AIRBORNE
Instant Evaluation of Beach Storm Damage
Using Airborne Laser Terrain Mapping
By Dr. Ramesh L. Shrestha and Dr. Bill Carter

Nearly 45 million Americans live along the Atlantic and Gulf coasts stretching from Maine to Texas. Several times each year high surf (driven by the north easterly winds generated as weather systems move offshore) causes substantial damage to hundreds of kilometers of beaches along the Atlantic Coast. Less frequent, but much more devastating, is the damage suffered in areas along both the Atlantic and Gulf coastlines that are struck by hurricanes. In this century alone, hurricanes have taken 14,500 lives and have been responsible for property damage in excess of $100 billion. Hundreds of thousands of people have been forced to evacuate, returning only to find impassible roadways and structural damage to their homes that make them unsafe until repairs can be made.
    Before the government can provide assistance and repair the damaged infrastructure, the extent and degree of the damage must be quantified. Engineers need to have reliable estimates of the quantity of sand displaced in order to determine the best corrective actions, prepare specifications, obtain bids, and monitor the restoration effort. Accurate pre-storm and post-storm maps are essential. Unfortunately, high accuracy mapping has always been a costly, time consuming process involving labor intensive ground surveys followed by airborne photogrammetry. Often material volumes are estimated based only on cross sections of the beach at spacings of 300 meters or more, based on classical leveling surveys. At a cost of $1,000 per mile for leveling, it can amount to hundreds of thousands of dollars and take months of field work before engineers have the information they need to rebuild or take other corrective actions in the aftermath of a major hurricane. A completely new approach is badly needed and now, thanks to the development of the Airborne Laser Terrain Mapper (ALTM) by Optech, one is available.
    The Optech ALTM 1020 system draws on several recent technological advances to create a compact, light weight, low power, and highly automated laser mapping system that can be operated from a light dual or single engine aircraft. A high repetition rate (typically 5,000 pulses per second, but 25,000 pulses per second is already feasible) laser ranging system is reflected from a precisely controlled scanning mirror and directed toward the ground in a saw tooth pattern that covers the ground at spacings of every few meters. An inertial navigation unit in the system and phase differenced kinematic Global Positioning System (DGPS) provide the orientation and position, respectively, of the aircraft at the exact instant of each range measurement. Only one known ground point, at which a GPS receiver is operated during the time that the aircraft is in flight, is needed as ground control. After some post flight processing, which can be completed within hours of the flight, a high resolution Digital Terrain Model (DTM) can be created. Comparisons between the ALTM and traditional surveying methods, necessarily confined to limited areas because of the time and cost to do the traditional mapping, show sub-decimeter agreement.
    Once the DTM is made, the data can be displayed in many forms, such as shaded relief and false color maps, traditional contour maps, and three-dimensional perspective views that can help both engineers and less technically qualified managers to comprehend the extent and degree of the damage. And, with a few strokes of a PC keyboard, engineers have the high resolution quantitative information that they need to design and carry out restoration and preventive measures.
    Researchers at the University of Florida (UF) see many potential uses for ALTM technology, including such diverse tasks as determining the rate of waste deposition at land fills, monitoring of sink holes, mapping highway right-of-ways and creating the DTMs needed to develop useful hydrologic models for environmentally sensitive areas, such as the Everglades. But in the state of Florida where the beaches are so important to the tourist industry, and where the beaches are so low and subject to damage by even moderate storms, not to mention the occasional hurricane, the benefits to be gained from ALTM are particularly attractive. For the first time ALTM makes it possible, indeed relatively easy, to map all one thousand kilometers of Florida beaches yearly, or even seasonally. Beaches damaged by storms can be re-mapped within a period of days, speeding restoration and minimizing the economic impacts and disruption of residents lives.
    In addition to accurate DTMs, other useful information is produced as a by-product in mapping the beaches. For example, the footprints and heights of beach front buildings are documented, information that had a variety of uses, including planning for new development.

About the Authors:
Dr. Ramesh L. Shrestha is a professor in the Geomatics Program at the Department of Civil Engineering, University of Florida. He may be reached at 352-392-4999. Dr. William Carter is an adjunct professor at the Department of Civil Engineering, University of Florida. He may be reached at 352-392-5003.

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