[an error occurred while processing this directive]

The CRAB

If you were able to visit the FRF, you might see a most unusual vehicle moving back and forth from the beach to deep water. This 10.7 m (35 ft) tall, motorized tripod is the Coastal Research Amphibious Buggy or CRAB. Its primary use is to accurately map the ocean floor. By being able to drive along on the bottom, the CRAB is unaffected by the wave and current motion that affect surveys from boats. The CRAB is one of only two in the world. The other one is owned by a private company that rebuilds beaches around the country. It is powered by a commercial Volkswagon Rabbit diesel engine which in turn runs three hydraulic motors, one for each wheel. Top speed is 3 kph (2 mph). To add stability and prevent freezing in the winter, the tires are filled with a mixture of water and anti-freeze.

If the water is calm, the CRAB can operate to depths of 9 m (30 ft) and in wave heights up to 2 m (6 ft). This ability to operate in breaking waves has allowed the CRAB to collect some of the first data to document the movement of sandbars during minor storms. These data are part of a unique record of bottom changes that the CRAB has collected since 1981.


CRAB photo . .
The CRAB, being delivered by helicopter, and being used as a work platform.


The primary use of the CRAB is to accurately map the ocean floor. Originally the mapping was done with an electronic surveying instrument located on the pier, which followed a reflective prism mounted to the crab. With this instrument the elevation and location of the CRAB was recorded once every second. By subtracting the height of the CRAB out of the recorded elevations, the actual elevations of the ocean floor were found. Changes in the floor can then be found by comparing surveys that are taken in the same location but at different times. This surveying procedure required two people, one to operate the CRAB, and the other to monitor the surveying instrument on the pier. In late 1996 a new procedure was adopted and still is in use today. This procedure involves monitoring and recording data from a GPS unit that is mounted on top of the CRAB. The GPS continuously provides an accurate measurement that is within 2 centimeters of the elevation and location of the CRAB. Another benefit of GPS is that one person can monitor the data and operate the CRAB at the same time. Besides its surveying tasks, the CRAB is also used as a work and diving platform and for precisely locating instruments in the water.



Survey plots taken before and after a storm; notice the big changes in the sand bar.


Next Page (Continue)