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A simple method for estimating flood discharge in gravel-bed channels with varied riverbed level
Get PDFThe information of flow discharge in channels is crucial for river planning and regulation, weir and reservoir operation, river crossing and encroachment remediation, and river restoration. It also affects the degree of erosion on the channel bed and river bank, and scour around structures in the riverine environment. During a flood event, large scour on the channel bed and rapid sediment transport along the channel reach occur at the flood rising stages. Significant changes of riverbed level appear consequently. However, the conventional method of on-site measurements is time-consuming and dangerous for the investigators. A simple method utilizing non-contact water-surface velocity radar (SVR) and ground-penetrating radar (GPR) was developed. Two rating curves, the relationships for the stream mean velocity and river cross section to the stream surface velocity, were investigated. The resultant stream discharge shows great improvement of the estimation of entire flood hydrograph, indicating that the proposed method has the capability to reasonably estimate the flood discharges of gravel channels with mobile bed
Simulation of sand particle transport by coupled CFD-DEM: first investigations
Get PDFThe continuum based Euler-Euler approach represents the main field of application for the simulation of sediment transport processes. Herein, the decisive phases of free water and soil are modelled by interpenetrating continua. Although mixing of the phases is possible, the multi-component character of the soil phase, as a mixture of solid grains and pore water, is neglected. Hence, a coupling between the free water and the pore water remains unnoticed as well. However, this coupling represents an important factor for determining the current state of the soil boundary near the transition zone. Due to water level changes caused by ship induced bow and stern waves, excess pore water pressure can occur in the upper soil layers. As a result, fluidisation effects can be initiated, which reduce the erosion resistance. To consider these fluidisation effects, the soil has to be treated as a mixture of dispersed grain particles and pore water by a Lagrange-Euler approach. The coupling of the Discrete Element Method (DEM) and the Computational Fluid Dynamics (CFD) approach offers this possibility. Thereby, the DEM is used for modelling the dispersed particles of the Lagrangian regime, while the CFD method models the continuum Euler-phase of the water. This paper introduces the coupled CFD-DEM method for simulating sand particle transport at the boundary layer transition zone. The model investigations and first results of simulations regarding the initiation of motion are presented
Laboratory investigation of backward erosion piping – effects of inclined exit face and constricted seepage exits
Get PDFWhile the Backward Erosion Piping (BEP) form of internal erosion is one of the least understood mechanisms in geotechnical engineering, a precise, mechanism-based analysis method for backward erosion piping (BEP) remains elusive for geotechnical engineers. BEP is still generally analyzed with empirically-based methods that do not consider many of the complexities of the soil, the subsurface geometry, the seepage regime, and the exit face conditions. Furthermore, many of the analysis methods commonly used for BEP were actually developed for other forms of internal erosion and adapted to BEP by correlating with crude empirical data. This paper presents a laboratory testing program performed to investigate the mechanisms of BEP under two conditions often encountered in the field: 1) exiting on a sloping exit face, and 2) exiting into a constricted exit. The study builds upon previous research on the mechanisms of piping initiation performed at Utah State University using a similar apparatus. A variety of soils representing a range of grain size, grain shape, and gradations are subjected to increasing hydraulic gradients under a variety of exit face condition including sloped exit faces and a range of constricted seepage exits. The results are compared with three-dimensional finite element analyses in order to develop a better understanding of the BEP initiation process
Experimental investigations of critical hydraulic gradients for a soil prone to suffusion
Get PDFThe presence of soils, which are at the limit state of internal stability, is a potential risk to earthworks under seepage flow. Therefore, it is necessary to identify unstable soils and to estimate hydraulic gradients at which the suffusion can be initiated respectively progressed. An experimental study has been carried out to quantify critical hydraulic gradients for a widely graded soil. For the tested soil, in downwards vertical percolation experiments, the global critical hydraulic gradients lie in the different ranges between icrit = 0:1 to 5:5 with dependency on the particle arrangement. The critical hydraulic gradient was investigated using various types of sample preparation technique. Moreover, suffusion tests using several types of samples with the same particle size distribution have been carried out. It states that for such a widely graded soil, the main problem is the particle arrangement. In other words, the suffusion might be not problematic if there is no segregation. Sometimes a specific amount of segregation also stabilizes the sample against suffusion. The common way of sample preparation delivers comparable results to the results of other researchers
Influence of uncertainties in internal erosion assessments of existing embankment dams
