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.

Popular Abstract

A river is a stream of water whose flow is driven by gravity in a channel to either another river, a lake or sea. It falls in the category of surface water that can easily be abstracted compared to groundwater. Further, it has numerous benefits to the environment and human life.
Rönne å (river) is the second largest river in Scania. It begins from Ringsjön to Kategat sea area and has many benefits to the inhabitants along the river. Unfortunately, the river undergoes bank erosion, and this has been taking place in the upstream of Rönne å at Ängelholm for many years. The sediment transported from upstream causes problems of sediment accumulation at the downstream. To address this problem, Ängelhom municipality engaged Norconsult in 2013 who produced a GIS layer of locations where bank erosion occurs and its level. The municipality also engaged another company called Marcon Teknik AB (MTE) in 2020 whose bathymetry survey report reveals that there is also riverbed scouring taking place in certain stretches of the river.
This master thesis therefore evaluates riverbank erosion and the stability of banks along Rönne å at Ängelholm municipality in Scania, Sweden. The main objectives cover building a hydrodynamic model and studying parameters that define sediment transport, modelling sediment transport within the river, mapping out erosive stretches and comparing them to earlier observational study conducted by Norconsult in 2013 and finally conducting a bank stability analysis in the most erosive stretches of the river.
There are many softwares that can be used to model sediment transport and conduct riverbank stability on erosive stretches. For this study, HEC-RAS one-dimensional model was adopted to build a hydrodynamic model and estimate sediment transport. For the bank stability analysis, BSTEM an integrated model in HEC-RAS was used.
The hydraulic parameter analysis done under steady state revealed that in portions where the river narrows and where bridges exist, the velocity and shear stress was high while in locations where deep holes exist and where the river widens, the velocities and shear stress were low. For same analysis, the study also revealed that at minimum sea water level, there was high velocity and shear stress due to no backflow effect from the sea. The sediment transport analysis revealed that at minimum sea water level, more sediment was transported compared to the average and maximum sea water level due to the same reason of no backflow effect from the sea. The analysis also revealed that high flows cause sediment to be transported compared to low flows. Further, the analysis revealed that the upstream catchment 541 was undergoing erosion at a rate of 197.8 tonnes/year while the downstream catchment 554 was undergoing deposition of sediment at a rate of 206.25 tonnes/year. The analysis further revealed that 127 cross sections out of 363 which is 35% were undergoing erosion. After a field inspection, it was disclosed that 8 cross sections where eroding faster than the others and these were considered in the bank stability analysis. The bank stability analysis itself found that all 8 cross sections had their safety factor above 1.0. However, 4 cross sections had their safety factor too close to 1.0 This implies that their chance of failure is very high compared to the rest.
The results are fairly good and useful for future studies. The study recommended Ängelholm municipality to first and foremost consider installing flow velocity measurement instruments and those that measure sediment transport. Secondly, conducting a refined established Rönne å model with better input on groundwater and geotechnical surveys close to the banks as well as use softwares that are possible to predict the time frame for when riverbanks will fail such as Slope/W. Lastly, creating a GIS layer for the erosion risk areas will be useful for follow up as well as monitoring.

• author: Kalimukwa, Arnold Mate and Mohamed, Abuzack Juma
• supervisor: Magnus Larson; Mohamed Fainaz Mohamed Inamdeen; Geraldine Thiere
• organization: Division of Water Resources Engineering
• course: VVRM01 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: HEC-RAS, BSTEM, cross section, erosion, sediment transport, deposition, bank stability, safety factor
• publication/series: TVVR21/5011
• report number: 21/5011
• ISSN: 1101-9824
• language: English
• additional info: Examiner: Rolf Larsson
• id: 9054259
• date added to LUP: 2021-06-15 08:58:23
• date last changed: 2021-06-15 08:58:23