Airborne Mapping Sheds Light on Hawaiian Coasts and Harbors
By Geraint West and Charles E. Wiggins

Hawaii is a Pacific paradise with rugged volcanic shorelines and breathtaking coral reefs. But to those who undertake the mapping of Hawaii's coastal zones, it can be an exceptional challenge.
      During 1999, the U.S. Army Corps of Engineers' (USACE) Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) system - operated by survey company John E. Chance and Associates Inc. - stopped in Hawaii on its way back to the U.S. mainland after completing New Zealand's first airborne Lidar nautical charting survey. While its principal task in Hawaii was to conduct comprehensive surveys of the coastlines of Maui and Kauai, SHOALS also undertook a number of port and harbor surveys in the area to assess navigation channel depths, identify shoal areas, and evaluate the condition of any rubble-mound structures. SHOALS is a bathymetric and hydrographic laser airborne mapping system that surveys above-ground features such as structures, beaches, and dunes.
      The state-of-the-art LIDAR (Light Detection and Ranging) system incorporates a 400Hz laser co-linearly scanning pulses of infrared and blue-green light across a swath. The infrared light is reflected from the sea surface, and the blue-green light is reflected from the seabed. The time difference between the two indicates depth. While maximum penetration is heavily dependent upon water clarity, measured depths in Hawaii of between 25m and 30m were common. For high resolution, a dense grid of depths every eight meters was produced across a scanned swath of 220m, while flying at a speed of 120 knots. Data collection and processing is fully digital, which allows the flexibility of product generation in order to meet requirements of the end-user.
      Coastline surveys were required to help improve storm-wave run-up prediction models for the Federal Emergency Management Agency (FEMA). These models are important for the planning of hurricane evacuation routes, since they greatly improve predictions of where destructive storm waves will strike and cause flooding in low-lying areas. This entailed collecting a comprehensive data set comprising both bathymetric data from the shore to the 30m (100-foot) contour, and topographic elevations to the 30-foot contour. SHOALS couples a unique ability to survey seamlessly across the sea/land interface with a data density sufficient for most coastal applications.
      Formerly, deployment of the SHOALS system to Hawaii would have required a complex and time-consuming delivery of a Bell 212 helicopter, upon which the system was then mounted. However, installation of a new 400Hz laser has now made fixed-wing, worldwide operations routine, and SHOALS is currently operating in a DHC-6/300 Twin Otter aircraft. The Twin Otter is a versatile short-take-off-and-landing (STOL) aircraft - commonly operated from jungle, dirt, and ice airstrips - that makes it ideal for remote operations. With endurance in excess of five hours, the aircraft could easily reach the Maui and Kauai survey areas from its base in Honolulu. By refueling locally at each island, it was possible to maintain an extremely rapid and economic survey schedule. Upon conclusion of the Hawaii project, ferry tanks were installed which more than doubled endurance, enabling the aircraft to return directly to North America in a single leg.
      The Maui and Kauai coast surveys comprised over 250 square miles of data compiled from 93 survey hours on-site, and flown over a period of 41 days. Much of this data was taken from complex, rocky coastal areas that are dominated by surf. Marking water depths from an aircraft allowed fast, accurate information to be collected along rocky coasts and over coral reefs, an environment generally too hazardous for survey boats. Careful planning was required to ensure that certain coasts were surveyed when prevailing winds allowed. Data with these characteristics are also particularly difficult to clean and "QC" (quality control) because the Lidar waveform of surf is difficult to distinguish from land.
     In this case, the main tool to aid the processor is the down-look video record collected concurrently with laser data. This record also aids the processor in resolving anomalies that may be caused by biological activity in the near-surface water column. It is not unknown for SHOALS to be a high-tech fish-finder!
     SHOALS' strength lies in its ability to rapidly survey areas ranging from small harbors to complete coastal regions, often when sea conditions are extremely hazardous for boats. This was particularly true in Hawaii where the surf, so popular with many visitors, presents significant problems for conventional survey vessels. Although deploying to Hawaii presented new logistical challenges for SHOALS, the types of projects conducted were not dissimilar from the work SHOALS has regularly performed across the U.S. mainland. Typically this has included regular re-surveys of navigation channels and structures, as well as regional sediment management surveys to monitor coastal erosion for the Florida Department of Environmental Protection. An often overlooked aspect of a survey as comprehensive as this one, is its inherent added value. In this case, not only did FEMA receive a coastline survey for its modeling requirements, but the Honolulu Corps of Engineers also received a base data set for coastal erosion management. Also, the National Ocean Service was able to check bathymetry of several areas that have not been surveyed since the 19th century. In summary, Hawaii has further proved the flexibility of SHOALS in meeting both regional needs such as storm wave modeling data, and to the specific needs of ports and harbors in the area.

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