Get PDFThe evaluation of a dam’s susceptibility to internal erosion is fraught with difficulties related to uncertainties in relation to the in-situ geotechnical properties of materials. These uncertainties include imperfect knowledge of actual soil conditions, imperfect representation of reality by models and variability of soil properties. Experience has shown that our current capability for mathematical analysis and modelling of potential seepage patterns far exceeds our capability to make judgments of comparable accuracy concerning the geology of a dam site or, for example, how the soil properties may be affected during construction of a dam. The uncertainties about the actual performance of embankment dams are discussed in this paper and illustrated by three case histories. These examples show the influence of various types of uncertainties in internal erosion assessments as well as efficient means to reduce them
Numerical simulation of scour below pipelines using flexible mesh methods
Get PDFEvaluating bed morphological structure and evolution (specifically the scoured bed level) accurately using numerical models is critical for analyses of the stability of many marine structures. This paper discusses the performance of an implementation within Fluidity, an open source, general purpose, computational fluid dynamics (CFD) code, capable of handling arbitrary multi-scale unstructured tetrahedral meshes and including algorithms to perform dynamic anisotropic mesh adaptivity. The flexibility over mesh structure and resolution that these capabilities provide makes it potentially highly suitable for coupling the structural scale with larger scale ocean dynamics. In this very preliminary study the solver approach is demonstrated for an idealised scenario. Discontinuous Galerkin finite-element (DG-FEM) based discretisation methods have been used for the hydrodynamics and morphological calculations, and automatic mesh deformation has been utilised to account for bed evolution changes while preserving the validity and quality of the mesh. In future work, the solver will be used in three-dimensional impinging jet and other industrial and environmental scour studies
Developments of low cost riverbank protection in Bangladesh
Get PDFOver the last two decades the Bangladesh Water Development Board (BWDB) has developed the widely accepted riverbank protection technology of geotextile bag revetments, consisting of multiple layers of systematically dumped geobags below low water plus a single layer of concrete blocks above low water. The original design characteristics from the early 2000s have proven sufficient for the most common application alongside largely agricultural land, with emphasis on the dimensions of the toe apron. In case of more severe design requirements, geotextile bags have been used as filter layer covered with rock. By and large geotextile bag revetments have become the backbone for a more stabilized Brahmaputra River System with a narrowed river corridor
Comparison of robust optimization and info-gap methods for water resource management under deep uncertainty
No full textThis paper evaluates two established decision-making methods and analyzes their performance and suitability within a water resources management (WRM) problem. The methods under assessment are info-gap (IG) decision theory and robust optimization (RO). The methods have been selected primarily to investigate a contrasting local versus global method of assessing water system robustness to deep uncertainty, but also to compare a robustness model approach (IG) with a robustness algorithm approach (RO), whereby the former selects and analyzes a set of prespecified strategies and the latter uses optimization algorithms to automatically generate and evaluate solutions. The study presents a novel area-based method for IG robustness modeling and assesses the applicability of utilizing the future flows climate change projections in scenario generation for water resource adaptation planning. The methods were applied to a case study resembling the Sussex North Water Resource Zone in England, assessing their applicability at improving a risk-based WRM problem and highlighting the strengths and weaknesses of each method at selecting suitable adaptation strategies under climate change and future demand uncertainties. Pareto sets of robustness to cost are produced for both methods and highlight RO as producing the lower cost strategies for the full range of varying target robustness levels. IG produced the more expensive Pareto strategies due to its more selective and stringent robustness analysis, resulting from the more complex scenario ordering process
Multi–scale modelling of effluent dispersion in the marine environment
Get PDFThis research aimed to investigate whether the unique numerical methods available within CFD model software Fluidity could progress the state– of–the–art in various aspects of modelling effluent dispersion within the marine environment. Fluidity contains a large library of models and numerical methods that enable modelling of flow processes at a wide range of scales. It has been proven to perform well when used for massively– parallel simulations (i.e. it scales well), and it has the un–common facility of unstructured mesh adaptivity, which has the prospect of significantly increasing the efficiency of CFD simulations when guided skillfully.
This research also forms part of a longer–term project to create a coupled (or even single) model of effluent dispersion that represents influencing factors from a wide range of scales (from tidal currents down to turbulent eddies) entirely using CFD techniques. As such, one aspect of the research was to validate the numerical methods available within Fluidity for use in modelling effluent dispersion. To facilitate this validation, some
of the model studies investigate aspects of effluent dispersion modelling from a hypothetical outfall site off the North–East coast of the United Kingdom.
