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International approaches to the hydraulic control of surface water runoff in mitigating flood and environmental risks
No full textThis paper compares and contrasts a number of international approaches to the hydraulic control of surface water runoff from new development and redevelopment, known as sustainable drainage systems (SuDS) or low impact development (LID). The paper provides a commentary on the progress and current status of national standards for SuDS in the UK to control the frequency, flow rate and volume of runoff from both frequent and extreme rainfall events, and the best practice design criteria presented in the revised UK CIRIA SuDS Manual, published in November 2015. The paper then compares these design criteria and standards with those developed and applied in China, USA, France and Germany and also looks at the drivers behind their development. The benefits of these different approaches are assessed in the context of flood risk mitigation, climate resilience and wider environmental protection objectives, including water quality, morphology and ecology. The paper also reviews the design approaches promoted by the new SuDS Manual and internationally for delivering additional benefits for urban spaces (such as recreation, visual character, education and economic growth) through multi-functional urban design
Incipient motion of sediment in rectangular open-channel flow with a submerged rigid vegetation zone
Get PDFAn experimental statistical method is here developed to investigate the incipient motion in open channel blocked with an artificial submerged rigid vegetation patch. We focus on the effects of vegetation elements on the incipient motion of sediment and the scour around the vegetation elements. The incipient motion of sediment is divided into three processes, the static, the partially motion, and the entirely motion, and the third process is defined as the criterion for incipient motion of sediment. Based on the experimental results, a semi-empirical equation for the sediment incipient velocity in the presence of submerged vegetation is derived, which is observed to be smaller than that without vegetation. A strong linear relationship between the average depth of the scour holes around the vegetation elements and the flow velocity is found
Stakeholders’ perceptions of the overall effectiveness of early warning systems and risk assessments for weather-related hazards in Africa, the Caribbean and South Asia
No full textLow-income countries are significantly more vulnerable than high-income countries to the risks posed by natural hazards. This paper describes the findings of research into stakeholders’ perceptions of the overall effectiveness of early warnings and risk assessments for weather-related hazards (i.e. cyclones, floods, droughts and landslides), for humanitarian and development purposes in low-income countries in Africa, the Caribbean and South Asia. New findings are derived from a survey and consultations with some 400 practitioners, scientists, researchers and decision makers in the regions. Our findings, which were based upon the collated results of a literature review, stakeholder interviews together with other reviews and surveys show that although there would appear to have been some progress in improving early warning and risk assessments for weather-related hazards, it is highly variable across the three regions. The findings are inconsistent with the self-reporting of progress against the Hyogo Framework for Action, which in many cases give a more positive view of their status. Significantly, more work is required to produce robust, reliable and accessible information to reduce vulnerability and manage risks, this should concentrate on understanding aspects of vulnerability, effective risk communication and community level actions, as well some well-focused improvements in technical, and underpinning scientific aspects of early warning systems and risk assessments
Modeling of particles migration in porous media: application to soil suffusion
Get PDFThe suffusion phenomenon occurs when fine soil particles are detached by seepage flow and transported away from the matrix. This process is one of the main causes of failure of hydraulic structures and road embankments. This study aimed to build a numerical model for simulating the particles suffusion within a porous medium. This model combines a flow law and an erosion equation related to the evolution of soil porosity. In addition the dispersion and the deposition kinetics processes of eroded particles were combined with detachability process. The equations describe the evolution of the instantaneous concentration of the fluidized solid, and the variation of eroded mass. Sensitivity analysis allows highlighting the influence of the different parameters on suffusion, particularly that deposition kinetics starts acting only below a threshold of hydraulic gradient and beyond a given sample length. The adjustment results indicate that the suffusion process is strongly related to hydraulic conditions, physical soil characteristics and pore water chemicals. The comparison of numerical results with experimental data from laboratory tests provides a quite good agreement
HAMMER: a tool to predict the impact of man-made noise on fishes
Get PDFFish and Noise: Why should we care?
All fish sense sounds and use natural
soundscapes (e.g. for finding food, choosing
mates, orientation and predator avoidance).
Fish can be impacted by noise (see next box).
Fish underpin many marine food webs.
Many fish species are commercially important
and provide food security for millions of people.
Several fish species are protected or are of
conservation concern (e.g. salmon and eels).
