Modelling alluvial channel dynamics in a river reach dominated by alternate bars

River morphodynamics and sediment transportRiver morphology and morphodynamic

The Acoustic Properties of Suspended Sediment in Large Rivers: Consequences on ADCP Methods Applicability

The use of echo-levels from Acoustic Doppler Current Profiler (ADCP) recordings has become more and more common for estimating suspended bed-material and wash loads in rivers over the last decade. Empirical, semi-empirical and physical-based acoustic methods have been applied in different case studies, which provided relationships between scattering particles features derived from samples (i.e., concentration and grain size) and corresponding backscattering strength and sound attenuation. These methods entail different assumptions regarding sediment heterogeneity in the ensonified volume (e.g., particle size distribution (PSD) and spatial concentration gradient). Our work was to compare acoustic backscatter and attenuation properties of suspended sediments, sampled in the rivers Parana and Danube that represented rather different hydro-sedimentological conditions during the surveys. The Parana represents a large sandy river, characterized through a huge watershed and the typical bimodal PSD of sediment in suspension, while the Danube represents in the investigated reach an exposed sand-gravel bed and clay-silt particles transported in the water column in suspension. Sand and clay-silt concentrations clearly dominate the analyzed backscattering strength in the rivers Parana and Danube, respectively, with an effect of PSD level of sorting in the latter case. This comparison clarifies the extent of assumptions made, eventually advising on the actual possibility of applying certain ADCP methods, depending on the expected concentration gradients and PSD of suspended sediment to be investigated

Computational Fluid Dynamics in Fluvial Sedimentation Engineering

The present dissertation describes the improvement of a numerical model when predicting sedimentation and erosion processes in fluvial geomorphology. Various algorithms and parameters were implemented in a computational fluid dynamic model for simulation of three-dimensional water flow and coupled sediment transport to gain an insight into the capabilities of the numerical model. Within the scope of the test cases the model simulated suspended load concentrations at a water intake, transient bed deformation in a 90º channel bend, grain sorting processes as well as an unsteady flow regime in a 180º channel bend, transient bed deformation in a sine-shaped meandering channel with occurring bed forms and the free-forming meander evolution of an initially straight channel. All results matched well with the measurements. The results also showed that using computational fluid dynamics for modeling water flow and sediment transport is one step closer of having a universal predictor for processes in fluvial geomorphology. However, there are limitations and some uncertainties in computing the water surface location and alluvial roughness as well as in turbulence modeling. These should be clarified in future investigations

Bank Retreat and Streambank Morphology of a Meandering River during Summer and Single Flood Events in Northern Norway

In recent years, advanced methods for measuring riverbank migration have been used to understand the process of river planform evolution. However, the role of the so-called outer secondary cell in the hydraulic pattern in bank erosion remains unclear. For this purpose, a natural river meander with high curvature bends and steep riverbanks was chosen to quantify bank migration by high-resolution terrestrial laser scanning of three patches along two river bends in four time intervals. The first two time intervals were seasonal, from spring to autumn, and with relatively few water level changes, whereas the third and fourth time intervals were short, just before and after single flood peak events. The yielded point clouds were filtered and digital elevation models (DEMs) were created. These DEMs were used to analyze bank retreat, riverbank morphology, and slope gradient changes in order to understand the role of the outer secondary cell in these processes. In addition, it is shown that storm events causing short peaks in river discharge are less important for river migration than longer-lasting medium discharge