In the light of the waterframework directive, the dutch organisation of rijkswaterstaat has decided to investigate the application of logs as a biological alternative for the protection and cover of both naturally developing scour holes. In this research a laboratory model was created that was designed to develop a scour hole by gradually contracting flow. Subsequent experiments with a cover of logs show mixed, but promising results where scour rates reduce up to 60% with respect to the baseline experiments.

The formation of scour holes is a vital thread of retaining the water infrastructure of rivers. Nowadays, protective filters are used to prevent washing away of the underlying material. Rijkswaterstaat started the program 'Stroomlijn’, in which trees (logs) are cut from the floodplains and placed in a sheltered zone of the river to test whether this would increase the flora and fauna. After some years, the logs might be re-used as bottom protection. This kind of bottom protection is formed by placing multiple logs next to each other, which together form a log carpet, and placing multiple log carpets on top of each other. This is called a log filter. Until now, no research has been carried out into the use of logs as bottom protection. The main function of a log filter is to reduce the hydraulic load so that the erosive capacity of the grains of the bottom will not be reached. Since a log filter has the same main function as a geometrically open filter, this research focuses whether the hydraulic load reduction of a log filter could be described by the current theory of granular open filter. In granular open filter theory, the hydraulic load is defined as the pore flow velocity and turbulent kinetic energy inside the filter layer. The hydraulic load is caused by the influence of the energy slope and the influence of the flow over the filter structure. The pore flow velocity and turbulent kinetic energy caused by the energy slope is always present, regardless of how thick the layer is. The influence of flow over the filter structure decreases exponentially and depends on the load damping length. In order to provide the hydraulic load reduction of a filter consisting of logs, experiments have been conducted in a flume of The Fluid Mechanics Laboratory at the Delft University of Technology. A filter consisting of logs has been mimicked and tested. The hydraulic load, i.e. increasing the depth average flow velocity and discharge, was increased until erosion occurred. During each increasing step, flow velocities above and inside the filter layer were measured. From the experiments, it is concluded that at least two layers are needed in order to ensure a significant decrease of the pore flow velocity and turbulent kinetic energy. Adding more than two layers will not lead to further reduction of the pore velocity or turbulent kinetic energy. The pore flow and turbulent kinetic energy beneath layer 2 are dominantly caused due to the influence of the energy slope. Hence, the equation describing the influence of the energy slope in a granular open filter is also appropriate in case of a log filter. However, the influence of the flow over the filter structure on the pore flow velocity and turbulent kinetic energy in a log filter does not decrease exponentially over depth, whereas in a granular filter it does. Therefore, we concluded that the current open granular filter models which describes the exponential decay of the pore flow velocity and turbulent kinetic energy due to the influence of the flow over the filter construction could not be applied in a log filter. In addition, we concluded that the flow in a log filter is very anisotropic. The flow characteristics differ in front, halfway and behind the log. In front of the log, stagnation is present, which leads to a vertical jet flow directed downwards. This flow causes erosion in front of the logs in the first layer. The erosion process in case of a log filter differs from granular open filters, which stated that erosion is predominantly caused by the hydraulic load, i.e. pore flow velocity and turbulent kinetic energy, in the filter and the bed pressure fluctuations due to low-frequency vortices. We also concluded that at locations where the porosity is higher, erosion occurred at lower hydraulic load conditions, i.e. discharge and depth-averaged flow velocity. This indicates that the porosity is an important variable when logs are used as bottom protection.

Currently multiple projects investigate the effect of lowering the crest of groynes in the Dutch rivers with the aim to lower the backwater effect of groynes under high water levels. There is a major uncertainty however in the modelling of groynes, and therefore in the prediction of the effect of groyne lowering. Currently submerged groynes are modelled subgrid as weirs. The flow processes differ however between groynes and weirs as discharge over a groyne is not purely defined by the contraction and expansion over the groyne, but also by lateral shear and interaction with the surrounding main channel and floodplain. The result is a lower resistance of the cross section than expected from weir modelling. For this reason a numerical investigation was started, comparing the modelling of groynes as different subgrid weirs in a 2D model and the modelling of groynes included in the bed topography in a 3D (non-)hydrostatic model. There were large differences between all different methods of groyne modelling. Furthermore the available experimental data did not suffice to explain the differences. For that reason a new physical model has been set up in the Delft University of Technology, at the hydraulic laboratory of the Faculty of Civil Engineering and Geoscience. The experiment includes a 1:30 scale model of a representative transect of the Waal river. The model is 5 m wide and 30 m long and includes 6 groynes and 5 groyne fields. It further includes a part of the main channel at one side and a part of the floodplain at the other side of the groyne fields. Gravel is fixed to the bed to ensure hydraulically rough flow conditions. In the experiment many measurements were done. The first aim of the measurements was to obtain a good spread of points to validate numerical models on. The second aim was to gain insight in the different flow processes in order to describe the differences between groyne and weir flow and quantify the resistance groynes have on a flow. An important observation was the existence of a region of low flow, at the tip of the groyne. The observation indicates a complex three-dimensional flow which effectively redistributes discharge over the transverse. The contribution of this flow to the two-dimensional momentum balance is then directly as a secondary circulation and indirectly by alternating the distribution of discharge. Complex flow furthermore invalidates the hydrostatic pressure assumption. The observed flow needs further investigation with either more accurate measurement devices or in a 3D non-hydrostatic model. Another observation includes the contraction and expansion of flow over the groyne crest. The expansion of flow over the main body of the groyne seems comparable to the expansion of flow over a weir, which can be predicted as Carnot head losses. The observation of expansion losses is however clouded by other flow processes, such as bed shear stress, the lateral flow around the groyne and possible three-dimensional processes induced by the groyne tip, so that the observed head loss over the groyne is only partially explained by weir-like expansion losses. The comparability between groyne flow and weir flow does not seem to hold at the groyne tip, where no separation of flow is observed. Measurements were performed 10 cm away from the transition between the groyne tip and main body of the groyne. This means that at full scale there is a region of 3 m on either side of this geometrical transition where the transition lies between weir-like overflow and non-weir-like overflow. With the here performed simulations it is possible to adapt weir head loss formulas on the observed head losses over groyne. It should be possible to validate a three-dimensional non-hydrostatic model, which can further quantify the effect of three-dimensional flow and differentiate the different contributions to groyne head loss. Based on these models proper tuning parameters can be chosen to represent the complex flow in a two-dimensional model and separately model the weir-like expansion.