Coupled hydraulic and energy relationships
Summary of key issues:
|Description||Examines the distribution of bed shear stresses and compares these with erosion thresholds for the types of sediment present.|
|Temporal applicability||Typically applied over scales of between 10 and 100 years.|
|Spatial applicability||Generally applied over the entire estuary, however, can be applied at a single point or cross-section.|
|Links with other tools||Hydrodynamic and sediment transport models, the hydrodynamic model provides the required flow information in which the bed shear stresses are derived.|
|Necessary software tools / sSkills||1D numerical model, historic bathymetry. A good understanding of the estuary hydrodynamic regime and the ability to interpret modelling results is essential.|
|Typical analyses||Estimate of changes in estuary form (width and depth).|
|Limitations||Calibration is extremely difficult; application and interpretation of the model results can also be difficult; lack of historic data.|
|Example applications||Humber Estuary|
Sediment transport is controlled by the velocity of water and its volume. The competence of the water allows sediment to be picked up and entrained within the water column. The force exerted by this entrainment is known as shear stress. If the shear stress exceeds the threshold for entrainment for a particular grain size, then the sediment will be picked up and transported.
Given the difficulty of predicting the changes that result from sediment transport processes, particularly in estuaries with mixed sediments, one option is to look at the changes in bed shear stress under different scenarios. A hydraulic model is used to predict the flows under the various scenarios under consideration and the changes in bed shear-stress patterns are used to infer the likely changes.