwater surfaces.
"EAARL is uniquely able to make measurements over ground that varies tremendously in reflectivity and complexity," says Wright. "We could fly over water, and one pulse could fall in the water and the next on a sandy beach." Water, Wright explains, tends to absorb the light and return a much weaker signal, while exposed sand is highly reflective and vegetation falls somewhere in between. "Other [lidars] tune for either weak or strong signals," Wright says. But EAARL can capture them all because it makes frequent measurements -- about four billion per second using multiple detectors. This trait makes it perfect for tracking hurricane damage in coastal areas, which are made up of water, sand and plant life.
 NASA's Experimental Advanced Airborne Research Lidar shows where sand and buildings on Hatteras Island disappeared in the wake of Hurricane Isabel. Click Image For Full Size Version.
EAARL's flexibility gives it another advantage for charting storm damage. Because it is already designed to measure all sorts of terrain, it can be kept on a dedicated airplane, ready to go at almost any time. It doesn't have to be re-calibrated for every project and every sort of terrain. "EAARL was on standby. It works on short notice," says Sallenger, and for his beach mapping project, time was in short supply.
Two days before Hurricane Isabel came ashore, Sallenger and Wright collaborated to survey the areas where the storm was predicted to make landfall. "We were able to survey 250-300 kilometers of shoreline in just a couple of days," Sallenger reports. They repeated the survey five days later to track the changes Hurricane Isabel made. Among other things, they mapped the new inlet that was cut across Hatteras Island, giving scientists the most detailed data set on storm inlets to date, says Sallenger. Those data will be used to build models of beach erosion to predict how and where inlets may form in the future.
Images courtesy of Asbury Sallenger and Amar Nayegandhi, USGS Center for Coastal and Watershed Studies, and Wayne Wright, NASA Wallops Flight Facility.
For more information, read "LIDAR In the Wake of the Storm" at:
http://earthobservatory.nasa.gov/Study/Lidar/
NASA's Earth Observatory |
