A model for high-frequency acoustic Doppler current profiler backscatter from suspended sediment in strong currents

Acoustic current Doppler profilers (ADCP's) have been used to measure currents for over 20 years. Although ADCP's are not designed to measure acoustic backscatter intensity accurately, several researchers have (successfully) related the backscatter intensity to suspended sediment concentration (SSC) using a random phase acoustic backscatter model. However, this paper shows experimental evidence that in a tidal inlet during high tidal current velocities, a random phase acoustic backscatter model overestimates the SSC by factors up to 60, which could not be explained by a time-varying particle size distribution. An acoustic backscatter model is developed that includes the effect of acoustic backscatter enhanced by coherence in the particles’ spatial distribution as a result of turbulence-induced sediment fluctuations. The model results are compared with field measurements, showing a good correspondence between measured and modelled SSC, including the strong high current conditions for which the random phase acoustic backscatter model was shown to fail. <br/

Consolidation and strength evolution of Caland-Beer Channel mud: Measurement report on laboratory experiments

Many harbours in the world suffer from high siltation rates in their basins. To guarantee safe shipping, harbour authorities have to maintain the navigable depth by having dredged large amounts of mud. Some authorities relate the navigable depth to a depth at which the density is equal to a certain value, e.g. 1200 kg/m3. However,the shear strength might be a more direct criterion to relate the navigable depth to. A research project is conducted to develop a model to describe the consolidation behaviour and strength evolution of mud layers. The second series of experiments for this project, executed at Delft University of Technology,are described in this report. The analysis of the data is left for future work. The sedimentation and consolidation of Caland-Beer Channel mud was simulated in segmented consolidation columns. The use of segments made it possible to obtain well-defined and undisturbed samples of the mud bed. For this reason, more accurate shear vane measurements of the samples could be done than if conventional consolidation columns had been used. Four segmented consolidation columns and one conventional consolidation column were set up. To study the time evolution of the strength of the mud bed, the segmented columns were dismantled at different times. After the dismantling, shear vane tests could be carried out and density measurements could be done with a conductivity probe. The density profiles of the mud layer in the conventional column were measured with a gamma-ray densimeter. Pore water pressures were measured at several times. From these measurements effective stresses and permeabilities were calculated. Various rheological parameters were derived from four different types of shear vane measurements. Flow curves were also measured. It turned out that the effective stresses could be approximated by a power law. Furthermore, the rheological parameters turned out to be approximately linearly interrelated, even though the parameters were derived from different types of rheological experiments. Small modifications of the experimental set-up are recommended for future experiments.Hydraulic EngineeringCivil Engineering and Geoscience

Consolidation theory and rheology of mud: A literature survey

In the framework of project Strength evolution of soft consolidating mud layers, financially supported by the Netherlands Foundation of Technology, a literature survey on consolidation theory and rheological modelling of mud was carried out. A consolidation theory, focused on the Gibson equation (Gibson et al., 1967), is presented. The solution of Gibson's one-dimensional finite strain consolidation equation requires closure relations for the effective stress and the permeability. It is assumed that both relations are functions of the void ratio only. However, the consolidation of soft soils cannot be modelled adequately with these assumptions, since effects as aging, channeling, creep, formation and break-up of flocs, for example, are not taken into account. The incorporation of a rheological model into the consolidation model can result in a better modelling of consolidating mud layers. From this point of view, various rheological models are presented, starting with simple models that become increasingly complex. The complex models incorporate thixotropy and yield stress to describe rheolocial complex fluids as mud. Experimental evidence of a relationship between effective stress and yield stress is presented. Topics for future research are indicated.Hydraulic EngineeringCivil Engineering and Geoscience

Consolidation and strength evolution of Dollard mud: Measurement report on laboratory experiments

