In Deeper Waters
Traditional methodology for collecting survey data in and around shoreline areas has typically been based on cross sections surveyed using traditional methods at regular intervals (typically 1000'). These cross sections include both the subaerial region (topographic) extending from the water line to an established baseline marked by survey monuments, and the nearshore region (hydrographic) extending from the waterline to a specified distance or depth offshore. Observations are made at regular intervals and at points of slope change along these cross sections, which are also often supplemented by aerial photography. Obviously, this methodology involves considerable time and cost including the maintenance of control. Intrinsic to this approach is the consequence that the majority of project areas were never surveyed so that detailed information was typically unavailable. Meanwhile, ever-increasing pressures on resources have led to a review of traditional methods of data collection and how these can be augmented by newer technologies.
In the past 10 years, the development of Airborne Laser Bathymetry (ALB) has revolutionized this scenario by providing a new capability to rapidly acquire comprehensive, high-density survey data over large coastal areas.
Project Tahoe: Proving SHOALS FlexibilityLake Tahoe, the largest alpine lake in North America, comprises 320 square miles, and with an elevation of 6,255 feet, it is also one of the world’s deepest and clearest lakes. This impressive and fascinating landscape offers such an attractive outdoor leisure environment that the region draws an estimated 3.5 million visitors each year. When compounded with decades of development, severe pressures are placed on a finely balanced environment. The result has been a reduction in water clarity in the lake from over 100 feet in 1969 to less than 70 feet in 1999. Concern over this led to a series of public workshops in 1997, culminating in a presidential forum, identified a number of measures required to prevent the permanent loss of the lake’s deep blue clarity. As a result, an executive order established a Federal Interagency Partnership to work with state and local entities to preserve this national treasure.
One member of this partnership, the U.S. Geological Survey (USGS), mapped the deeper portions of the lake in 1998, using a multi-beam sonar system. However, this technology is ineffective in very shallow depths towards the shoreline where the sonar’s swath becomes progressively narrower. In the case of Tahoe, these areas, comprising some 72 miles of shoreline, are increasingly becoming the focus for studies to understand and arrest the processes that are leading to the lake’s decreasing clarity. This is because soil washed into the lake from surrounding land contains nutrients, nitrogen and phosphorus that encourage the growth of light-absorbing algae. Therefore, another member of the partnership, the Sacramento District of the USACE (United States Army Corps of Engineers), decided to utilize SHOALS (Scanning Hydrographic Operational Airborne LiDAR Survey) these critical shoreline areas and finally complete the map of the lake’s subsurface.
SHOALS flew 30 hours of survey over five days. This short but demanding project was a graphic demonstration of the flexibility of SHOALS in adapting to a rugged, high-altitude environment. Also highlighted was the complementary use of diverse technologies along with the use of SHOALS to bridge the gap between existing topographic data and that collected in the deeper water by the multi-beam survey. When combined, the Corps and USGS data will be invaluable in understanding the cause and effect of natural and man-made lakeshore erosion processes. This knowledge will be a fundamental step in halting the decline in Lake Tahoe’s water clarity.
Project Vieques: Cutting Survey TimeSituated seven miles off Puerto Rico, Vieques possesses a variety of unique flora and fauna, including one of the world’s most beautiful bioluminescent (light by living organisms) bays. However, it is also the home of the Atlantic Fleet Weapons Training Facility (AFWTF), the U.S. Navy’s most extensive live-fire training site, used to “train up” fighter and attack jet pilots flying from aircraft carriers before they deploy to the Middle East, Persian Gulf and the Mediterranean.
Although the U.S. Navy owns 22,000 acres (two-thirds of the island), the weapons range itself is located on the eastern tip of the island and only comprises 900 acres. More than half of the remainder is managed under seven conservation zones. In such a setting where the U.S. Navy is highly sensitive to criticism of its environmental stewardship, increasing emphasis is placed on the development of scientific knowledge and databases.
One of the consequences of the 2000 crisis was a $40 million program to meet the health, safety, environmental and economic concerns of Vieques residents. Among the environmental steps proposed was creation of an artificial reef and fish aggregation program, as well as programs to preserve the Puerto Rican? Mosquito Vieques bay, maintain the ecosystem and conservation zones, and implement wildlife management plans.
As one consequence, SHOALS deployed to the area in June 2000, flew 17 hours of survey time in five days, completely mapping the coast of Vieques. This comprehensive data, which would have taken months to collect by boat, is vital to the base lining the bathymetry around the islands prior to the establishment of the artificial reef, as well as updating navigational charts for the safety of visiting tourist vessels.
Project Molokai: Saving the Coral ReefsIn 1998, President Clinton issued an executive order “to preserve and protect the biodiversity, health, heritage, and social and economic value of U.S. coral reef ecosystems and the marine environment.” The order established the interagency U.S. Coral Reef Task Force with the aim of protecting coral reef ecosystems, and the development of coordinated, science-based plans to restore damaged reefs as well as to mitigate current and future impacts on reefs. One of the priorities was to coordinate a comprehensive program to map and monitor the reefs as well as to evaluate existing navigational aids, including charts, maps, day markers and beacons to determine if marking the locations of coral reefs can be improved. At the second meeting of the Task Force in Maui, Hawaii, in early 1999, this requirement focused on reefs in the Pacific through the launch of a comprehensive mapping effort.
