1642 research outputs found
Sort by
Developing a national programme of flood risk management measures: Moldova
No full textA Technical Assistance project funded by the European Investment Bank has been undertaken to develop a programme of flood risk management measures for Moldova that will address the main shortcomings in the present flood management system, and provide the basis for long-term improvement. Areas of significant flood risk were identified using national hydraulic and flood risk modelling, and flood hazard and flood risk maps were then prepared for these high risk areas. The flood risk was calculated using 12 indicators representing social, economic and environmental impacts of flooding. Indicator values were combined to provide overall estimates of flood risk. Strategic approaches to flood risk management were identified for each river basin using a multi-criteria analysis. Measures were then identified to achieve the strategic approaches. A programme of measures covering a 20-year period was developed together with a more detailed Short-Term Investment Plan covering the first seven years of the programme. Arrangements are now being made to implement the programme. The technical achievements of the project included national hydrological and hydraulic modelling covering 12,000 km of river, the development of 2-dimensional channel and floodplain hydraulic models from a range of topographic and bathymetric data, and an integrated flood risk assessment that takes account of both economic and non-monetary impacts
CFD modelling of marine discharge mixing and dispersion
No full textMany municipal and industrial outfalls release effluent into water bodies. Temperatures and/or salinities of the effluent and receiving water may be different, and other dissolved/suspended constituents may be present. Away from the outfall, mixing processes usually reduce constituent concentrations to acceptable levels for local water quality. However, at the outlet, concentrations may be sufficient to cause environmental concern. Accurate dispersion prediction is therefore important. Two-stage modelling approaches are typically used: (1) a near-field dilution assessment, based on mixing zone or empirical models; and (2) a mid- /far-field dispersion assessment, using hydrodynamic models. As computational and numerical methods improve, computational fluid dynamics (CFD) can increasingly model dispersion across both regions. These methods require validation of the underlying discretisation and turbulence schemes. Preliminary validation is presented for near-field simulations of buoyant and dense jets. Buoyant jet predictions compare well with established results. Preliminary simulations under-predict entrainment into the dense jet, overly-predicting near-field concentrations
Operation of the Leigh Flood Storage Area in the December 2013 flood
Get PDFThe December 2013 flood on the Medway was large and unusual in its formation as the jet stream pushed several low pressure weather systems across the UK. The worst storm occurred on 23 to 24 December when up to 76 mm of rain fell in 24 hours, on already saturated catchments. There was wide-scale flooding in the Medway catchment on Christmas Eve and Christmas Day of comparable severity to the 1968 and 2000 floods but with a different distribution of flows from these earlier events. It is understandable that after the event there was public concern and criticism from those whose lives had been turned upside down during the Christmas holidays
Use of agent-based modelling in emergency management under a range of flood hazards
No full textThe Life Safety Model (LSM) was developed some 15 years ago, originally for dam break assessments and for informing reservoir evacuation and emergency plans. Alongside other technological developments, the model has evolved into a very useful agent-based tool, with many applications for a range of hazards and receptor behaviour. HR Wallingford became involved in its use in 2006, and is now responsible for its technical development and commercialisation. Over the past 10 years the model has been applied to a range of flood hazards, including coastal surge, river flood, dam failure and tsunami, and has been verified against historical events. Commercial software licences are being used in Canada, Italy, Malaysia and Australia. A core group of LSM users and analysts has been specifying and delivering a programme of model enhancements. These include improvements to traffic behaviour at intersections, new algorithms for sheltering in high-rise buildings, and the addition of monitoring points to allow detailed analysis of vehicle and pedestrian movement. Following user feedback, the ability of LSM to handle large model ‘worlds’ and hydrodynamic meshes has been improved. Recent developments include new documentation, performance enhancements, better logging of run-time events and bug fixes. This paper describes some of the recent developments and summarises some of the case study applications, including dam failure analysis in Japan and mass evacuation simulation in England
Interpreting filtration-based suffusion criteria using micro-Computed Tomography and laboratory filter tests
Get PDFIn a cohesionless soil containing a mixture of coarse and fine particles, suffusion is the erosion of the fine particles which can be transported through the void space between the coarse particles. Two commonly used criteria for suffusion are Kézdi (1979) and Kenney & Lau (1985), both of which were determined empirically and both include a limiting factor of 4 between fine and coarse particle sizes. This factor of 4 is often interpreted based on simple filter criteria, as the ratio between the size of coarse particles and the size of constrictions in the void space, with some factor of safety. To fully understand the suffusion mechanism it is necessary to consider the size of void constrictions, but also how far fine particles can move before they are retained and the potential for the fine material to self-filter. Several researchers have used empirical and numerical results to show that the criteria are conservative, however these studies could not clearly distinguish between the issues of constriction size, particle movement and self-filtration. This paper uses micro-Computed Tomography (microCT) images to examine the underlying filter criteria, by measuring constriction sizes relative to particle sizes for three materials. Laboratory filter tests, with minimal self-filtration, are then presented to observe particle movement. The results provide a new means to interpret the physical significance of the filter criteria, on which suffusion criteria are based
