Sediment

The unit models the transport, erosion and deposition of suspended sediment. It should be noted that the predicted erosion and/or deposition is not taken into account in the 1D hydraulic simulation.

Description

Mud transport is simulated in the 1D Water Quality Solver by dividing the model element into four vertical sub-compartments: the water column, the fluffy bed layer, the consolidated bed layer and the pore water.

The water column is the main body of water through which the suspended sediment or any other suspended or dissolved substance is transported. Sediment may settle out of the water column into the fluffy bed layer. Erosion of either the fluffy bed layer or the consolidated bed resuspends sediment into the water column. Sediment becomes part of the consolidated bed layer by consolidation of the fluffy bed layer.

The fluffy bed layer lies on top of the consolidated bed and has a lower density. Mud falling out of the water column initially settles into this layer. It is limited in size by a maximum thickness. Once this thickness is exceeded any additional material that settles out of the water column is assumed to result in consolidation of the lower part of the fluffy bed layer into the bed layer. Erosion of the fluffy bed layer resuspends sediment into the water column.

As mud consolidates into the bed, water is trapped within its pores. This feature of the model has no bearing on the mud transport. It does, however, effect the rate of transfer of dissolved substances into or from the bed.

In the 1D Water Quality Solver, the term 'mud' represents cohesive sediments. Traditionally these are characterized as consisting of particles of mean diameter less than 60μm. Mineral particles of this size do not settle individually, but tend to form flocs which fall at a faster rate. The tendency to flocculate increases with sediment concentration. Consequently the settling velocity increases with concentration. At high sediment concentrations, the settling velocity can reduce due to a process known as hindered settling. A wide variation of settling velocities has been observed at different sites.

As no general relationship exists between settling velocity and concentration, the 1D Water Quality Solver requires that the settling velocity is given as a series of values for suspended solid concentration at intervals of 50ppm. The data can be obtained from laboratory analysis of mud from the area modelled. Two sets of values are needed, one for settling in fresh water and the other in salt water (>2ppt).

Deposition from the water column into the fluffy bed layer can only occur if the shear stress at the bed is less than some critical stress. The critical deposition stress is typically less than 0.1N/m2.

Erosion of the bed is controlled by critical stresses, which when exceeded allow erosion to occur. The critical erosion stress of the fluffy layer is less than the critical erosion stress of the consolidated bed reflecting the greater shear strength of the consolidated bed. If the bed stress exceeds the critical erosion stress of the fluffy bed layer then all of the sediment in the fluffy bed layer is immediately re-suspended. When the critical erosion stress of the consolidated bed is exceeded, erosion is controlled by an erosion rate constant. The erosion rate constant is an indication of the hardness of the bed (ie how easy it is to erode). It typically has a value of 0.001 kg/N/s.

Equations

The amount of sediment depositing on to the bed is represented by the equation:
                                                                                                              (1)

Where:

m = mass of sediment per unit bed area (kg/m²)

w = settling velocity (m/s)

c = sediment concentration (kg/m³)

The bed stress, τ (N/m²), is calculated using the equation:
                                                                                                         (2)

Where:

f _bed= overall friction factor

u = velocity of the flow in the river (m/s)

ρ = average density of the water (kg/m³)

The rate of erosion from the bed of the river is controlled by the equation:
 for  (3)

Where:

M _e= erosion rate constant (kg/N/s)

τ _e= critical erosion stress of the bed (N/m²)

General

The SEDIMENT module may be run independently of the other water quality modules If the simulation of the OXYGEN WITH SEDIMENT module is required then the SEDIMENT module must be included in the simulation. The SEDIMENT module simulates the following transported variable name:

• Suspended sediment (mg/l)

The SEDIMENT module simulates the following variables on the river bed:

• Pore water (m³ pore water/m² bed area)
• Fluffy bed layer (g/m²)
• Consolidated bed (g/m²)