Fluvially-Dominated Deltas

Fluvially-dominated deltas are primarily controlled by the water density difference between the inflowing river water and the standing water on the basin. Different flow types that determine the distribution of sediment and sedimentary structures formed in the delta are homopycnal flow, hyperpycnal flow, and hypopycnal flow. When investigating coastal areas with low tide and wave energy, the deltas in this situation can be inertia-dominated, friction-dominated, or buoyancy-dominated.

Homopycnal flow: occurs when the density of the river water is equal to the density of the standing water in the basin. This type of flow is associated with rapid mixing throughout the general flow. There is also abrupt deposition of the sediments carried to the basin by the river as the two water sources meet.

Hyperpycnal flow: produced when the density of the river water entering the basin is greater than the density of the standing water in the ocean basin. This higher density river water will flow below the standing water in the basin because of the difference in density. A zone of mixing occurs along the outer edge of the flow. As the river water flows beneath the standing water, it erodes the previously deposited bottom sediments. Further out in the basin, the flow eventually deposits the eroded sediments as turbidites.

Hypopycnal flow: is associated with a lower river water density entering a higher density standing water in the basin. Under these conditions, the river water will flow out over the standing water, gradually depositing the suspended clay portion of the sediment load on the prodelta. The clay particles settle out of suspension through the process of flocculation (the clumping of clay particles together due to a positive-negative charge relationship created by the seawater).

Inertia-dominated deltas are considered to be an uncommon dominance for deltas. They are associated with high flow velocities and large amounts of turbulence. Sediments are deposited close to the main flow of the channel as it enters the basin. In other words, the deposition of sediments in inertia-dominated deltas does not have a large lateral component.

Friction-dominated deltas are associated with high bed friction and shear stress. These stresses cause the flow to slow down rapidly and deposit sediment with a wider lateral extent than the inertia dominated deltas. The river/basin setting for this type of situation is more common. It consists of a shallow water level where the river flow and basin meet. This shallow area is a major factor in producing a friction-dominated delta. Features associated with this type of delta include subaqueous levees, middle ground bars (fining seaward), and bifurcated channels.

Buoyancy-dominated deltas occur where the river/basin setting consists of a deeper water level when compared to the friction-dominated delta. This situation leads to the formation of subaqueous levees (parallel banks extending out from the channel). Other deposits common with this type of delta are distributary mouth bars (grading seaward), bar finger sands, distal bars, and prodelta clays.