A 2-D LAND-BUILDING MODEL FOR SUSPENDED SEDIMENT IN COASTAL DIVERSIONS

We use sediment cores, grain-size data and time-lapse bathymetry maps from Cubit’s Gap and the West Bay Diversion on the lowermost Mississippi River, to inform a simple 2-D advection settling model for land construction at diversions. In this model, sediment used for land-building is suspended sediment sourced from the upper fraction of the river’s water column. We couple the record of deposition in Cubit’s Gap, which opened in 1862, with the shorter record available from the West Bay Diversion, which was opened in 2003. Bathymetry and grain-size data from sediment cores show that mud-rich riverine sediment was distributed as a blanketing deposit over a low-sloping subaqueous clinoform.

Using flow hydraulics and channel geometries at these sites, and neglecting the effects of waves and tides, computed advection lengths of well-suspended sediment range from just under 1 km for 100µm sand to more than 8km for 44µm silt. This is in good agreement with grain-size patterns and deposition rates at both sites. The settling velocity of the median particle size in deposits is used in a 2-D advection settling model to compute deposition rates. We use published values and field observations to constrain suspended sediment concentrations, outlet depth and current velocity, the other variables required in this model. Sediment concentration near the bed is taken to be twice the average sediment concentration for a turbulent, well-mixed water column in the Mississippi River. We use the pre-existing bathymetry at these two sites as the basal surface upon which we build deposition.

Contrary to the traditional prograding delta model, blanketing deposition and net shallowing of the basin is dictated by the hydrodynamics of well-suspended sediment at the diversion outlet. This lengthens the time-scale associated with land emergence, but once the basin is sufficiently shallow emergent deposits can “pop up” over a very short span of time.