Coral reefs are central to the recreation industry in Hawaii and are amongst its most valuable resources with estimated revenues from ocean recreation in excess of $745 million. In addition to recreation, the importance of these reefs lies in their protection of the shore from storm damage; provision of a diverse habitat for sea creatures; and for their importance as fisheries (1997 estimates place their value at $20 million annually). The USGS is currently leading a multi-institutional research study of the coral reefs of southern Molokai where the fringing reef on the southern coast is the most extensive in the USA, stretching for some 60 to 70 km (30 miles), and extending from sea level to depths of over 30 m (100 ft). One major focus of this work concerns thematic mapping, using state-of-the-art technologies, including aerial and satellite imagery, hyperspectral and multispectral imaging and LiDAR. Baseline mapping will assist marine biologists in assessing both the current and future health of these coral reefs.
In early 1999, SHOALS flew 215 hours over 40 operational days in the Hawaiian Islands. While the primary mission was to collect data for USACE basemaps used for hurricane evacuation management numerical simulations, a secondary mission was to provide high-density bathymetry data for USGS coral reef mapping studies. A total of 25 million soundings were extracted from the LiDAR data. The data collected for the USACE comprised the entire shorelines of Maui (200 km) and Kauai (150 km), extending from the shoreline to offshore depths of 30 m. Additionally, the data were requested by the National Ocean Service (NOS) to update the 1927 data on nautical charts for the islands of Maui and Kauai. Most recently, SHOALS completed supplementary work in Hawaii during the later part of 2000 for the U.S. Navy, NOS, USACE and USGS, taking in additional islands as well as providing more detail on some areas previously surveyed in 1999.
Of particular interest to USGS is the amount of muddy sediment delivered to the reef by erosion and runoff, since they can affect the health and growth of corals, which in turn can affect the health and abundance of fish and crustaceans on the reef. By integrating the SHOALS data with aerial photography and USGS DLGs (Digital Line Graphs), the USGS has gained a significant advance in its ability to assess both changes in bathymetry and the amount of sediment on the reef.
ConclusionSince becoming operational in 1994, SHOALS has surveyed over 300 projects ranging in nature from small navigation, beach and structure projects to large nautical charting projects. Although only a fraction of these surveys have been environmentally motivated, the utility and flexibility of the technology is increasingly recognized for its suitability for such work. The reasons for this can be summarized as follows: first, the speed with which data can be collected for large areas provides a snapshot on a regional scale. Consecutive surveys may be compared to monitor changes in bathymetry and topography that occur over time, such as beach and cliff erosion, coral reef damage, and navigation channel and harbor shoaling. Second, because LIDAR is non-intrusive, remote, shallow, rocky shorelines and coral reefs that present extreme hazards for survey vessels, are easily surveyed without imperiling the environment. And finally, the density of SHOALS survey data reveals linkages between subaerial and marine processes that affect the nearshore environment.
Sidebar: The SHOALS SystemBuilt by Optech Inc., Toronto, Ontario, Canada, SHOALS is the result of a development effort begun in 1988, and cost-shared with the Canadian government under the U.S./Canadian Defense Development Sharing Program. Following extensive field testing, which established that it met or exceeded all its design goals, SHOALS was accepted as an operational system in 1994. It is operated by John E. Chance & Assoc. Inc. of Lafayette, La., and owned by the USACE. Operations are administered by the Joint Airborne LiDAR Bathymetry Technical Center of Expertise (JALBTCX), which is a partnership with the Naval Meteorology and Oceanography Command and located at the Corps’ Mobile District Office.
SHOALS incorporates a 400 Hz laser, co-linearly scanning pulses of infrared (l = 1064 nm) and blue-green
(l = 532 nm) light across a swath. The infrared light is reflected from the water surface and the blue-green from the seabed; the time difference between the two indicates the depth. The limiting factor on performance is water turbidity, but the ability to detect the seabed in depths in excess of 30 m is usual in optically clear waters. Operating at an altitude of 300 to 500 m and at speeds up to 70 m/s, SHOALS provides measurements on a 4 to 8 m horizontal grid, covering up to 35 km2 per hour. Data density can be adjusted by flying higher or lower at different speeds or by selecting different scan widths. SHOALS is currently operating in a DHC-6/300 Twin Otter aircraft contracted from Ken Borek Air Ltd. of Calgary, Canada. The Twin Otter is a versatile short take-off and landing (STOL) aircraft with endurance in excess of five hours. Commonly operated from jungle, dirt and ice airstrips, it is an extremely maneuverable aircraft, ideally suited for operating around steep slopes coastal slopes, while its quietness makes it well-suited for carrying out low-level work in environmentally sensitive areas.
A down-looking video provides a record of the area surveyed and assists with classification of anomalous data, while the audio track is a continuous record of both the pilots and operator’s real-time observations. Continuous annotation of the time, latitude, longitude, altitude, and the pitch, roll and heading of the aircraft is incorporated, while a set of nadir-aligned cross hairs can be used to assist with positioning objects of interest.
SHOALS’ primary method of positioning is differential global positioning system (DGPS), and in this mode, vertical measurements are referenced to the local water surface. In response to the USACE’s need to map the upland beach, dunes and above-water portion of coastal structures, KGPS technology was incorporated in 1996 to provide full topographic capability, and therefore, the ability to rapidly survey entire shoreline regions seamlessly across the land/water interface.