Analytical design for stable channel geometry of Naesung Stream in Korea
Get PDFA stable channel is one in which channel dimensions of slope, width, and depth remain balanced with equilibrium condition between input and output sediment discharge and erosion and sedimentation of channel beds and banks. In the analytical method of stable channel design, stable channel slopes and depths with given channel widths are determined by satisfying the condition that the upstream incoming sediment rate is equal to the sediment transport rate at the design channel. Therefore, the most sensitive parameter when designing a stable channel is the choice of a sediment transport equation applied for the channel design. The Stable Channel Analysis and Design (SCAD) program has been developed for the better option of various sediment transport equations and for calculation of a unique solution considering design constraints such as fixed width and depth as well as various solutions for stable channel condition with a wide range. The study reach of Naesung Stream in Korea has been selected for this study. The Naesung Stream is a tributary of Nakdong River and mostly sand bed. The stable channel analysis and design for the study section has been performed using the SCAD program with different sediment transport equations for sensitivity analysis and design constraints
Effect of vibration on the scour process around cylindrical structures under unidirectional flow in a sandy bed
Get PDFThe structures that support wind turbines in offshore wind farms are dynamically sensitive and can vibrate as a consequence of their slenderness and their location in severe environments subject to strong wind and wave load. The granular soils in which such structures are often located are highly responsive to vibrations; depending on their initial state, these soils might experience processes such as compaction, dilation, and liquefaction. Their behaviour in response to structural movement and the effect on the rate of scour is the subject of the present work. Laboratory experiments have been conducted to investigate the effects of vibration on the scour process in granular soil. A series of storms were simulated by a continuous sequence of periods with and without vibration applied to a model pile. The results show that although scour depths are initially reduced by vibration (backfilling), the lateral extent of the scour hole grows and the final scour depth and extent can be significantly greater than for an equivalent test without vibration
Research to improve the applicability of ICOLD Bulletin 164 on internal erosion
Get PDFICOLD Bulletin 164 on internal erosion in existing dams, dikes and levees and their foundations is available in two volumes from www.icold-cigb.org. The Bulletin has been prepared because internal erosion results in many failures, causing numerous fatalities, but has not been well understood. The Bulletin has drawn on research and practice to develop an understanding of the mechanics of internal erosion. Using this new knowledge, the Bulletin makes it possible for engineers to estimate the water level at which internal erosion will lead to failure by each of the four initiating mechanisms: concentrated leak erosion, backward erosion, suffusion and contact erosion. Further research may provide knowledge to improve on engineers’ ability to predict the response of dams to the hydraulic forces that initiate internal erosion. Examples from the Bulletin are used to show where more laboratory and field research may contribute to expanding the applicability of the new understanding of internal erosion mechanics, including methods to extend the range of soil gradings of vulnerable soils, to determine the extent of crest desiccation and cracking, and improved methods to determine in-situ permeability
Experimental and numerical investigation of backward erosion piping in heterogeneous sands
Get PDFBackward erosion piping is a relevant failure mechanism for water–retaining structures that determines uncertainty in residual flooding hazard and risk mapping. The occurrence of piping can be predicted using the Sellmeijer model, which is developed and validated for homogeneous granular layers. However, the subsurface encountered below levees can be far from homogeneous. Previous laboratory tests indicated a substantial increase of piping resistance in sand samples with variation of properties in the path of the pipe. In the research described in this paper the process of backward erosion piping in heterogeneous sands was investigated by means of small–scale experiments to study the influence of micro–scale and macro–scale heterogeneity. Numerical simulations of piping experiments with macro–scale heterogeneity were performed using a 2D groundwater flow model extended with a piping module and a 3D groundwater flow model
Induced shear stress measurement in overtopped floodwalls
Get PDFThe scour generated by water overtopping floodwalls results in tilting and sometimes failure of floodwalls. The water free fall induces shear stress, which is difficult to measure, at the levee surface and if it exceeds the critical shear stress, scouring initiates. It is perceived that once a pool of water is generated at the levee crest due to scour, the induced stresses on soil surface at the pool bottom will decrease. The scour progress is stopped once the induced shear stresses at the pool bottom are less than critical shear stress; however, due to the low values of critical shear stress, the scour most likely continues for a long time. A methodology was developed in this study to measure the induced shear stresses. Based on the tests conducted in this study, the generated pool will decrease the induced shear stress until constant erosion rate is achieved. This shear stress is defined as ‘equilibrium’ shear stress herein