Bedform dimensions under supply limited conditions

The volume of mobile sediment on the river bed is the primary control on the dimensions of the bed forms that develop. If an unlimited supply of sediment is available the bed form dimensions which develop are determined by the water depth, the flow velocity and the sediment characteristics. This relation is described by the various models for the prediction of bed form dimensions which have been formulated (e.g. Van Rijn, 1984, Zhang, 1999). When the volume of mobile sediment is smaller than the volume required for equilibrium dimensions, the bed forms are supply limited and will remain smaller

Free surface effect on dune morphology and evolution

Our aim in this paper is to illustrate the importance of free water surface effects and sediment transport mode in the morphological evolution of sand dunes to upper stage plane beds. We have analyzed a large number of bed form data, 414 experiments from flumes and field, showing significantly different evolution of dune height and length in shallow (high Froude numbers) and in deep flows (low Froude numbers). In shallow flows, dune heights are observed to grow only in bed load dominant transport regime and start to decay for Suspension numbers greater than 1. Dunes in this case are not observed for Suspension number greater than 2.5. For low Froude numbers, dune heights continue to grow from bed load to suspended load dominant transport regime. Dunes in this case are not observed for Suspension number greater than 5. Furthermore, dunes in shallow flows reach significantly greater heights compared to dune heights in deep flows and dune lengths are generally larger in shallow flows

Development of supply-limited transport due to vertical sorting of a sand-gravel mixture

If a partially mobile sediment is transported an immobile sediment layer can form below the bedforms. This immobile layer can cause a supply-limitation, i.e. the volume of mobile sediment on top of the layer available for transport becomes limited. This causes the bedforms, roughness and sediment transport to be reduced compared to alluvial conditions, i.e. the situation where all bed surface material consists of mobile sediment. We studied the development of the bed stratification in a series of flume experiments with different initial sand-gravel mixtures. In all experiments a thin immobile gravel layer developed with supply-limited bedforms on top; showing a strong similarity with the supply-limitation as observed in situations of sand transport over pre-installed flat immobile beds. Two phases were observed in the temporal development of the stratified bed: I) first a relatively quick development of dunes with\ud gravel accumulating in the dune troughs, followed by II) a slower development of the level of the immobile gravel layer in the bed. In the final equilibrium situation the thickness and composition of the immobile layer appeared to be more or less independent of the initial sand-gravel mixture composition. However, the thickness of the mobile sediment layer (active layer) and the average dune height strongly reduced with increasing gravel concentration in the initial mixture

Comparisons between sediment transport models and observations made in wave and current flows above plane beds

As a part of the MAST2 G8-M Coastal Morphodynamics project, the predictions of four sediment transport models have been compared with detailed laboratory data sets obtained in the bottom boundary layer beneath regular waves, asymmetrical waves, and regular waves superimposed co-linearly on a current. Each data set was obtained in plane bed, sheet flow, conditions and each of the four untuned numerical models has provided a one-dimensional vertical (1DV), time-varying, representation of the various experimental situations. Comparisons have been made between the model predictions and measurements of both time-dependent sediment concentration, and also wave-averaged horizontal velocity and concentration. For the asymmetrical waves and for the combined wave-current flows, comparisons have been made with vertical profiles of the cycle-averaged sediment flux, and also with the vertically-integrated net sediment transport rate. Each of the turbulence diffusion models gives an accurate estimate of the net transport rate (invariably well within a factor of 2 of the measured value). In contrast, none of the models provides a good detailed description of the time-dependent suspended sediment concentration, due mainly to the inability of conventional turbulence diffusion schemes to represent the entrainment of sediment into suspension by convective events at flow reversal. However, in the cases considered here, this has not seriously affected the model predictions of the net sediment flux, due to the dominance of the near-bed transport. The comparisons in this paper are aimed not only at testing the predictive capability of existing sediment transport modelling schemes, but also at highlighting some of their deficiencies

Hydrological and morphological effects of stream restoration in Twente

Due to canalization and implementation of weirs in river systems, water managers in the Netherlands were able to control the rivers. The current changes in river discharge however need new approaches. Changing rivers into more natural, dynamic water systems seems to be a possible answer. However, there is not much experience yet on how to realize this in practice. In this study the effects of removing weirs and implementing measures, such as small dams and meandering of the stream path, is being investigated. Removing weirs can only be done when other measures are taken in order to control the water flows, otherwise risks for flooding or drought become even larger. An important question is how ‘natural’ a river might be in the perspective of other constraints such as safety and agriculture. Most of the times a combination of measures is chosen in order to fulfil multiple goals