The beach-nearshore portion of the profile extends from the toe of the dune to approximately 

 the 9-m depth. After each beach-nearshore survey, the two parts were combined. The 

 following sections describe the CRAB, identify the sources of errors, and discuss the 

 correction of errors. 



The CRAB 



The unique three-wheeled vehicle, the CRAB (Figure 4), was built by the Wilmington 

 District of the US Army Corps of Engineers, and was modeled after a vehicle originally built 

 by Marine Travelift & Engineering of Sturgeon Bay, for Chicago Dredge and Dock 

 Company, to monitor a Corps of Engineers beach nourishment project. 



The CRAB consists of a tripod of 0.2 m schedule-80 aluminum tubing, connected at the 

 base by horizontal members 2.1 m (7 ft) above the ground, and an operations platform 

 10.7 m (35 ft) above the ground. Power is supplied by a 53-hp Volkswagen engine on the 

 deck which drives a variable stroke hydraulic pump. This pump transfers hydraulic fluid at 

 800 psi or higher to hydraulic motors at each of the wheels. The variable stroke feature of 

 the pump allows an infinitely variable gear ratio in either forward or reverse and constant 

 engine speed. For strength and corrosion resistance, all hydraulic lines are stainless steel, 



except for short flexible sections at the 

 front wheel, which is used for steering. 



Total vehicle weight is about 8,200 kg 

 (18,000 lb); the distance between the rear 

 wheels is 8.2 m (27 ft). Though it 

 appears top-heavy, the liquid-filled tires 

 and wide wheelbase make it very stable. 

 The CRAB has passed a 20-deg tilt test 

 and is designed to withstand even steeper 

 angles. 



Top speed of the CRAB is 3.2 km/hr 

 (2 mph) on land and somewhat less in the 

 water. Since the maximum significant 

 wave height for operation is 2 m (6 ft), 

 the CRAB is capable of operating in all 

 but the most severe storms. The large 

 tires have a negligible effect on a hard 

 rippled sand bottom; however, scour 

 around the tires has been observed in areas 

 of active wave breaking or strong currents 

 if the CRAB remains motionless. The 

 CRAB cannot be used on soft silty or 

 loose bottoms. The position of the CRAB 

 is determined by using an array of prisms, 

 which serves as a target for the Zeiss Elta- 

 2s Electronic Survey System and the 



Chapter 2 Profile Data 



Figure 4. The CRAB 

 6 



