Evaluation of bank erosion and stability analysis along Rönne å at Ängelholm, Sweden

Abstract: This master thesis report presents the results of evaluation of bank erosion and stability analysis along Rönne å (river) at Ängelholm, located in south-west of Sweden within Scania. The main objectives cover building a hydrodynamic model, studying parameters that define sediment transport, modelling sediment transport within the river, mapping out erosive stretches and comparing to earlier observational study conducted by Norconsult in 2013, and finally conducting analysis of bank stability in most erosive stretches. The study section considered begun from E6 and E20 bridge to the river mouth, a total distance of about 12 km. In HEC-RAS, the one-dimensional model was employed. To build the model on the already provided DEM, the HEC-RAS Mapper was used. After building the model, steady flow and sediment transport simulations were conducted to attain the objectives mentioned above. For the maximum, minimum, and average downstream sea water level, the steady flow simulation revealed that having a minimum sea water increases velocity and shear stress along the river. This was due to the no backflow effect from the sea. The sediment transport simulation on the other hand also revealed that at minimum downstream sea water level, more sediment is transport during this scenario. Further, it revealed that high flows also contribute greatly to more sediment discharge, and this was seen in 2007 when there were high flows even in summer. For this year, the highest sediment discharge of 1516 tons/year was attained for the most downstream cross section 21. Of the 363 cross sections assigned on the river, 127 cross sections representing 35% showed to be undergoing erosion and the rest 65% showed either no change or deposition. From the 127 cross section that showed erosion, 8 i.e., 9364, 9279, 9255, 9228, 9156, 7594, 7392 and 6580 were eroding faster than the others. These were selected and analyzed for bank slope stability using HEC-RAS BSTEM. Results from BSTEM simulation revealed that all the 8 cross sections had their safety factor above 1.0 signifying that the banks were all stable. However, some cross sections i.e., the left and right bank of XS 7594, the left bank of XS 9228 and XS 9255 had its safety factor (FS) too close to 1.0. The conclusion drawn and deduced from this is that their probability of failure will increase.