Coastal engineering applications of high-resolution lidar bathymetry
Introduction
Obtaining accurate, high-density measurements of bathymetry and topography in the coastal environment is a challenging task. Typical hydrographic surveys of coastal projects include the use of acoustical techniques operated from launch-type vessels or the use of an electronic distance-measuring device or stadia rod positioned on a sea sled. These surveys are then coupled with wading-depth surveys which cover the dry beach and shallow waters. Typically, data are collected along transects spaced anywhere from 8 m to 300 m apart. Furthermore, data at navigation projects are usually collected only within the maintained navigation channel alignment rather than throughout the entire project. Systems are now available which collect full coverage bathymetry with a vessel-mounted multi-beam acoustic fathometer. These systems provide accurate bathymetry over swath widths as great as 4 times the water depth; however, they are slow, particularly in shallow areas where the spatial coverage is reduced.
In the early 1980s, the US Army Corps of Engineers (USACE) began investigating technologies to augment its existing hydrographic survey capabilities at comparable cost providing fast, accurate, high-density surveys. In 1988, the USACE began a cost-shared program with the Canadian government to design, construct, and field-verify an airborne lidar (Light Detection And Ranging) hydrographic survey system. As a result of this effort, the USACE introduced the Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) system in 1994. The SHOALS system, operating from a helicopter, is designed to be a highly mobile system capable of quickly collecting accurate, high-density bathymetry remotely. SHOALS uses a scanning, pulsed infrared and green laser transmitter, 5 receiver channels, inertial reference and differential global position system (DGPS), and real-time and post-flight data processing techniques to produce survey depths accurate to within ±15 cm. A nominal depth-and-position measurement spacing of 4 m yields high-density bottom coverage, at a rate of 8 km/h, producing soundings from the above-water beach, or coastal structure, to a maximum depth of 40 m. The system is currently used by the USACE to monitor coastal navigation and erosion-control projects.
Since becoming fully operational, SHOALS surveyed over 2000 km2 along the Atlantic Ocean, Pacific Ocean, Gulf of Mexico, Great Lakes, and Caribbean Sea. Survey projects varied from tidal inlets and navigation projects to beach nourishment and monitoring. SHOALS was also used for emergency-response surveys after Hurricanes Opal and Fran. Consecutive SHOALS surveys are used to assess accurately natural morphologic changes and response to man-made alterations. With these unique data sets, new research is possible, such as improved methods for coastal sediment transport prediction. Today's existing tools for predicting bathymetric change are impractical when applied to areas with complex bathymetry. With a detailed history of sediment transport patterns, such as consecutive SHOALS surveys provide, more accurate predictive tools can be developed.
The following sections provide an overview of lidar bathymeter technology and a detailed description of the SHOALS system. Several survey sets, covering a variety of project types, are presented to introduce the benefits of lidar bathymetry to the coastal community. Furthermore, ongoing research in sediment transport, initiated by these unique data sets, is described showing the future benefits of lidar technology.
Section snippets
Lidar technology
An airborne lidar bathymeter uses lidar (Light Detection And Ranging) technology to measure water depths in optically clear waters. A laser transmitter/receiver (transceiver) is mounted on an airborne platform and transmits laser pulses towards the water surface (Fig. 1). The energy reflected back toward the airborne platform is continuously recorded to develop a lidar waveform (Fig. 2). When the laser pulse reaches the water surface, a fraction of the energy is reflected back to the airborne
Lidar history
According to Guenther (1985), the concept of using lidar technology to measure water depths formed during the early 1960s with initial theoretical studies and system modeling by the mid-1960s (Ott, 1965; Sorenson et al., 1966; and Prettyman and Cermak, 1969). These investigations along with field experiments undertaken by the Scripps Institute of Oceanography in the late 1960s provided sufficient information to realize the potential success of an airborne lidar bathymeter (Duntly, 1971). The
SHOALS system
There are 2 components to the SHOALS system: the airborne data-collection system and the ground-based data-processing system. The airborne system operates from a Bell 212 helicopter provided by NOAA's Aircraft Operations Center. Since SHOALS rapidly moves from one survey project to another, so too must the ground-processing system. The ground-processing system operates from a fully mobile trailer providing the capability to support successfully a survey mission and produce final products
Structure evaluation
The USACE routinely evaluates its coastal navigation structures as part of its maintenance management program (US Army Corps of Engineers, 1996). Since SHOALS collects high-density bathymetry and topography remotely, it can completely map these difficult-to-survey structures. The SHOALS data of a rubble-mound structure identifies and quantifies any breaches or loss of elevation along a structure's length, displacement and settling of groups of armor stone, and side-slope defects such as
Computing sediment transport
Estimating sediment transport is another coastal-engineering application for which high-resolution bathymetric data is also applicable. At inlets and harbors, coastal structures alter the flow of sediment and cause erosion and accretion problems. Analyzing or even anticipating such problems is possible, if the directions and magnitudes of sediment transport can be estimated. Longshore-transport equations, such as the wave-energy-based formulas in the Shore Protection Manual (1984)are
Future applications
Two of the variables used in topographic-change numerical models are the angle of wave approach αb and wave height Hb. Because of lack of data, these variables are generally assumed to be either constant in the longshore direction (e.g., no variation in breaker height) or vary simply and in a known manner, such asRather than making such simplifying assumptions in the models, SHOALS could be used to determine the longshore variability in these wave variables. The IR
Summary
SHOALS provides a tool which allows one to more accurately quantify geomorphic changes, structure conditions, and project performance. The use of SHOALS at East Pass allowed for on-site precision engineering evaluation of the west jetty following a massive storm event. SHOALS data at King Harbor allowed a detailed engineering assessment of the breakwater's condition, both above and below water. Consecutive SHOALS surveys at navigation projects, such as those at Moriches inlet, allow coastal
Acknowledgements
The projects, analyses, and resulting data described herein, unless otherwise noted, were obtained from work performed at the Waterways Experiment Station of the US Army Corp of Engineers. The use of trade names does not constitute an endorsement in the use of these products by the US government. Permission was granted by the Chief of Engineers to publish this work.
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