Flow Structure and Channel Stability at the Site of a Deep Scour Hole, Mackenzie Delta, Canada

Unusually deep scour holes in distributary channels of the Mackenzie Delta are of concern for oil and gas resource development, particularly with respect to buried pipeline crossings. Surveys of one such hole, carried out in 1985 and 1992, indicated vertical stability and slight lateral movement. The present study examines how the hole may have changed by the mid-2000s and documents the complex local velocity field and related bed material properties. Small discrepancies between isobaths of different years suggest a dynamic stability that involves short-term fluctuations in erosion and deposition. This suggestion was corroborated by detailed measurements of the highly three-dimensional velocity field, which revealed major eddy structures and flow reversals that help maintain sizeable velocity magnitudes despite low mean velocities. The composition of the bed material suggests cohesive behaviour, but the literature indicates a range of critical shear stresses that spans two orders of magnitude. The more probable lower end of this range is consistent with the observed dynamic stability of the scour hole.Des fosses d’affouillement exceptionnellement profondes se trouvant dans les effluents du delta du Mackenzie risquent de poser des problèmes en matière de mise en valeur des ressources pétrolières et gazières, surtout en ce qui a trait aux pipelines enfouis. Selon les levés d’une de ces fosses effectués en 1985 et en 1992, cette fosse serait verticalement stable et aurait un léger mouvement latéral. La présente étude se penche sur la façon dont la fosse pourrait avoir changé une fois rendu vers le milieu des années 2000 et documente le champ de vitesse local complexe de même que les propriétés des matériaux de fond connexes. De petits écarts entre les isobathes de différentes années laissent entrevoir une stabilité dynamique assortie de fluctuations à court terme sur le plan de l’érosion et de la déposition. Cette suggestion a été corroborée à l’aide de mesures détaillées du champ de vitesse hautement tridimensionnel, qui a révélé d’importantes structures à grands tourbillons et des écoulements de retour qui favorisent le maintien d’intensités de vitesse appréciables malgré des vélocités moyennes faibles. Bien que la composition des matériaux de fond suggère un comportement cohésif, la documentation indique une gamme de contraintes de cisaillement critique qui s’étend sur deux ordres de grandeur. Le bas de cette gamme plus probable est conforme à la stabilité dynamique observée à l’égard de la fosse d’affouillement

Review of a Semi-Empirical Modelling Approach for Cohesive Sediment Transport in River Systems

In this paper, a review of a semi-empirical modelling approach for cohesive sediment transport in river systems is presented. The mathematical modelling of cohesive sediment transport is a challenge because of the number of governing parameters controlling the various transport processes involved in cohesive sediment, and hence a semi-empirical approach is a viable option. A semi-empirical model of cohesive sediment called the RIVFLOC model developed by Krishnappan is reviewed and the model parameters that need to be determined using a rotating circular flume are highlighted. The parameters that were determined using a rotating circular flume during the application of the RIVFLOC model to different river systems include the critical shear stress for erosion of the cohesive sediment, critical shear stress for deposition according to the definition of Partheniades, critical shear stress for deposition according to the definition of Krone, the cohesion parameter governing the flocculation of cohesive sediment and a set of empirical parameters that define the density of the floc in terms of the size of the flocs. An examination of the variability of these parameters shows the need for testing site-specific sediments using a rotating circular flume to achieve a reliable prediction of the RIVFLOC model. Application of the model to various river systems has highlighted the need for including the entrapment process in a cohesive sediment transport model

Cohesive sediment transport in a rotating circular flume

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732In this paper, a laboratory study to examine the flocculation mechanism of cohesive sediments is described. A rotating circular flume was used to carry out the study. Two types of sediments were tested; a river sediment and a pond sediment. Deposition experiments were carried out using the sediment-water mixture in the flume, and in-situ size distributions of sediment flocs were measured to assess the flocculation of sediment. The data collected from these experiments were used to calibrate and test a mathematical model of flocculation and settling in a rotating circular flume. This model was developed by the author from a previous study. Results showed that the model predicted the flocculation and the transport of cohesive sediment in the rotating circular flume well for both type of sediments

Modelling of River Flows, Sediment and Contaminants Transport

Economic development projects in river basins, involving mining, forestry, agriculture and urban developments, invariably impact the aquatic ecosystems of the basin [...

Evaluating Effective Particle Size Distributions of Cohesive Sediment under Varying Shear Stress and Bed Configurations in a Rotating Annular Flume

Despite the environmental significance and ecological importance of cohesive sediment (<63 μm), improved knowledge of how effective particle size distributions (EPSDs) change due to flocculation under different conditions of shear stress and bed configuration is required to better understand in situ transport and storage properties and refine existing sediment transport models. Here, a rotating annular flume was used to (i) evaluate EPSDs under different shear stress and bed types (plane-impermeable and -porous gravel bed) for deposition and erosion experiments; (ii) assess flocculation processes with EPSDs; and (iii) compare flume and field EPSDs observations with respect to measured shear stress. While deposition experiments over the impermeable bed led to an EPSD equilibrium in all shear conditions (constant EPSD percentiles), the ingress experiment over the gravel bed resulted in varying EPSDs, and no equilibrium was observed. During the erosion experiment, deposited flocs became coarser due to bed consolidation, and no particle breakage was observed once particles were resuspended. The ingress experiment showed high efficiency in entrapping suspended particles (~95% of initial suspended sediment), and no exfiltration or resuspension was recorded. Flocculation ratios calculated using EPSDs showed negative correlations with shear stress, indicating that increasing flow energy promoted flocculation for flume and field observations. Our results showed that both suspended and bed sediments can flocculate into coarser flocs that, in turn, are preferentially ingressed and stored in the substrate when in suspension. These findings have important implications regarding legacy impacts, as substrate-stored particles can potentially extend the effects of upstream landscape disturbances

Estimation of Fractal Dimension of Suspended Sediments from Two Mexican Rivers

Sampling programs for suspended sediment were carried out in the Usumacinta River and its tributary Grijalva River in Mexico during the years 2016 and 2017. Suspended sediment samples collected during these sampling programs were analyzed in the laboratory using a Rotating Annular Flume (RAF) fitted with a Particle Tracking Velocimetry (PTV) to obtain the 2D images of the suspended sediment particles as they were undergoing floc reconstruction, and subsequently using a glass settling column fitted with inline digital holography set up to obtain 3D holograms of the fully flocculated sediment particles. From these high-resolution hologram images, the fractal dimension of the flocculated sediment particles was obtained using the classical box-counting method and an improved Triangular box-counting method. The estimated fractal dimension of flocculated sediment, which is a measure of floc compactness and structure that control the settling velocity of flocculated sediment was used to validate two empirical models to estimate the fractal dimension in terms of the floc sizes of suspended sediments of these two rivers. It is shown in this study that the floc characteristic can be analyzed in laboratory experiments after floc reconstruction with the use of an RAF and it offers a viable alternative to the costly in-situ sampling that is often carried out in ocean research. The digital holography method employed in this research offers an efficient methodology to obtain the floc fractal dimension. Regarding the innovative aspects and new contribution to science, we can say that we have developed a laboratory protocol to test river waters to establish floc properties such as fractal dimensions of flocs in this research which will help to test river waters on a routine basis with manageable costs. We can also say that we have developed models to predict the relationship between floc fractal dimension and floc size, which did not exist before