SEABED DRAG COEFFICIENT UNDER TIDAL CURRENTS IN THE EASTERN IRISH SEA

Over a 2-month period the seabed drag coefficient C-100 at a site in t he eastern Irish Sea (26-m water depth) had a mean value of 0.0025, wh ich is the value typically assumed, but was highly variable. The objec tive of the analysis was to unravel the causes of the variability in C -100. An error analysis revealed that a considerable part of the varia bility could be due simply to random errors in the estimates of fricti on velocity, from which C-100 was deduced, but the error bounds did no t enclose all of the data. Wave-current interaction was ruled out as t he principal cause of the variability because waves were small and the re was no noticeable correlation between C-100 and ratio of wave-orbit al speed to tidal-current speed. Classifying the boundary-layer flow i nto smooth-turbulent, transitional, and rough-turbulent regimes implie d that flow-regime transitions could explain much of the variability, however, inspection of the data revealed a contradiction that made tha t explanation unlikely: the classification implied that the boundary-l ayer flow was rough turbulent during neap tides but was smooth turbule nt under peak spring-tide currents. The classification scheme was base d on the assumption of neutral stability, which, according to the stab ility parameter z/L, was violated for a considerable portion of the ti me. Boundary-layer stratification by suspended sediment was found to p rovide a coherent explanation for drag coefficient variability; when z /L < 0.03, C-100 was relatively constant, which reflected a relatively constant underlying bed roughness, but when z/L > 0.03, the buoyancy flux due to the suspended sediment caused C-100 to vary widely. The an alysis of the stratified-flow dynamics includes the development of an equation (correct to second order in z/L, where z/L is small) for esti mating bed shear stress from the velocity spectrum, which extends the applicability of the well-known ''inertial-dissipation'' method into t he region z/L > 0.03.