Many harbours in the world suffer from high siltation rates in their basins. To guarantee safe shipping, harbour authorities have to maintain the navigable depth by having dredged large amounts of mud. Some authorities relate the navigable depth to a depth at which the density is equal to a certain value, e.g. 1200 kg/m3. However, the shear strength might be a more direct criterion to relate the navigable depth to. A research project is conducted to develop a model to describe the consolidation behaviour and strength evolution of mud layers. The third series of experiments for this project, executed at Delft University of Technology, are described in this report. The analysis of the data is left for future work. The sedimentation and consolidation of Dollard mud was simulated in segmented consolidation columns. By using segments well-defined and undisturbed samples of the mud bed were obtained. For this reason, more accurate shear vane measurements of the samples could be done than if conventional consolidation columns had been used. Four segmented consolidation columns and one conventional consolidation column were set up. To study the time evolution of the strength of the mud bed, the segmented columns were dismantled at different times. After the dismantling, shear vane tests were carried out and density measurements were done with a conductivity probe. The density profiles of the mud layer in the conventional column were measured with a gamma-ray densimeter. Pore water pressures were measured at several times. From these measurements effective stresses and permeabilities were calculated. Various rheological parameters were derived from four different types of shear vane measurements. Flow curves were also measured. It turned out that significant segregation occurred, resulting in a mud bed formed on top of a layer of approximately 5-8 cm with a relatively large coarse silt fraction and high densities. For the mud layer it turned out that the effective stresses could be approximated by a power law. Furthermore, the rheological parameters turned out to be approximately linearly interrelated, even though the parameters were derived from different types of rheological experiments. Both the relationships between peak shear stress and density, and between peak shear stress and effective stress show time dependency.Hydraulic EngineeringCivil Engineering and Geoscience

Consolidation and strength evolution of soft mud layers

Civil Engineering and Geoscience

Laboratory experiments on consolidation and strength evolution of mud layers

Many harbours in the world suffer from high siltation rates in their basins. To guarantee safe shipping, harbour authorities have to maintain the navigable depth by dredging large amounts of mud. Some authorities relate the navigable depth to the depth at which the density is equal to a certain value, e.g. 1200 kg/m3. However, the shear strength might be a more direct criterion to relate the navigable depth to. Presently, a research project, which is financed by The Netherlands Technology Foundation, is being conducted to develop a model that can be used to translate results from laboratory experiments to field conditions. To gain knowledge that is required for the model formulation, a first series of laboratory experiments was carried out at the University of Oxford and reported herein Continuous bulk density profiles were measured with a very accurate, non-destructive X-ray densimeter, which is available at the University of Oxford. At discrete levels the pore water pressure was measured. From these quantities accurate effective stress data and fairly accurate permeability data could be obtained. It turned out that both effective stress and permeability can be related to volume fraction of solids according to a power law. The concept of a fractal structure also implies power laws between the quantities mentioned. The fractal dimension obtained equals 2.71 ± 0.05. In contrast with the density and pore water pressure measurements, the vane tests are destructive. In order to study the evolution of yield stress in time, the three columns were started under the same initial conditions. After 8, 13 and 24 days, a constant shear rate vane test was carried out. The peak shear stress, calculated from the torques measured, shows a more or less linear relationship with the effective stress, and apparently exhibits a yield stress at zero effective stress, the so-called true cohesion. Because of the linear relationship with the effective stress, the peak shear stress reduced by the true cohesion can also be described by a power law holding for a fractal dimension of 2.71. The results of this study indicate that the structural properties that determine effective stress, permeability and peak shear stress, can be characterized by a fractal dimension.Hydraulic EngineeringCivil Engineering and Geoscience

Long-term ferry-based observations of the suspended sediment fluxes through the Marsdiep inlet using acoustic Doppler current profilers

Long-term measurements with a hull mounted acoustic Doppler current profiler (ADCP) under the ferry, crossing the Marsdiep inlet between the mainland and the island of Texel (the Netherlands), were used to determine the volume flux and the flux of suspended particulate matter (SPM) through this inlet for the period 2003-2005. Profiles of the SPM concentration were estimated from profiles of the acoustic backscatter intensity in which the shift between the low and the high turbulent regime is taken into account. Calibration constants and tuning parameters were estimated by using data collected during 7 different 13 hour anchor stations. The residual (water) volume flux through the inlet appears to vary strongly on a variety of time scales from daily to inter-annual. A regression analysis indicates that the daily residual volume transport correlates well with the daily mean wind component from the south; the latter likely drives the residual flow along the coast of Holland. The observed residual SPM transport of 7 to 11 Mton/yr is,dominated by the correlation between tidal velocity and SPM concentration variations. This leads to an import as currents and SPM concentrations during flood were higher than those during ebb, a process generally known as tidal asymmetry. Our analysis has shown that regular observations with a ferry mounted ADCP is an effective method to monitor the volume and SPM transport processes in an estuary

Measurement and modelling of the properties of cohesive sediment deposits

Research studies undertaken as part of the "Bed Dynamics" Task D of the EC funded COSINUS project are described. The studies undertaken involve the reformulation of sediment exchange equations, in situ field measurements of bed strength, laboratory settling column experiments, bed consolidation modelling, the development of a model of bed dynamics based on generalised Biot theory and the testing of an integrated erosion/entrainment model against laboratory experiments. The results of the various studies are synthesized and overall conclusions drawn