Presentation on theme: "Buoyant Plumes Positively and negatively buoyant plumes contribute to particle transport across and along shelves and to density stratification of coastal."— Presentation transcript:
Buoyant Plumes Positively and negatively buoyant plumes contribute to particle transport across and along shelves and to density stratification of coastal waters. Plumes are normally sharply bounded offshore by fronts, which can serve as permeable barriers to across-shelf transport.
Positively buoyant plumes are the most common and are attributable to low salinity river mouth or estuarine effluents. Negatively buoyant plumes can also occur and are caused by high suspended sediment concentration, brine extrusion from freezing sea ice, and intense cooling of coastal waters by cold air outbreaks. On leaving the confines of a river, a positively buoyant plume will initially expand horizontally, and thin vertically, and eventually detach from the seabed. Further, the plume spreads more gradually and undergoes anticyclonic Coriolis turning – tending toward alongshore coastal currents.
Plume generally constrained close to the coast - for many shelves this is on the inner shelf.
Sediment settling from the plume tends to do so in shallow water, on the inner shelf. For energetic coastal environments, this sediment may then undergo secondary processes, e.g., Cross-shelf diffusion Advection in bottom boundary layer Advection in density underflows (still within bottom boundary layer, but different mechanisms at work).
Negative buoyancy (less prevalent than positive buoyancy) responsible for the downslope transport of dense plumes, including - in the extreme case, turbidity currents. although previously thought to only occur in submarine canyons and on the continental slope, they are presently being investigated in many inner shelf environments. can be formed in situ (e.g., ‘wave-supported’ fluid mud) or by direct discharge of high concentrations into the marine environment (hyperpycnal plume)
Plume velocity can be evaluated using: Chezy Equation F g = F d Where C d = bottom drag coefficient (0.0025 – 0.0050) E t = interfacial drag coefficient (0.0004 – 0.0015)
Simple Chezy model ignores: Coriolis turning Plume interactions with tidal or currents (including waves) More complete (yet still relatively simplistic) analytical modeling is being undertaken by Friedrichs & Wright. They imply that density-driven processes may control the across-shelf transport (these processes move a large amount of sediment in directions perpendicular to prevailing currents) and, the location of many flood deposits around the world.
Forms of negatively buoyant sediment flows: Fluid Mud - Conc = 10-300 g/l of sediment, downslope flow is non-erosive. Convergent processes – Wave-supported - Direct Hyperpycnal Plume – Conc > 40 g/l of sediment in fresh water plume to overcome fluid density difference. Turbidity Currents – after initial trigger, downslope flow sustained by erosion of the seabed (generally coarse-grained)