Sediment Transport

Sediments are a major component of the suspended load and sediment bed in many rivers, lakes, estuaries, and coastal waters. These sediments can generally be classified as cohesive and noncohesive. Cohesive sediments are primarily composed of clay, silt, and organic matter, which readily adsorb contaminants introduced into an aquatic system. Heavy metals and organic chemicals are particularly susceptible to finegrained sediments. Noncohesive sediments, primarily fine and medium sands, are generally not associated with contaminants. However, understanding their transport and fate is essential to adequately characterize the sediment and contaminant distributions in water bodies.

Exponent personnel are skilled in the development and application of two- and three-dimensional models of sediment transport. Examples of such models include the finite element model STUDH (which is part of the USACE TAB II suite of models) and finite difference models such as EFDC and ECOMSED. These models are capable of realistically modeling a wide variety of aquatic systems. The sediment transport computations are coupled to hydrodynamics and waves. The finite difference models use an orthogonal, curvilinear coordinate system so that a spatially variable numerical grid can be applied. For many studies involving the transport and fate of chemicals in the aquatic environment, sediment transport model results are coupled to one or more pollutant transport models.

Processes Modeled

• Hydrodynamics
• Advection and dispersion
• Resuspension, aggregation (of cohesive sediments), and deposition
• Wave–current interaction on bed shear stresses
• Sediment-bound tracer transport

Model Inputs

• Time-varying flow rates, water surface elevations (tide/seiche), wind forcing, and sediment loads
• Diffuser inputs
• Sediment resuspension and depositional parameters, bed sediment properties

Model Outputs

• Spatial and temporal distributions of:
  • Suspended sediment concentrations
  • Tracer concentrations
  • Erosion/deposition and subsequent change in bed elevations
  • Fraction of cohesive and noncohesive sediments constituting the bed

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