Coupled hydraulic and regime relationships
Summary of key issues:
|Description||This model couples both hydraulic models and regime models together. Hydraulic models predict the flows under the various scenarios under consideration and the changes in bed shear-stress patterns are used to infer the likely changes.
Regime theory characterises links between hydrodynamics and estuary morphology by a simple empirical formula (or formulae), describing an estuary equilibrium (or quasi-equilibrium) and subsequent evolution following a disturbance to the system.
|Temporal applicability||Typically used to model medium to long-term events (e.g. 100 years)|
|Spatial applicability||Generally applied to estuary wide models extending from the upstream tidal limit to the mouth of the estuary|
|Links with other tools||Hybrid Model Interface (HMI) aka 'SHELL' (coupling tool between 1D hydrodynamic models and regime equations); hydrodynamic tools, regime tools, Entropy and energy based tools.|
|Necessary software tools / skills||Hydrodynamic model, regime algorithms, understanding of modelling techniques and estuary processes. Programming skills would be advantageous to allow modification of underlying code and assumptions|
|Typical analyses||Change in estuary morphology (area and volumes) and hydrodynamics (water levels, discharges and speeds)|
|Limitations||Expert judgement is required. Model calibration is difficult to achieve without historic information and/or a good understanding of the system.|
|Example applications||Hybrid Regime models of five estuaries have been constructed to investigate estuarine response to changes in morphology as a result of sea level rise these include, the Blackwater, Humber, Mersey, Southampton Water and Thames Estuary.|
This sub-set of hybrid models is perhaps both the simplest and most well-developed. They make use of some form of regime relationships linked to a hydraulic model by a scheme to update the estuary bed (Gerritsen et al., 1990; O'Connor et al., 1990; Spearman, 1995; van de Kreeke, 1996; Tzanetatou, 2000) (Figure 1).
Figure 1 Flow diagram for operation of regime model
In most cases, the hydraulic model is 1-dimensional, allowing long-term simulations to be readily undertaken. Regime modelling tools have been used to investigate the long-term implications of sea level rise, engineering changes and managed retreat by Spearman et al. (1998), Pethick (2002) and in the EMPHASYS guide - PDF 3.97MB (EMPHASYS Consortium, 2000). A similar approach has been adopted for the modelling of an outer delta (de Vriend et al. 1994), again using the sort of regime relationships. Readily available versions of this type of model include HYMORF from HR Wallingford and EstReg from ABPmer.