Tom De Mulder

Validation of large-scale particle image velocimetry to acquire free-surface flow fields in vegetated rivers

The reliability of large-scale particle image velocimetry (LSPIV) methodology to measure a 2D surface velocity field in a vegetated lowland stream is evaluated. To this end, measurements of the free-surface flow field obtained with LSPIV are compared with measurements with an electromagnetic current meter (ECM) close to the surface at four different locations. The measurements were performed monthly, allowing the evaluation of the LSPIV measurements in relation to different vegetated conditions. The difference observed between the mean velocities measured with ECM and LSPIV remains low in winter, whereas an increase is observed in summer. Inappropriate particle seeding density and unsteadiness of the flow are the main sources of LSPIV reliability reduction. Nonetheless, the seasonal average frequency of reliable LSPIV measurements is 97%, 95% and 78% in winter, spring and summer, respectively. The results illustrate that LSPIV is an inexpensive methodology, which provides high-resolution and reliable data to study the flow-field distribution in vegetated rivers, provided some considerations are taken into account to deal with the added complexity of the vegetation presence and the field conditions.

Modelling of the tributary momentum contribution to predict confluence head losses

ABSTRACT This paper proposes a new model to determine the head losses at confluences in one-dimensional models of open channel networks, making use of a momentum conservation approach. Momentum conservation has been applied in several theoretical models for confluence head losses, giving satisfactory results in general. However, for larger confluence angles between the main channel and the incoming tributary, the model accuracy diminished. Many authors identified that a correct estimation of the tributary momentum contribution is a prerequisite for accurate results. This work reports on the development and application of a theoretical model for the tributary momentum contribution, based on similarities with the flow upstream of a circular bend in a straight open channel. It describes the two-dimensional depth-averaged flow features in the tributary under the assumption of a 90° angle confluence in which all channels have equal widths, in order to obtain the resulting momentum contribution. The proposed model predicts head losses within the same order of accuracy as a numerical model solving the shallow water equations in two dimensions throughout the confluence.

New methods for ADV measurements of turbulent sediment fluxes – application to a fine sediment plume

ABSTRACT New methods have been developed to extract turbulent fluxes of momentum and fine sediments from Acoustic Doppler Velocimeter (ADV) data. The methods were validated with turbidity plume experiments. The ADVs backscatter amplitude signal was used to determine the sediment concentration and its turbulent fluctuations. However, different kinds of noise are found in the backscatter amplitude and the velocity signals, which are polluting the turbulent fluxes results. Therefore, spectral noise correction methods have been developed, which allow more accurate quantification of turbulent velocity and sediment concentration fluctuations. The techniques are applied to two benchmark cases of a vertical sediment-laden jet. Reynolds stresses, turbulent intensity of velocity and sediment concentration as well as turbulent sediment fluxes are shown to agree well with two-fluid plume measurements reported in the literature. The methods presented in this paper can be applied to the processing of measurements in cohesive sediment plumes, turbidity currents or mixing layers in the absence of, or in stable, flocculation.

Numerical simulation of near-field dredging plumes: efficiency of an environmental valve

AbstractNumerical simulations of the sediment-air-water buoyant jet released through the hopper dredgers’ overflow shaft have been performed. The release of sediments into the marine environment due to skimming the excess water from the dredging vessel’s hopper can lead to increased turbidity and adverse effects on the adjacent environment. Base-case simulations have been validated using in situ field observations. Simulations have been performed using the large-eddy simulation technique, which allows including the effect of large turbulent structures on the sediment dispersion. The complex nature of the flow field poses challenges for numerical simulations, such as the presence of propeller jets and three different phases: water, sediment, and air bubbles. The model has been applied to simulate the effect of a so-called environmental valve, which reduces air inclusion. This valve has been used in the past, but its efficiency as a function of the boundary conditions was never analyzed before. It is shown ...

A parameter model for dredge plume sediment source terms

The presented model allows for fast simulations of the near-field behaviour of overflow dredging plumes. Overflow dredging plumes occur when dredging vessels employ a dropshaft release system to discharge the excess sea water, which is pumped into the trailing suction hopper dredger (TSHD) along with the dredged sediments. The fine sediment fraction in the loaded water-sediment mixture does not fully settle before it reaches the overflow shaft. By consequence, the released water contains a fine sediment fraction of time-varying concentration. The sediment grain size is in the range of clays, silt and fine sand; the sediment concentration varies roughly between 10 and 200 g/l in most cases, peaking at even higher value with short duration. In order to assess the environmental impact of the increased turbidity caused by this release, plume dispersion predictions are often carried out. These predictions are usually executed with a large-scale model covering a complete coastal zone, bay, or estuary. A source term of fine sediments is implemented in the hydrodynamic model to simulate the fine sediment dispersion. The large-scale model mesh resolution and governing equations, however, do not allow to simulate the near-field plume behaviour in the vicinity of the ship hull and propellers. Moreover, in the near-field, these plumes are under influence of buoyancy forces and air bubbles. The initial distribution of sediments is therefore unknown and has to be based on crude assumptions at present. The initial (vertical) distribution of the sediment source is indeed of great influence on the final far-field plume dispersion results. In order to study this near-field behaviour, a highly-detailed computationally fluid dynamics (CFD) model was developed. This model contains a realistic geometry of a dredging vessel, buoyancy effects, air bubbles and propeller action, and was validated earlier by comparing with field measurements. A CFD model requires significant simulation times, which is not available in all situations. For example, to allow correct representation of overflow plume dispersion in a real-time forecasting model, a fast assessment of the near-field behaviour is needed. For this reason, a semi-analytical parameter model has been developed that reproduces the near-field sediment dispersion obtained with the CFD model in a relatively accurate way. In this paper, this so-called grey-box model is presented.

