Bottom Stress Measurements on the Inner Shelf
Bottom stress shapes the mean circulation patterns, controls sediment transport, and influcences benthic habitat in the coastal ocean. Accurate and precise measurements of bottom stress have proved elusive, in part because of the difficulty of separating the turbulent eddies that transport momentum from inviscid wave-induced motions. Direct covariance measurements from a pair of ADV has proved capable of providing robust estimates. Sherwood et al. designed a mobile platform coined the NIMBBLE for these measurements, and deployed two of them and two more conventional quadpods at seven sites on the inner shelf over a period of seven months. Analyses of these data suggest the NIMBBLEs may provide an accurate and practical method for measuring bottom stress.
The New Instruments for Making Bottom Boundary Layer Evaluations (NIMBBLEs) are low-profile platforms that support a pair of 6-MHz Nortek Vector ADVs and 2-MHz Nortek Aquadopp upward-looking ADCPs. Instrument and battery cases and lead weights are sandwiched between two open grates on a steel frame to form a sled 20 cm high, 1 m wide, and 1.5 m long. A rigid lifting bridle allows the sled to be lifted and towed to a new location by a small vessel without necessarily brigning it aboard. The ADVs are separated by 147 cm and measure three components of flow 29 cm above the deck of the NIMBBLEs, or about 49 cm above the bottom before settling.