Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics

Closed conduits model

Two types of energy losses in closed conduits can be studied in this model:

1. friction losses (caused by friction at the inner wall of the conduit) and;
2. local losses (caused by sudden changes in the conduit).

Small demonstration flume

This 2.5 m long and 0.08 m wide flume provides students with the ability to study fluid flow phenomenon associated with weirs and sluice gates. Sediment can also be applied to the bed to demonstrate bedform dynamics and sediment transport. The flow system is entirely self-contained and the flume is portable meaning it can be move into any suitable laboratory or class environment for use.

Large demonstration flume

This 5 m long and 0.3 m wide flume allows students to study a wide range of topics including the hydraulics associated with different types of weir and sluice gate. Bed roughness and channel slope can also be adjusted and the apparatus includes a closed sediment circuit which allows a sand sediment load to be fed into the flow enabling bedload transport rates and phenomenon such as local scour around bridge piers to be investigated.

Venturi meter and orifice

Measuring discharges in closed conduits is possible with an orifice or Venturi meter. Students learn how these widely used systems are working by applying the energy conservation law and the equation for continuity of mass.

The experimental research mainly focuses on morphological responses to channel flow and overland flow. Current research topics are in the fields of stream restoration, gully erosion, side channels created by longitudinal dams, plastic transport and bedform development.

Physical scale model with a mobile bed composed of lightweight sediment to establish morphodynamic behaviour around a training dam (River Rhine - The Netherlands).

To manage the expected extremity in high and low river discharge, the state authority for infrastructure Rijkswaterstaat in the Netherlands, is searching for an alternative river design. Currently, the banks on both sides of the River Rhine are protected by groynes. Besides bank protection, the groynes keep the cross-section of the river relative narrow, to ensure water depth for navigation. During high water, however, the groynes are flooded and cause an increase in hydraulic roughness and because of this an increase in water level.

A possibility to ensure the navigation depth during low flow, and reduce the water levels during a high discharges, is the replacement of the groynes by a training dam at the inner bend of the river. The training dam will be placed at 30 meter from the head of the removed groynes and parallel to the river bank. Only at the shallow inner bend there is space for a new construction without reducing the width of the fairway. Between the training dam and the bank a new channels is created. The flow into this side channel is regulated by a fixed weir, so only a limited discharge will flow through the side channel during a low flow situation. During a flood the large amount of water can discharge trough the side channel, because extra space is created since the groynes are removed.

An uncertainty in the new dam and weir design is the behaviour of the river bed morphology during low and high flow. To adequately test the unknown morphological effects at local scale (dam size scale) a physical scale model is built in a flume (Figure 1). The scale model captures both the bed levels around the intake point of the side channel and the effect of the bed levels in the navigation channel. The model has a mobile bed composed of light weighted polystyrene to simulated the bed load transport in the channel. The relative density of the material in water is 1.055. In polystyrene dunes are developing in equal proportions to dunes in the prototype river. The applied flow velocity in the model is based on a scaling analysis of the dimensionless bed shear stress.

Laser scan results of the bed level shows that during low discharge the training dam has a positive effect on the bed levels in the navigation channel. The interpretation of the model results and the translation to the prototype is mainly focussing on the spatial pattern of erosion and sedimentation and the relative bed levels.

A flume model to test the influence had by silt concentration in sand-silt mixtures upon channel bedform geometry and sediment transport rates.

Subaqueous geometric bedform properties such as height, length and leeside angle are crucial in determining hydraulic form roughness and interpreting sedimentary records. Traditionally, bedform existence and geometry are predicted with phase diagrams and empirical equations, which are mostly based on uniform, cohesionless sediments. However, mixtures of sand, silt and clay are common in deltas, estuaries, and lowland rivers where bedforms are ubiquitous. Bedform dimensions may decrease when clay (<4μm) is present and when high suspended sediment concentrations (SSCs) suppress bedform growth. Non-cohesive silt (~30 - 63μm) is mainly transported in suspension and is therefore expected to limit bedform height and length. Weakly-cohesive silt (4-~30μm) is expected to limit bedform development similar to clay. Evidently, it is unknown what the exact influence is of silt on bedform dimensions. 

In this flume model we tested the influence of silt in sand-silt mixtures on bedform geometry and sediment transport characteristics. The 15m tilting, recirculating flume in the Kraijenhoff van de Leur Laboratory was used for this analysis. Sand and silt content were systematically varied for various discharges and the resulting dynamic near-equilibrium bed geometry was measured with a line laser scanner (see Figure 1).

Water levels, 3D velocity profiles, suspended sediment concentration (SSC) and grainsize distributions were also recorded. The data collected will be used to seek relationships between bedform geometry and silt content in the riverbed, bed shear stress and SSC. It is hypothesised that with increasing silt concentration, SSC increases and the hydraulic roughness decreases, resulting in a decrease in bedform height and length. Figure 2 shows an example of the dune beforms generated during the experimental runs, prior to laser scanning.

Several courses, given by the chairgroups HWM and SLM from Wageningen University offers make use of the facilities from WSL. The lab also provides a practical space for both BSc and MSc students, to carry out experiments for their thesis.

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