View source for Morphodynamic Models - Bedload Transport

[[de:Morphodynamische Modellverfahren - Feststofftransportmodelle]]
In the last few years bedload management for the purpose of creating a dynamic bottom balance in federal waterways has increased significantly in importance. For the Federal Waterways and Shipping Administration (WSV), active bed load management has now become an essential tool for safeguarding competitive fairway conditions. In addition, as a result of the legal developments in the environmental sector, problems requiring an integrated, interdisciplinary approach have become vastly more important. The orientation of the maintenance measures of the German national government to management objectives and the programmes of measures relating to the implementation of the Water Framework Directive, which aim for improvements in the diversity and quality of the structure of the water and the bank zones, call for simulation tools that enable the WSV to implement the measures that will be required in the coming years effectively and without negative consequences for the transport function of the waterways. Planning, assessment and optimisation of the corresponding measures therefore rely on predictions which, in view of the complexity of the processes, can only be achieved through the use of numerical models for solid matter transport. 
The simulation parameters of morphodynamic methods consist essentially of two physical processes: the transport of suspended matter in the body of water and the near-bottom bed load transport. While the transport of suspended matter can be described by a hydrodynamic transport equation (advection-diffusion equation), there is a large number of empirical equations (Meyer-Peter-Müller, Bagnold, Engelund-Hansen, van Rijn, Zanke, etc.) for the bed load transport which critically influences the morphological development of the river bed in free-flowing inland waterways. These equations frequently have only an extremely restricted area of validity and must be adapted to the specific conditions of the waterbody by means of time-consuming calibration work. 
The mathematical-numerical description of transport processes, whether referring to suspended matter or to bed load, is always based on a previous flow simulation. The coupling of the hydrodynamic and morphodynamic methods therefore require special attention, in order to ensure the feedback between changes in the river bed caused by bed load (geometry, roughness) and the flow simulation. Against the background of very long simulation periods for morphological problems ranging from several years to decades, the support of time steps of differing lengths is important in the morphodynamic and hydrodynamic methods in order to be able to take into account the widely varying time scales between flow and morphology and, in this way, to achieve acceptable computing times. 
One-dimensional, morphodynamic models, which because of their basic equation of hydrodynamics, are limited in application to long-term and large-scale indication of trends may be regarded as the state of the art technology. In this field, the BAW uses the method HEC-6T developed by the company Mobile Boundary Hydraulics from the USA for large-scale models with flow lengths of more than 100 kilometres. The demands of spatial differentiation and precision of the model predictions along with continually growing computing resources are now leading to a focus on the application and further development of multi-dimensional morphodynamic models. 
For applications on free-flowing waterways in the inland area, various requirements for applications with relevance to practicality and forecasting ability must be accounted for by the use of multi-dimensional morphodynamic modelling methods: 
* fractional calculation of sediment transport
* vertical discretisation of the sediment inventory, e.g. using a multi-layer model
* various coupling options between the morphological and hydrodynamic model to allow for the differing time scales of morphodynamics and hydrodynamics
* calculation of the beginning of the motion of the bed load fraction (e.g. when the critical bottom shear stress is exceeded)
* selection of suitable bed load transport formulae
* effects resulting from differences in particle size (hiding and exposure effects)
* influence of bottom inclination on the bed load transport 
* influence of secondary flows in bends when coupled with two-dimensional hydrodynamic methods
* subsoil resistant to erosion
* roughness effect of transport bodies (e.g. dunes, ripples)
* calculation methods for the erosion and deposition of fine sediments
* calculation methods for the settling rate of fine sediments
The BAW mainly uses the morphodynamic method Sisyphe (Villaret, 2005).  The calculation method for the transport of bed load and suspended particles is one module of the comprehensive system of programmes known as Telemac developed by the company Electricité de France (Hervouet and Bates, 2000; Hervouet, 2007) and can be coupled both with the two and three-dimensional hydrodynamic method of the programme system. An additional morphodynamic method developed jointly by the BAW and the University of the German Federal Armed Forces in Munich known as SediMorph (Malcherek et al., 2005) is used to calculate bed load transport. This method can be coupled with both Telemac2D and UnTrim, the three-dimensional hydrodynamic method from the University of Trento (Casulli and Walters, 2000). The routine application of multi-dimensional morphodynamic methods and the proof of their prediction ability still require further development work in regard both to methods and to work organisation.
 
Contact person: Dr.-Ing. Thomas Brudy-Zippelius 

==Bibliography:==
* Casulli, V. and R.A. Walters (2000): An unstructured, three-dimensional model based on the shallow water equations. International Journal for Numerical Methods in Fluids 2000, 32: pages 331-348.
* Hervouet J.-M. and P. Bates (Ed.) (2000): The Telemac Modelling System, Special Issue of Hydrological Processes. Volume 14, Issue 13, pages 2207-2363.
* Hervouet J.-M. (2007): Hydrodynamics of Free Surface Flows: Modelling with the finite element method. Wiley, Chichester.
* Villaret, K. (2005): User Manual Sisyphe Release 5.5. HP-76/05/009/A, Department National Hydraulics and Environment Laboratory, Electricite de France.
* Malcherek, A. et al., (2005): Mathematical Module SediMorph - Standard Validation Document Version 1.1, Technical Report, Bundesanstalt für Wasserbau, 2005.

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