Studies were performed in a series of stages in which key aspects of effluent dispersion modelling were addressed. CFD models were created of near–field jet dispersion, tidal motion, and far–field plume dispersion. Idealised test cases were also performed to investigate the performance of advection–diffusion solver methods. At each stage the aim was to investigate the benefit of novel numerical modelling techniques and compare their accuracy and efficiency to existing methods.
A set of near–field buoyant jet dispersion CFD models were created, one representing conditions associated with power, and combined power and desalination plants, and one representing conditions typically associated
with desalination discharge. These CFD models utilised a mesh adaptivity algorithm to optimise mesh resolution during the course of the simulation. Model predictions were compared with published laboratory data and the predictions from validated integral models. An assessment was made of when CFD offers a benefit over other modelling options, and when it might be sufficient to use cheaper tools. There was also a discusii sion of the effectiveness of mesh adaptivity in increasing model efficiency, together with advice for how and when it is best to use mesh adaptivity when modelling buoyant jet dispersion. Model results showed that with modest parallel computing resources and expertise, high–resolution simulations of jet dynamics can be achieved with reasonable accuracy using CFD modelling.
A model was created of tidal flow within the European continental shelf and results were compared to a large database of tide gauge measurements. This model took advantage of recently published methods for ocean model meshing and coastline resolution reduction. The purpose of this study was to confirm that these methods offered a benefit to model accuracy and efficient, and also that Fluidity could be used to accurately generate the tidal forcing boundary conditions for a far–field model of effluent dispersion at a hypothetical outfall site.
The predictions of M2 tide amplitude in the vicinity of the outfall site had an average error of 10.1% compared with tide gauge measurements. The predictions of S2 tide amplitude in the vicinity of the outfall site were even closer to tide gauge measurements, with an average error of 3.7%. The speed of the model solve showed a vast improvement over a previous comparison model study, with 37 days of tidal motion being simulated in 15.2 hours (58.4 seconds of simulation for each second of solving), compared to the comparison simulation with a similar level of accuracy, which simulated 2 seconds of tidal motion for every second of solver time.
A series of simplified test cases were run to assess a commonly–used advection–diffusion solution method from the library of those available within Fluidity. This work was intended to give general confidence that the numerical methods available within Fluidity are suitable for modelling coastal processes and so give confidence in later multi–scale results. The test cases chosen were relevant to coastal dispersion, including those testing tracer advection, diffusion, point sources and stratification. The method compared well with results published using world–leading free surface modelling software, Open TELEMAC.
A model was created of the dispersion of neutrally–buoyant dissolved pollutant from a hypothetical outfall. The assumed effluent is typical of that released from a manufacturing plant. The aim of this modelling was to validate the use of Fluidity for modelling effluent dispersion within the coastal zone, and also investigate the benefit of using 2–d horizontal mesh adaptivity to optimise model mesh resolution during the course of the simulation. It was shown that the use of mesh adaptivity improved model efficiency, significantly lowering the effect of numerical diffusion.
Finally, a short outline was given of a prospective strategy for producing a coupled–model of effluent dispersion, using as a basis the techniques developed within this thesis. The proposed coupled model of effluent dispersion would include a near–field jet model two–way (i.e. “fully– coupled”) to a far–field plume model. Tidal forcing would be provided by a one–way coupled tidal model. Fluidity is capable of modelling all of these processes and so third party coupling software would be unnecessary
Model experiments and simulations on influence of liquefaction on scour at a landward toe of coastal dykes due to tsunami overflow
Get PDFIn the 2011 off the Pacific Coast of Tohoku Earthquake, massive tsunami caused serious damage to coastal dykes. One of the failure mechanisms of a coastal dyke has been considered as the local scour at the landward toe of a coastal dyke. Furthermore, there is a possibility that liquefaction occurs around a coastal dyke by aftershocks when tsunami strikes. In this paper, flume experiments were conducted to clarify the influence of liquefaction on the scour at a landward toe of coastal dykes due to tsunami over flow. Moreover, simulations were conducted to evaluate the influence of the upward seepage on the scour. In the experiments, scour process and scour profile is different according to the degree of liquefaction. In the simulations, the vertical velocity at the bottom of scour hole increases by the upward seepage. However, there is little influence of the vertical velocity on the scour because the uplift force of the vertical velocity is small