Near-bed turbulence measurements, stress estimates and sediment mobility at the continental shelf edge

As a contribution to ocean-shelf exchange studies, the Sediment Transport and Boundary Layer Equipment STABLE has been deployed west of Spain near 42degrees40'N in 200m water depth, in December and August. and at three other continental shelf edge and slope locations off north-west Europe. Turbulence at 8Hz and acoustic back-scatter at 4Hz were measured at three levels between 0.3 and 1 m above the bed. Typically the speed profile shows little shear; the turbulence data are necessary to estimate near-bed stress. Reynolds stress and total turbulence energy approaches give consistent results; total turbulence results appear more robust but can be affected by long surface waves. Typical peak values of friction velocity u* per tide were 10 mm/s off Spain and 7-18 mm/s at the other locations, and they were asymmetric owing to along-slope flow. Record maxima were 30 mm/s, 11-18 mm/s respectively, sufficient to move local sediment except for a short 'event-free' record at Goban Spur. Acoustic backscatter (SPM) showed peaks matching peaks of of or waves in the case of winter deployments off Scotland and Spain. Comparison of these STABLE measurement periods with contemporary current records of longer duration suggests that bed stresses in all these upper-slope locations are often sufficient to move the local sediment. Off Spain. shelf sediments are re-suspended and transported down-current. The sandy fraction settles soon. but the fine fraction (w(s) < 0.1 mm/s) is progressively winnowed down-current to the shelf edge, suggesting export of the bound organic matter. (C) 2002 Elsevier Science Ltd. All rights reserved.