Calibration of acoustic doppler current profiler apparent bedload velocity to bedload transport rate

The bias in aDcp bottom tracking induced by near-bed particle motion provides a measure of apparent bedload velocity (va ). Previous site specific field and laboratory calibration exercises to relate bedload transport rate (gb ) to va are reviewed. Good linear relations have been obtained, with r2 ranging from 0.30 to 0.91. The calibration relations differ as a function of both bed material and aDcp operating parameters. In general, for a given value of va , higher mass transport is observed in gravel-bed rivers than sand-bed rivers. The influence of bedload grain size is evaluated in detail using a new data set of 144 paired va and gb samples obtained at the braided gravel-bed Rees River, New Zealand. The bed material of the Rees River includes both sand and gravel, with subsurface median (D50) grain size of 7.5 mm, and the D50 of collected bedload samples ranging from 0.2 mm to 13 mm. Relatively poor gb-va correlation was observed for the entire Rees River data set (r2 = 0.29). However, excluding samples with <69% good bottom track data and classifying the remaining samples into subpopulations by bedload grain size (sand, sand with > 10% gravel, gravel with > 10% sand, and gravel) improved gb-va calibration relations (r2 ranging from 0.45 to 0.98). The implications for utilization of such size-dependent gb-va calibration relations in rivers with mixed gravel-sand beds are discussed

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

The presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at (57°550N, 1°380E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is still being released since a blow out in 1990, have been used to identify the generation of the seasonal thermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derived from two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depth recorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the nearbottom temperature. The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration. Besides that, the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima. Results show that the thermocline broke down between mid-October and early November, several weeks earlier than the breakdown of the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site. The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-like structure, associated with the seasonal tidal mixing front