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Measurement of constriction size distributions using three grain-scale methods
Get PDFThe grain-scale justification for empirical rules for granular filters has largely been based on simplified models of sphere packings. The development of discrete element modelling (DEM) and micro computed tomography (μCT) enables a more scientific appraisal of the void space within granular filter materials. The constrictions or pore throats that govern the filter’s performance can now be directly measured. However, definitive partitioning of the void space is not possible for realistic grain packings and so multiple methods, with differing theoretical bases, have been proposed to identify constrictions. This contribution compares three such methods, each of which results in a constriction size distribution (CSD). The methods considered are the triangulation based weighted Delaunay method (Reboul et al. 2010), a contact based method (O’Sullivan et al. 2015) and an image analysis method based on watershed segmentation (Taylor et al. 2015). Each model, along with its relative advantages is introduced. Then CSDs resulting from applying each model to the same virtual filter samples created using DEM are presented. It is shown that there is reasonable agreement despite the different basis of each approach.
A consideration of empirical filter rules is carried out by normalising the full CSDs by the characteristic diameters typically used to represent the retention capacity of granular filters in design. It is shown that similar CSD curves are obtained for different particle size distributions (PSDs) when curves are normalised by characteristic diameters, irrespective of the method used to identify the constrictions. This gives fundamental support to filter rules using characteristic particle diameters to represent the filtration capability of granular filters
Understanding wave generation in pneumatic tsunami simulators
Get PDFTsunami crest only (elevated) and trough led N-waves have been generated using an improved pneumatic Tsunami Simulator. The crest only wave periods range from 20s to 160s, and 20s to 240s for the trough led N-waves. The length of flume in which these waves are generated was found to have an influence on the measured wave profile at a particular location for waves with period between 40s and approximately 120s. For waves less than 40s the wave generation is not affected by reflections. For waves greater than 120s the variation in free-surface elevation along the flume at any given instant is small resulting in negligible variation in measured profile from different positions within the flume. Outlines for further investigations and improvements to the Tsunami Simulator are given, including initial developments for an active wave absorption system for the Tsunami Simulator
Simplified Comprehensive Scour Model compared to Erodibility Index Method
Get PDFThis paper presents a simplified application of the Comprehensive Scour Model (CSM) developed by Bollaert (2002, 2004). The CSM is a physics-based theoretical method used to estimate the scour potential in fractured rock. It allows the user to determine the time evolution of scour formation as well as the ultimate scour depth and the shape of the scour hole in a semi-3D manner. Despite its successful application to many dams worldwide over the last 15 years, one of the drawbacks of the method is its relative complexity of application and the wide range of values that can be used for basic input parameters. Hence, without any experience, sound application of the method may rapidly become challenging. Based on advanced modelling experiences acquired during the last 15 years with the CSM, this paper proposes a less complex and more straightforward application of the model. This Simplified Comprehensive Scour Model (SCSM) allows estimating the ultimate scour depth and its time evolution in a simple but theoretically still defendable manner. SCSM is compared with the popular and easily applicable semi-empirical Erodibility Index Method. Guidelines are provided for sound application of the method in practice
Changes in soil deformation and shear strength by internal erosion
Get PDFInternal erosion is a major cause for failures and incidents in slopes, embankment dams, landslide dams and dikes. After the loss of some fine particles, the microstructure and mechanical behaviour of the soil change. In this study, a series of tests was conducted on a gap-graded soil using salt to replace part of soil particles to investigate soil deformations and shear strength changes caused by the loss of a predefined amount of fine particles. The dissolution of predefined amounts of salt in the soil specimen during saturation process successfully simulated different degrees of erosion. Drained triaxial compression tests were performed on the samples already subject to internal erosion to study the changes in the mechanical behaviour of the soil. After loss of a significant amount of fine particles, the void ratio became larger, the critical state line rose substantially, the soil behaviour became less dilative, and the shear strength decreased significantly
Exploring the bifurcation between sedimentation versus scour onset below pipelines in unidirectional currents
Get PDFThe onset of tunnel scour beneath offshore pipelines has been previously documented to result from a phenomenon known as piping, in which the hydrodynamic pressure difference across a pipeline causes a sufficient pressure gradient within the soil under the pipe to result in floatation or suspension of downstream sediment particles. The change in propensity for onset of tunnel scour due to sedimentation around the pipe has also been previously noted. This paper explores the potential for predicting the cumulative sedimentation around a pipeline in unidirectional currents using a more fundamental approach than has previously been described in the literature. The results of this approach are compared to model pipeline experiments to see how well the predictions explain the observed changes in sedimentation and the associated effects of sedimentation on the potential for tunnel scour
Flood threshold value for bridge scour prediction and warning
