Author(s): Felix Beckers; Najibullah Sadid; Stefan Haun; Markus Noack; Silke Wieprecht
Keywords: Sediment transport; Morphological modelling; Relevant discharge for bed load transport
Abstract: The presented study aims at evaluating the possibility to reduce the total simulation time of long term morphological modelling on the example of the river Saalach. Two methods, the coarsening of an existing mesh formerly used for flood modelling and the reduction of the duration of the inflow hydrograph by compressing it to only bed load relevant discharges, are discussed and presented. Mesh coarsening is conducted with respect to the demands of long term morphological modelling to a minimum number of cells required for an accurate morphological simulation. This procedure leads to a reduction of 85%for both, total nodes and elements. For the timewise downsizing of the hydrographs relevant discharges for bed load transport have to be investigated. Two hydrographs of the years 2005 and 2007 are selected and each of them are compressed to discharges only above 100 m³/s, 130 m³/s, 150 m³/s, 180 m³/s, 200 m³/s and 250 m³/s, respectively. With each resulting hydrograph a 2D morphological simulation is conducted. The results are evaluated in terms of erosion/deposition volume and sorting effects in the active layer. Furthermore, bed load transport rates for a morphological simulation with a linearly increasing input hydrograph are evaluated at characteristic river sections to verify the findings from the erosion and deposition volume balance. The investigations of relevant discharges for bed load transport identify a significant threshold discharge in the range between 100 m³/s und 150 m³/s which corresponds to a reduction of the natural hydrograph of 98% (2007) and 95% (2005) ,respectively. The presented methods proved to be applicable to reduce the total simulation time for a 2D morphological investigation without severe loss of information or deficiency in quality of modelling results and can thus be recommended for the application to similar tasks in comparable studies.