Experimental Investigation of Free Surface Gradients in a 90° Angled Asymmetrical Open Channel Confluence

State of the art numerical models have come up with a number of possibilities to treat the free surface (e.g. rigid-lid approach, interface-tracking methods, interface-capturing methods). Depending on the case at hand, the complexity of the free surface treatment can be changed. To make a well-informed choice, the modeller should be able to test the model performance in a selection of cases with different relevant processes. In this paper, an experimental set-up is exploited to provide a selection of cases with high-resolution data of the free surface levels in an open channel confluence. This will allow to evaluate the relative importance of the surface gradients and presents data for numerical modellers to assess the capabilities of their chosen free surface treatment for the adopted flow conditions. The selected case is a 90° angled asymmetrical confluence, with subcritical flow, but with a downstream Froude number high enough to have important contributions of the free surface gradients to the overall momentum balances in the measurement domain. Complementary, Large Eddy Simulations with a horizontal rigid-lid treatment of the free surface are performed. This will allow to evaluate the performance of this numerical methodology in the selected case of the confluence flow. In general, the free surface levels are found to be of major importance to the overall momentum balances in the Confluence Hydrodynamics Zone, and thus correct modelling of the effects of the free surface proves to be a prerequisite for correct simulation of the flow. With the data presented in this paper, the performance of the numerical models can be further tested with respect to the treatment of the free surface.

Experimental determination of free surface levels at open-channel junctions

The Author is acknowledged for extending knowledge of open channel confluences by providing additional experiments. More specifically, the Authors research offers a dataset with inclined channels, including not only fully subcritical flows (Type I flow), but also flows with supercritical flow in one upstream branch (Type II flow) or upstream and downstream branches (Type III flow) of the confluence. Moreover, the experimental data are used to assess the accuracy of Taylors model for predicting the increased water levels over a confluence. In this Discussion, we would like to address some issues that are not fully covered in the paper. First, the Authors experimental data will be compared with existing datasets available in the literature. Secondly, it will be verified whether the hypotheses on which Taylors model is based are met in the Authors experiments, in order to allow a fair comparison between the model and experiments. The Discussion is focused on the Type I and II flows.

Closure to “On analytical formulae for navigation lock filling-emptying and overtravel” by LAURENT SCHINDFESSEL, TOM DE MULDER, STEPHAN CREELLE and GERALD A. SCHOHL, J. Hydraulic Res. 53(1), 2015, 134–148

In the original paper, approximate explicit formulae were proposed for the following dimensionless quantities: the filling time , the amplitude and time of the first overtravel peak, as well as the frequency of the chamber surface oscillations around the equalization level. For several reasons, the Authors only discussed the first overtravel peak: (i) conciseness; (ii) the increasing difficulty of experimentally validating gradually smaller water surface deviations of subsequent extremes; and (iii) the changing oscillation period during the overtravel due to the nonlinear damping. Therefore, the frequency was defined in Eq. (13) to be only representative for the first overtravel peak.

On analytical formulae for navigation lock filling–emptying and overtravel

ABSTRACT In culvert-based navigation lock filling–emptying systems, inertia effects have a significant influence on the filling–emptying time and cause a (damped) oscillation of the water surface in the lock chamber around its equalization level, referred to as the overtravel phenomenon. In this paper, the derivation of analytical formulae for the lock filling–emptying time and overtravel peak of systems consisting of a number of identical culverts is revisited. In comparison to earlier publications, the underlying assumptions are made explicit and the importance of accounting for the surface area ratio of lock chamber to upper reservoir in case of filling (or lower reservoir in case of emptying) is pointed out. Additionally, it is shown how the applicability of the analytical formulae can be extended to lock filling–emptying systems with more complex lay-outs by using an “equivalent culvert” approach. The validity of the analytical formulae is thoroughly assessed, first by comparing to an accurate numerical solution of the governing non-linear second-order differential equations, and second, by means of experiments in a physical model.

The effect of geometry and tidal forcing on hydrodynamics and net sediment transport in semi-enclosed tidal basins

A new depth-averaged exploratory model has been developed to investigate the hydrodynamics and the tidally averaged sediment transport in a semi-enclosed tidal basin. This model comprises the two-dimensional (2DH) dynamics in a tidal basin that consists of a channel of arbitrary length, flanked by tidal flats, in which the water motion is being driven by an asymmetric tidal forcing at the seaward side. The equations are discretized in space by means of the finite element method and solved in the frequency domain. In this study, the lateral variations of the tidal asymmetry and the tidally averaged sediment transport are analyzed, as well as their sensitivity to changes in basin geometry and external overtides. The Coriolis force is taken into account. It is found that the length of the tidal basin and, to a lesser extent, the tidal flat area and the convergence length determine the behaviour of the tidally averaged velocity and the overtides and consequently control the strength and the direction of the tidally averaged sediment transport. Furthermore, the externally prescribed overtides can have a major influence on tidal asymmetry in the basin, depending on their amplitude and phase. Finally, for sufficiently wide tidal basins, the Coriolis force generates significant lateral dynamics.