Morphodynamic Simulation of River Bend Retreat (erosion)
The theory and algorithm of riverbank erosion and failure was developed in form of a novel mathematical model of bank retreat process in consolidated non-cohesive soils. It hae been implemented in RMA-Kalypso. Simulation of riverbank erosion due to bank failure mechanisms and bed evolution using the new developed model has been successfully conducted in a meander reach of Hardebek-Brokenlander Au. Modeling the failure and erosion of riverbank in bends by considering a 3D flow and simulation of riverbank retreat due to erosion and failure mechanism. Monitoring the necessary parameters in field and also doing experimental test in laboratory. Measurement of shear stress at bank and exploration of turbulent structure responsible for erosion of sediment at riverbank-toe are other aims of this research. In year 2005: A review on erosion and failure mechanism together with mathematical models of flow and fluvial processes has been achieved and prepared in 70 pages. A new formula for computation of critical shear stress at side slope of the river was developed by consideration of hydrodynamic pressure forces.Hardebek-Brokenlander Au was selected for the monitoring purposes. Required instruments have been studied and will be ordered in the coming year. Detail drawings for experimental set up in the mentioned river have been drawn. The first laser scanning from eroded part of riverbank has been achieved. Having received RMA suit models (RMA10, RMA11, RMAAGEN) from Prof. King, the first steps in implementations of new morphodynamic model were taken, that is getting through the code.Sediment transport and erosion model for the bank-toe of the riverbank in a meandering channel has been developed. It is being implemented in RMA10s model. The necessary instruments has been purchased and are going to be installed in the river. River survey, TIN model, soil mechanical test, erosion test has been achieved. The instrument support structure has been built and prepared. An innovative monitoring program was developed to understand river bend meander migration process. StreamPro mini ADCP was modified in the institute to be applicable for the first time for flow discharge monitoring in very shallow waters. Water level and flow discharge were measured from January to April 2007 continuously. Suspended sediment transport was measured by ASM turbidity profiler. Riverbank was scanned by 3D laser scanner intermittently from Dec. 2006 to Dec.2007. A simple method was developed to solve the complex problem of quantify bank erosion /deposition in folding surfaces (undercut or overhang zones). Bed bathymetry was integrated with 3D bank surfaces to generate seamless complex 3D surfaces of river bend .Flood frequency and magnitude were correlated qualitatively with bank erosion. The bank failure process was additionally recorded by an innovative method using remote camera to record the moment of failure. A rigid traverse system for the measurement of velocity distribution in the river bend was constructed and tested. Simulation and calibration of the 2D numerical model of the flow in the mentioned river was achieved in Kalypso for three flood events. River bank erosion causes loss of fertile lands and is a threat to the infrastructures along its margin. Major bank erosion occurs during flood events, which have been intensified (both in frequency and magnitude) in last decades due to the climate change. The aim of this project is to contribute and improve the edge of knowledge in processes involved in bank erosion mechanism and develop a tool to predict this process, using mathematical and numerical modeling. The complex process of bank erosion was studied in Hardebek- Brokenlander Au in Schleswig-Holstein in last two years by scanning of bank surface and measuring detailed 3D flow filed in river bend. It was concluded that the mechanism of bank failure in non-cohesive soil is much more complex than commonly believed. A new physically-based concept for bank evolution in non-cohesive soils was developed based on the high resolution 3D laser scanner images and detailed flow field measurements. This will be implemented in RMA-Kalypso model in next year as a prediction tool of riverbank evolution. Moreover, it was concluded that current bed shear stress models are not capable of computing bed shear stresses properly in river bend, which is a significant issue in bank-toe erosion. The latter is a key process in triggering bank failure. Publikationen
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