Underwater noise is included in national and
international legislation (e.g. EC 2008; DEFRA 2009)
Micromechanical features of jet erosion - A numerical perspective
Get PDFThe erosion mechanisms driving the particle detachment of a soil under an impinging jet are here analysed from a micromechanical perspective combining the numerical Discrete Element and Lattice Boltzmann methods (DEM-LBM). Firstly, the local hydrodynamic conditions of both free and impinging jets are examined and briefly discussed, particularly on the relevance of the superficial irregularities of the granular assembly, which can dominate the local distribution of hydrodynamic variables over the impingement area. Profiles of both macromechanic and discrete (micromechanic) variables will be put forward. Then, the onset of erosion is reviewed from a topological point of view (locality of first grain motion) on the basis of a parametric study. Finally, the variation of the critical inflow velocity for the initiation of erosion in dependence on the particle size is also examined and briefly discussed
Hydrodynamic effects of debris blockage and scour on masonry bridges: towards experimental modelling
Get PDFThis paper describes the preliminary stage of an ongoing project investigating the hydrodynamic effects of debris blockage at masonry bridges. Debris blockage is cited as one of the primary causes of bridge failures in the UK and around the world. Masonry bridges, many of which are valuable historical assets, are particularly vulnerable to debris blockage due to their short spans and low clearance. This paper presents work done as part of the first phase of the project involving experimental research to understand the scientific relationships between debris characteristics and flow conditions. The study, being carried out at Centre for Water Systems at University of Exeter, utilizes a 0.6m-wide and 10m-long flume to run hydraulic experiments in order to characterize the impact of debris blockage on flow hydrodynamics, scour, and hydrodynamic pressures and forces at masonry bridges. This paper outlines the design of the experimental setup and the reasoning behind the choices for preliminary experimental parameters. The experiments are to include testing of bridge models and various 3D-printed debris shapes under realistic flow conditions. Geometry of the bridge and debris models are kept approximately similar to prototype conditions, with hydraulic conditions of the experiments designed to the degree that experimental constraints allow based on Froude similarity. Velocities, scour and hydrodynamic pressures are measured using an Acoustic Doppler Velocimeter, echo-sounding concept and pressure sensors, respectively. Preliminary results indicate that the designed experiments have the potential to enhance our understanding of the hydrodynamic effects of debris blockage
Hydraulic failure by heave and piping
Get PDFIt is generally agreed the same criterion for hydraulic heave and for piping can be used. In vertical upward flow the theory is well established. The weight of the soil must be greater than the seepage force of the water. Despite this well established theory piping and sand boiling occurs at much lower hydraulic gradient than the theoretical value. It is concluded that this discrepancy resulting from a conceptual error by not making distinction between the local and the global equilibrium. In the case of hydraulic heave the equilibrium of the entire soil prism (global equilibrium), while for the initiation of piping on the surface of the soil the equilibrium of one grain (local equilibrium) should be investigated. Investigating the local equilibrium of a single grain physical model for the initiation of piping is proposed. Criterion for piping on the top of a horizontal surface in granular soil subject to an upward flow deduced from first principles. It is concluded that hydraulic gradient alone does not sufficiently describe the hydraulic stability of the soil. The particle size and its distribution also have effect on the hydraulic stability of the soil. Critical grain diameter corresponding to the critical hydraulic gradient is introduced. If the diameters of the grains in a soil matrix are bigger than the critical diameter then the Terzaghi criterion is valid. Grains with smaller diameters lose their stability at lower hydraulic gradient than the critical one. The removals of grains from the soil matrix increase both the hydraulic gradient and the permeability of the soil. Thus the initial grain removal from the surface of the soil can progressively lead to sand boiling; therefore, the stronger criterion of piping should also be required for hydraulic heave
Analysing scour interaction between submarine pipelines, valve stations and mechanical protection structures
No full textSubsea pipeline networks with components such as Wye-pieces and Pipeline End Manifolds (PLEMs) require protection from mechanical impacts. Pipeline scour is well understood and scour at solid caissons/gravity base foundations and piled foundations has been studied (Whitehouse, 1998; Whitehouse, et al., 2011; Sumer and Fredsøe, 2002). The scouring at small subsea gravity protection structures has received less attention than those with piles (Ottesen Hansen, 1997; Fog and Krogh, 2001) and hence there is uncertainty about predicting scour at such structures (Whitehouse, et al., 2010).
The design of subsea structures requires a composite assessment of scour and stability for the protected asset and the protection structure itself. A closed structure may lead to the formation of a scour footprint that is quite similar to a solid caisson or gravity base. For an open structure, the contribution to scour interaction from the different components is more difficult to assess.
Large-scale mobile bed laboratory experiments evaluated scour for a surface laid pipeline, with a spur and valves, and a generic subsea protection structure with mudmats and a perimeter skirt. Bathymetry mapping was achieved with a high precision underwater laser scanner. The experiments evaluated the contributions to scour arising from the different subsea components.
Sediment mobility was a key factor, with the perimeter skirt able to resist scour over part of its length when the mobility was low/moderate, whereas in a high mobility environment the skirt was completely undermined. The foundation beams were underscoured quite quickly and the eroded sediment was flushed out through the perforated mudmats, highlighting the importance of mudmat design. The scour pattern and rate was similar with or without the pipeline in place. The underscouring of the beams caused settlement and tilting of the structure, highlighting the importance of assessing scour and scour countermeasures for these types of structure. The scour assessment needs to be integrated into the structural, geotechnical and functional performance study for any structure and the new results illustrate the importance of understanding effects related to sediment mobility, the prevailing direction of currents, the design of mudmats and skirts, and the need for additional scour countermeasures
A model for scour around bridge piers caused by flood waves
Get PDFThe local scour process during flood waves is modeled introducing the idea of an effective work by the flow on the sediment bed around the pier. Dimensional considerations show different possible formulations for the dimensionless effective flow work W* that, in each case, is shown to be a generalization of the flow intensity concept, commonly used in existing scour formulas. A novel experimental installation able to reproduce any hydrograph with high precision in the laboratory flume is used to carry out steady and unsteady flow experiments in order to calibrate and validate the mathematical model. Results confirm the uniqueness of the relationship between the dimensionless, effective flow work W* and the relative scour depth Z*, highlighting the high predictive capacity of scour depth caused by any hydrograph. The proposed model provides good performances allowing a straightforward prediction of maximum scour depth and its evolution