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

River morphodynamics and sediment transportRiver morphology and morphodynamic

Numerical study of discharge adjustment effects on reservoir morphodynamics and flushing efficiency : an outlook for the Unazuki Reservoir, Japan

The Unazuki Reservoir is located on the Kurobe River, which is influenced by a catchment with one of the highest sediment yields in Japan. Due to a sufficiently available discharge during flood events, annual sediment flushing with full water-level drawdown (i.e., free-flow sediment flushing) is conducted to preserve the effective storage capacity of the reservoir. Nevertheless, the upstream half of the reservoir (i.e., study segment) suffers from the excessive deposition of coarser sediments. Remobilization of these coarser materials and their transportation further downstream of the reservoir is a priority of reservoir owners for sustainable reservoir functions, such as flood-risk management and hydroelectric energy generation. In this paper, an already conducted sediment-flushing operation in the Unazuki Reservoir is simulated, and its effects on sediment scouring from the study segment of the reservoir together with changes in bed morphodynamics are presented. A fully 3D numerical model using the finite volume approach in combination with a wetting/drying algorithm was utilized to reproduce the hydrodynamics and bed changes using the available onsite data. Afterwards, the effects of discharge adjustment on the morphological bed changes and flushing efficiency were analysed in the study segment using an additional single-discharge pulse supplied from upstream reservoirs. Simulation results showed that an approximately 75% increase in the average discharge during the free-flow stage changed the dominant morphological process from deposition into an erosive mode in the study segment. If the increase in discharge reaches up to 100%, the flushed volume of sediments from the target segment can increase 2.9 times compared with the initiation of the erosive mode.Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT

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