In addition to the random processes described as horizontal turbulence, there are certain more regular processes which may contribute to horizontal mixing. One of these occurs in a shearing current, where the vertical gradient of velocity combined with vertical turbulent mixing leads to an effective diffusion in the horizontal direction. It is shown that this effect occurs in an alternating flow, such as a tidal current, as well as in a steady flow. In estuaries and coastal waters horizontal mixing by the shear effect may be associated with tidal currents, density currents or wind-driven currents. In each case the effective coefficient of horizontal diffusion, Kx or Ky, is inversely proportional to the coefficient of vertical eddy diffusion Kz. The occurrence of a stable gradient of density therefore increases the effective horizontal mixing very considerably. Results obtained from observations in the Mersey estuary and Irish Sea are compared with theoretical estimates of these effects.

Observations were made in a tidal current off Red Wharf Bay, Anglesey, North Wales. The frictional stress at the bottom, Fb, was determined from the velocity profile within the first z m above the bottom and found to be related to the velocity at I m by a quadratic law, Fb = kpUl2, where K has the value 3-5 x 10-3. The corresponding value of the roughness length xo is 0-16cm. Current meter measurements at a number of depths between surface and bottom were made at half-hourly intervals, enabling the acceleration terms in the equations of motion to be determined. From the bottom stress and the acceleration terms, the shearing stress in the water was computed as a function of depth and as a function of time during the tidal period. While at the times of maximum current the shearing stress increased approximately linearly from surface to bottom, as in the case of steady flow in a channel, at other times the acceleration terms caused the stress to deviate considerably from a linear variation. Estimates of the vertical eddy viscosity, N,, indicated that its value was somewhat higher at middepth than nearer the surface or bottom. N , varied during the tidal period, tending to reach maximum values when the current was at a maximum and to be larger during the flood than during the ebb. The numerical values of N , were of the order of 270cm2/s during the flood and 130cm2/s during the ebb, corresponding to depth-mean currents of 35 cm/s and 39cmls respectively. The depth of water averaged 22 m. The observed distributions of velocity and shearing stress are compared with those obtained from a theoretical model, in which the eddy viscosity is taken as constant above a friction layer near the bottom.

Abstract.Since the boundary layer at the sea bed has a number of features in common with boundary layers found in laboratory scale flows and in meteorology, a brief review is given first of the properties which may be inferred from experience in these fields or from theoretical studies. Measurements of velocity profiles, turbulence, and shearing stress which have been made near the bottom, in deep water, and on the continental shelf, are described in relation to this background. In particular, the logarithmic form of the velocity profile near the bed and deductions from it appear to be valid in certain conditions, but the occurrence of ripples and other bed forms is a complicating feature. The relation of the dynamical aspects of the flow to the transport of sediment as bed load and in suspension is discussed. The diffusive properties of the layer are then considered, in relation to fluxes near the sea-sediment interface and to the formation of nepheloid layers or layers well mixed in temperature and salinity.

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Measurements have been made at a number of stations in the Narrows of the Mersey estuary of the residual, or nontidal, flow and the salinity distribution under a wide variety of conditions. The data have been analyzed in terms of a stratification‐circulation diagram as introduced by Hansen and Rattray. In this way the characteristics of the Mersey estuary have been compared with those of other estuaries and with the features of Hansen and Rattray’s theoretical model. In the central stretch of the Narrows, the density current circulation is highly developed and the advective mode accounts for about 75% of the upstream transfer of salt. Toward both the upstream and downstream ends of the Narrows, beyond which the estuary becomes wider and shallower, the advective transfer becomes less important than the diffusive processes. The value of the stratification‐circulation diagram in representing the conditions in an estuary and interpreting them in terms of the underlying processes is confirmed.