Get PDFIn Taiwan, owing to deep slope, frequency of extreme rail fall, typhoon attacking and earthquake impacts, huge amount of sediment would generate from mountain area and flow with flood toward downstream river. Then, the bridge safety issue is serious discussed during flood event. Therefore, bridge-scour problems have attracted considerable attention in Taiwan, spatially in Zhuo-shui River. In this study, the effects of bend and contraction scours could be neglected because of the river reach near the bridge is roughly straight and the channel width are substantially larger than the pier diameters, respectively. In addition, according to the river bathymetry survey, the bed elevation is generally steady around the bridge area. Therefore, the total scour depth of monitoring bridge is dominated by the local scour. It indicates that a two-dimensional numerical model is adopted to simulate flow field and water depth without sediment transport calculation for collected local scour depth formulas. An appropriate turbulence model, K-epsilon (k-ε) turbulence model, is the most common model used in Computational Fluid Dynamics (CFD) to simulate mean flow characteristics for turbulent flow conditions, spatially near bridge piers. Herein, the adapted empirical equations have been validated experimentally; using return-period hydrograph events, and they can give satisfactory simulation results. Then, flood threshold value for bridge scour prediction and warning can be established in the future. In the present study, based on the experimental results, those empirical equations will be suggested to compute the local scour depth evolution under unsteady flow caused by rapid changes of flow depth and velocity in field
Thermomechanical Erosion Modelling of Baydaratskaya Bay, Russia with COSMOS
Get PDFRapid coastal erosion threatens Arctic coastal infrastructure, including communities and industrial installations. Erosion of permafrost depends on numerous processes, including thermal and mechanical behaviour of frozen and unfrozen soil, nearshore hydrodynamics, atmospheric forcing, and the presence of sea ice. The quantification and numerical modelling of these processes is essential to predicting Arctic coastal erosion. This paper presents a case study of Baydaratskaya Bay, Russia, using the COSMOS numerical model to predict thermal-mechanical erosion. In particular, this study focuses on thermoabrasional rather than thermodenudational processes. A field dataset of onshore thermal and mechanical soil characteristics was supplemented by sources from the literature to serve as input for the model. A detailed sensitivity analysis has been conducted to determine the influence of key parameters on coastal erosion rates at the study site. This case study highlights the need for expanded data collection on Arctic coastlines and provides direction for future investigations
Effect of bottom roughness on sediment transport due to streaming beneath linear propagating waves with an angle of attack on current
Get PDFThe effect of bottom roughness on sediment transport due to three-dimensional wave-induced streaming in the seabed boundary layer has been investigated for following and opposing linear propagating waves and current where the wave propagation forms a non-zero angle with the current. Visualizations are given by mean Eulerian wave-averaged suspended flux profiles, as well as the time series of bed shear stress over a wave period. The bedload transport rate along with suspended flux and total sediment transport rate have been presented. For linear propagating waves, the turbulence is induced by the Longuet-Higgins streaming and the classical wave-current interaction. Sediment transport is always in the wave propagation direction for the Longuet-Higgins streaming and increases with decreasing bottom roughness, or as the mean grain diameter decreases
Lime treatment of slightly clayey coarse soil for the control of internal erosion by suffusion
Get PDFThe cost of an earth dike is even lower than soils used for its construction came from deposits close to the site. In parts of France, stocks of coarse and slightly clayey soils are not used because they are prone to internal erosion by suffusion. In order to reduce or control internal erosion in this kind of soils containing relatively low amounts of fines, a lime treatment was considered. For this purpose, an experimental laboratory test, based on the use of a soil column, has been developed. It reproduces the flow of water through a reconstituted soil, having characteristics similar to that of natural soils, and compacted at 95% of the optimum Proctor. The soil was treated with two percentages of lime (1%, 3%) and erosion tests were performed at different curing times (1, 7, 28 and 90 days). The developed device has been instrumented in order to measure different parameters such as turbidity, flow and pore water pressure.
The results are expressed in terms of eroded mass and soil permeability if erosion occurs or, alternatively, in terms of fracturing conditions. A comparison is performed between the results of different tests carried on non-treated and treated soils in different conditions. The lime treatment induces a significant change in the hydraulic behavior of the soil, significantly reducing the phenomenon of suffusion and fracturing the soil at high hydraulic gradients
Scour potential at Laouzas Dam
Get PDFLaouzas Dam, owned and operated by Electricité de France (EDF), is a 52 m high double-curvature arch dam in a large valley located on the Vèbre River in the Languedoc-Roussillon Midi Pyrénées region of France. The dam is founded on heavily fractured granite and migmatite rock. It houses a surface spillway equipped with 3 radial gates. The radial gates are positioned such that their gate lip is systematically located about 1m upstream of the spillway crest itself. This particular design generates aerated jets that do not behave like normal falling jets. Despite the absence of severe spillages since dam construction in 1961-1965, the ones observed in the past have allowed creating locally a 5 m deep scour hole along the right hand side of the rock mass. The granite and migmatite have significant unconfined compressive strength. The rock mass fracturing is composed of 3-4 joint sets and shows desquamation joints affecting both abutments. EDF has commissioned a numerical study to assess scour potential downstream of the dam as well as the positive influence of rock anchors, by using the Comprehensive Scour Model (CSM)