Surface winds and currents, temperature, and water transparency at heights of 1 and 5 m above bottom in 60 m of water were measured during the period from February 23 to May 2, 1984, at a site on the Mississippi delta. The objective was to better understand the influence of near-bottom flow on the character of sedimentation on the prodelta shelf. Data analysis showed currents to be parabathic, with periodicities of 5 and 20 days. Flow showed a strong east-west polarization, with net flow 0.4 cm/s onshore at the lower level and 0.5 cm/s offshore at the upper. With easterly flow, water temperature declined (indicating upwelling), and current veering between 1 and 5 m was 12°–15° in the sense of a bottom Ekman layer. With westerly flow, water temperature increased (indicating downwelling), and veering was of the same magnitude but in the sense of a surface Ekman layer. Flow and suspended sediment data suggest that during the period of study, deposition of sediment from the turbid plume of the Mississippi River was continuous. Calculations of sediment-settling velocities indicate that the surface plume of turbid river water transported, in suspension, particles with hydraulic equivalent diameters ranging from clay to fine-sand size. Computations of friction velocity indicate that here on the outer prodelta shelf, the observed bottom currents were not competent to erode bottom sediment. Short-period (1–3 days) fluctuations in mean bottom turbidity may be due to variations in the supply of suspended sediment as the surface plume moves laterally in response to the wind and, in at least some cases, to a vertical redistribution of sediment in suspension by currents too weak to initiate resuspension. These fluctuations are probably responsible for the silty and sand laminae (generally a millimeter thick or less) observed in bottom cores. The thickness of the annual flood deposit may approach 1 m in the vicinity of the plume.

Bottom currents and fluviomarine sedimentation on the Mississippi Prodelta Shelf: February-May 1984

Suspended Sediment Transport in Rivers and Estuaries

Ongoing and recently completed efforts in cohesive sediment transport research in Europe are described, with emphasis on the use of models as research tools Numerical models are increasingly used as virtual laboratories to study complex phenomena in which sediment-turbulence interactions play a central role A related research area is the study of the strength development in cohesive sediment beds This paper attempts to indicate where European research stands and what it hopes to achieve in the coming years

Cohesive sediment transport modeling: European perspective

Although the barrier, which is to be constructed about 15 km seawards of London Bridge, need only be closed to exclude exceptionally high tides to fulfil its primary function as a flood prevention device, its design is such that it could be operated as a tide-control structure, closing at a particular stage during each ebb and opening at a similar stage on the succeeding flood, unless a storm surge is forecast. This paper outlines a theoretical study, undertaken at the request of the Greater London Council, to assess the effect on the condition of the water of the Thames Estuary of operating the barrier on a regular basis in such a way as to prevent water levels landwards of it falling below Newlyn Datum.

Pollution problems in relation to the Thames barrier

The Thames barrier

The Greater London Council have commissioned the Hydraulics Research Station to carry out a comprehensive investigation of the hydraulic aspects of means of preventing flooding in Central London. The mathematical model of silt movement in the Thames Estuary described in this paper forms part of that investigation. The main purpose of the model was to attempt to reproduce the periodic pattern of suspended silt concentrations, and the location of the main areas of deposition of silt in the estuary: hence to test the effects of silt movement in the estuary of various tidal barriers proposed for flood protection.

M. W. Owen

Papers

A Mathematical Model of the Effect of a Tidal Barrier on Siltation in an Estuary