Effect of a Submerged Vane-Field on the Flow Pattern of a Movable Bed Channel with a 90° Lateral Diversion

This laboratory study focused on the effect of a submerged vane-field on the flow pattern and bed morphology near and inside the entrance reach of a movable bed 90° lateral diversion. The system was modelled under live bed conditions for a water discharge ratio of ≈0.2. Two experiments were run until bed equilibrium was reached: with and without a vane-field installed close to the diversion entrance to control the transfer of sediments into the diversion channel. The equilibrium bed morphology and the associated 3D flow field were measured in great detail. The bed load diverted into the diversion was reduced by approximately one quarter due to the action of the vane-field. The vanes prevented the formation of the diversion vortex in the main channel, upstream of the diversion’s entrance, thus contributing to that decrease. They also created a main channel vortex that started at the most upstream vanes and further decreased the amount of bed load entering the diversion. The flow separation zone inside the diversion was larger with vanes, but conveyance was balanced through a slightly deeper scour trench therein. The flow structures described were confirmed through the measurements of the turbulent kinetic energy.

Quantifying hydraulic roughness from field data: can bed morphology tell the whole story?

<p>Bedforms are thought to be a major cause of hydraulic roughness in channels. The geometry of the river bed, shaped by bars, dunes, and ripples, and the spatial and temporal distribution of these, influence the resulting roughness variations. Roughness is a fundamental parameter for understanding river flow behaviour by influencing sediment transport and water level.</p><p>Quantification of roughness is challenging since it is not directly measurable in the field. It is therefore inferred from hydrological characteristics, -including water depth, water surface slope, flow velocity, discharge-, as well as morphological characteristics, -such as bedform height-, or derived from calibration of a hydraulic model.</p><p>This study contributes to the elucidation of factors influencing hydraulic roughness, and its quantification from field data. Proper quantification of roughness and its spatiotemporal behavior will increase our knowledge in river behavior and will lead to improvement of river management strategies and operational models.</p><p>In this research, three methods will be explored, to quantify the spatial distribution of hydraulic roughness in the field. We aim to state the importance of bed morphology for hydraulic roughness and we pursue the auxiliary aim to explore the spatial distribution of bedforms and roughness in our case study area river Waal, the Netherlands.</p><p>Method 1 uses the St. Vernant equations (better known as the Chezy equations) to quantify roughness, with as input among others flow velocity, bed slope and water surface slope. This value is seen as the ‘true’  roughness of the river system. Method 2 is a traditionally often used method, where form roughness is obtained from dune characteristics such as height and length via empirical predictors. Method 3 makes use of characteristics of the bed itself, not strictly related to 2D bedform geometry, specifically the inclination of the streamwise local elevation profile, i.e. local topographic leeside angle. Doing so eliminates the necessity of defining dune characteristics, and therefore taking one, often arbitrary, step out of the procedure to quantify roughness.</p><p>The three methodologies show the same general trend and order of magnitude of roughness (C=30-70 m<sup>0.5</sup>/s, mean 42 m<sup>0.5</sup>/s) however kilometer-scale variations show contrasting patterns. Nor dune geometry neither local topographic leeside angle manage to fully explain the variations in the roughness as obtain from the st. Vernant equations. From this we conclude that bed morphology does not seem to be the only explaining factor for roughness variations. Possible explanations include the low leeside angle of dunes (mean <10°), the influence of man-made structures such as groynes and longitudinal training dams, the influence of fixed gravel layers in sharp bends, river curvature, and cross-sectional variation in river depth (bars) and flow velocity. Further steps will be made to unravel the contributing factors for spatial variation in roughness.</p>

Particles bed morphology under an oscillating stiff and flexible plates

We analyse the different morphologies induced by an oscillating plate above an erodible bed. We present some data describing how the shape and the stiffness of the plate affects the main features of the generated heap. We investigate several configurations with different geometries, frequencies and the amplitudes. Some preliminary results are available, in which the role of the flexibility of the plate is taken in account. Unlike the rigid plate in which a proper oscillation induces the formation of one heap, the morphology of the bed is now characterized by more than one heap due to a different pressure profile induced by a flexible plate.

Flow Characteristics and Bed Morphology in a Compound Channel between Two Single Channels

In mountainous areas, a river can widen from a single channel to a compound channel under the influence of geological conditions or human impacts, bringing about challenges in terms of flood control and channel regulation. This paper reports the results of tests conducted in a 26 m long flume with a uniform sediment bed (grain size = 0.5 mm), investigating the flow characteristics and bed morphology in a compound channel between two single channels. The stage‒discharge relationship in the compound channel and the longitudinal and cross-sectional bed profile in the compound channel between two single channels are presented and analyzed. The experimental results indicate that the flow characteristics and bed morphology in a compound channel between two single channels are significantly different from those in a normal compound channel. Based on the experimental data and observations, the mechanisms of flow and sediment transport in the compound channel between two single channels are illuminated.

Effect of Inclination Angles on the Local Scour around a Submerged Cylinder

In ocean engineering and coastal environmental studies, local scour around a submerged structure is a typical issue, which is affected by the inclination of the structure. To investigate the effect of inclination directions and angles on flow structure and the bed morphology, a three-dimensional numerical model of a submerged inclined cylinder was established. In this model, the hydrodynamics are solved from the RANS (Reynolds-averaged Navier–Stokes) equations closed with the RNG k-ε turbulence model, while the bed morphology evolution is captured by the sediment transport model. In the case of vertical-cylinder scour, the simulation results agree well with existing laboratory experiments. In the cases of inclined-cylinder scour, the results show that the inclination direction not only changes the intensity and the location of the downflow but also modulates the pattern of the horseshoe vortex in front of the cylinder, thus influencing the local scour depth and the morphology of the bed. Compared with the case of vertical cylinder, the scour around an upstream-inclined cylinder is deeper, mainly due to the enhancement of downflow in front of the cylinder. The scour around a downstream-inclined cylinder is shallower and broader due to the weakened downflow and accelerated incoming flow. The maximum scour depth decreases with the inclination angle in the downstream-inclination case. In the upstream-inclination case, the maximum scour depth does not vary monotonously with the inclination angle, which results from a competitive effect of the horseshoe vortex and downflow in the front of the cylinder.