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

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

Dynamics of granular avalanches caused by local perturbations

Surface flow of granular material is investigated within a continuum approach in two dimensions. The dynamics is described by a non-linear coupling between the two `states' of the granular material: a mobile layer and a static bed. Following previous studies, we use mass and momentum conservation to derive St-Venant like equations for the evolution of the thickness R of the mobile layer and the profile Z of the static bed. This approach allows the rheology in the flowing layer to be specified independently, and we consider in details the two following models: a constant plug flow and a linear velocity profile. We study and compare these models for non-stationary avalanches triggered by a localized amount of mobile grains on a static bed of constant slope. We solve analytically the non-linear dynamical equations by the method of characteristics. This enables us to investigate the temporal evolution of the avalanche size, amplitude and shape as a function of model parameters and initial conditions. In particular, we can compute their large time behavior as well as the condition for the formation of shocks.Comment: 25 pages, 12 figure

Bed shear stress measurements over rough fixed and mobile sediment beds in swash flows

Direct measurements of bed shear stress have been conducted over rough fixed and mobile sediment beds in dambreak driven swash flows. The comparison between rough fixed and mobile bed results indicated the significant importance of grain borne shear stress component, induced by increased dispersive stress and the momentum transfer by moving sediment grains to the bed. The increase of the averaged peak bed shear stress under mobile sediment beds can be up to 100% of that for fixed beds. The direct incorporation of the shear stress data into the classic MeyerPeter&Muller (1948) bed load model leads to over-estimate of bed load transport rate and reveals the fact of starved bed conditions applied in the present experiments

Turbulence in mobile-bed streams

This study is devoted to quantify the near-bed turbulence parameters in mobile-bed flows with bed-load transport. A reduction in near-bed velocity fluctuations due to the decrease of flow velocity relative to particle velocity of the transporting particles results in an excessive near-bed damping in Reynolds shear stress (RSS) distributions. The bed particles are associated with the momentum provided from the flow to maintain their motion overcoming the bed resistance. It leads to a reduction in RSS magnitude over the entire flow depth. In the logarithmic law, the von Kármán coefficient decreases in presence of bed-load transport. The turbulent kinetic energy budget reveals that for the bed-load transport, the pressure energy diffusion rate near the bed changes sharply to a negative magnitude, implying a gain in turbulence production. According to the quadrant analysis, sweep events in mobile-bed flows are the principal mechanism of bed-load transport. The universal probability density functions for turbulence parameters given by Bose and Dey have been successfully applied in mobile-bed flows

Modem design for a MOBILESAT terminal

The implementation is described of a programmable digital signal processor based system, designed for use as a test bed in the development of a digital modem, codec, and channel simulator. Code was written to configure the system as a 5600 bps or 6600 bps QPSK modem. The test bed is currently being used in an experiment to evaluate the performance of digital speech over shadowed channels in the Australian mobile satellite (MOBILESAT) project

Organised turbulence over mobile and immobile hydraulically rough boundaries

33rd IAHR Congress: Water Engineering for a Sustainable EnvironmentThe present work is aimed at the study of near-bed organised turbulence over mobile and immobile, porous, hydraulically rough boundaries. The bed was permeable and composed of non-cohesive sediments. Two data sets were analysed, characterised by the same u*. The mobile bed data featured generalised sediment transport for all size fractions smaller than the d90. The fixed bed was obtained as result of an armouring process. Comparison of these data sets reveals differences on such parameters of the bursting cycle as the maximum shear stress and the transported momentum. These results point to a reorganization of turbulence, in the near-bed region, when the bed is mobile. The impacts of these reorganization are discussed, namely in what concerns the third order moments of the distributions of the velocity fluctuations

Biomimetic Algorithms for Coordinated Motion: Theory and Implementation

Drawing inspiration from flight behavior in biological settings (e.g. territorial battles in dragonflies, and flocking in starlings), this paper demonstrates two strategies for coverage and flocking. Using earlier theoretical studies on mutual motion camouflage, an appropriate steering control law for area coverage has been implemented in a laboratory test-bed equipped with wheeled mobile robots and a Vicon high speed motion capture system. The same test-bed is also used to demonstrate another strategy (based on local information), termed topological velocity alignment, which serves to make agents move in the same direction. The present work illustrates the applicability of biological inspiration in the design of multi-agent robotic collectives