Tidal processes • Waves • Sediments • Flocculation

Sediments in Estuaries & Bays

Erosion

The initial movement of sediment begins when the shear stress becomes sufficiently great to overcome the frictional and gravitational forces holding the particles on the bed. This value is known as the critical shear stress ( c) and can be determined for any given particle size.

The relationship between particle size and critical shear stress is not linear, but is complicated by the cohesiveness of the sediment. Cohesion mainly results from the presence of platy clay minerals in the sediment, which are held together by a combination of electrostatic attraction and the surface tension of the water surrounding the particles (see Flocculation). Non-cohesive sediments contain a majority of coarser particles, which are often more near-spherical than cohesive sediments. They lack the physico-chemical interactions that exist between clay particles, and so are free to move independently.

Erosion is more rapid under accelerating or decelerating currents, such as those set up within waves, as compared to steady tidal currents. Initial erosion caused by wave generated currents followed by transportation of the eroded sediment by tidal currents usually produces the most rapid bed erosion.

Deposition

In the case of bedload, particles of a certain size will be deposited once the bed shear stress (o) falls below the critical shear stress (c) that was needed to start them moving.

The settling velocity (ws) (the rate at which a particle settles out of suspension), and a process known as settling lag, control deposition from suspension back to the bed. As the flow carrying the sediment begins to slacken, the particles begin to settle from suspension as soon as the forces due to upward and stream-wise components of turbulence fall below those associated with downward components of turbulence and gravity acting on the particle mass. However, at this stage, the particles do not settle vertically through the water but continue to be carried as they sink, by the still-moving flow. This means the particles may not reach the bed until some time after the flow transporting them has dropped below the critical shear stress. This process enables the mud to be deposited at some distance inland of the point at which the critical settling velocity is reached. Assuming that the ebb tide and flood tide show a similar time-dependent velocity distribution, when the tide turns the deposited sediments will not be re-suspended until much later in the flow, since they have gained some distance due to the settling lag.

For particles smaller than 0.1 mm, the settling velocity is proportional to the square of the particle diameter:

w s d²

where d = particle diameter where the particle is a perfect sphere in an infinite volume of fluid.

This means that for only a small change in diameter there is a significant change in the settling velocity, and so the smaller the particle size the more significant the process of settling lag becomes. For particles greater than 0.2 mm, the settling velocity is proportional to the square root of the particle diameter:

w s d^1/2

This means that large changes in diameter will only result in a small change in settling velocity. For particle sizes between 0.1 and 0.2 mm, the settling velocity is proportional to gradually decreasing powers of the diameter, from d² to d^1/2.