Suspended Sediment Supply Research Articles

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

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.

The Paths of the Suspended Particulate Inorganic and Organic Matter in a Small Urban Estuary: The Aarhus Harbour Estuary

The nverine supply of suspended inorganic and organic sediment, its temporal and spatial distribution and its exchange with the bay, were estimated for an urban estuary (Aarhus Harbour Estuary, Denmark) during 1983-84. The river supplies high levels of particulate matter to the estuary. Export from the estuary averages one fifth of the river introduced particulate matter resulting in the accumulation of terrigenous material in the estuary. River discharge determines whether the high depletion of particulate matter in the upper estuary follows an exponential(flocculation) or a lineary curve (dilution). Preferential deposition of organic as opposed to inorganic matter in the upper estuary depletes the particulate matter of organic matter and associated pollutants. The estuary is divided into three depositional zones identified on the basis of sediment activity, grain-size and organic content. A sediment budget is precented for the estuary. Good agreement is shown between the actually dredged sediment quantity and the mass to sedimentation from the budget.