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Parametric equations for Shields parameter and wave orbital velocity in combined current and irregular waves
Get PDFA fundamental requirement for any scour assessment and scour protection design is the ability
to determine the Shields parameter for combined wave and current conditions.
The Shields parameter can be calculated for current combined with monochromatic waves using the approach
of Soulsby (1997) in combination with the wave friction factor concept. For current in combination with irregular
waves, the same approach is suggested using a wave orbital velocity, Um, for representation of the irregular
sea state. Um is defined as 1.41 times the standard deviation of the near bed wave orbital velocity.
The Soulsby (2006) expression for Um is compared with a hyperbolic expression and validated using numerical
methods and laboratory measurements.
A large number of expressions exist for the wave friction factor as a function of relative bed roughness. From
a literature study, the paper proposes a combination of existing expressions to cover relative bed roughnesses
from sand over gravel to coarse armour rock
Experimental findings of soil particle movement in 2D seepage failure of soil using Particle Image Velocimetry
Get PDFSeepage failure is one of the most important issues associated with the performance-based design of soil at high groundwater sites. We discuss the movement of soil particles with increase in the hydraulic head difference, H, in half 2D model tests on the seepage failure of soil in front of sheet piles using PIV analyses (Particle Image Velocimetry). The following conclusions were obtained: (1) At a certain value of Hpiv lower than Hy, the micro movement of soil particles is found around the bottom tip of a sheet pile wall, where Hpiv and Hy are the hydraulic head differences at the start of soil particle movement using PIV analysis, and at the onset of deformation of the soil surface, respectively. (2) The location of the micro movement of soil particles corresponds reasonably well with the net < 0 region, where net is the net body force exerted on a unit volume of soil. (3) Micro movement of soil particles occurs at a hydraulic head difference of 73 - 100% of Hy. (4) PIV analyses show the boundaries between regions where soil particles do or do not move as well as the movement of sand particles. (5) The region of soil particle movement proves the validity of the prism of failure for Terzaghi’s method and the prismatic failure concept
A multivariate extreme value analysis for the design of coastal structures in England.
No full textThis paper describes the application of a state-of-the-art multivariate extreme value statistical model to offshore winds, waves and sea levels around the coast of England and Wales. The output of the extreme value statistical model is a Monte-Carlo (MC) simulation of extreme offshore events. To undertake robust risk-based design of coastal structures, it is necessary to assess the performance of existing and proposed new structures against all of these events. A series of SWAN wave transformation models of the coastline have been established, Figure 1. It is, however, computationally impractical to transform all of these MC events from the offshore to the nearshore, particularly when covering the coastline of England and Wales.
A computationally efficient statistical method has therefore been employed. The statistical method, known as an emulator, has been used to replicate the behaviour of the SWAN wave transformation model. The emulators translate the thousands of MC events from offshore to the nearshore. The nearshore results have been stored on a 1km mesh.
This nearshore dataset has the potential to overcome many of the limitations of the existing joint probability methods based on exceedences. The method can be implemented for wide range of uses, including the robust, risk-based, design of coastal structures, climate change impact assessment, nearshore wave climates for detailed local flood risk assessments and coastal flood forecasting.
This paper describes the practical application of the data as developed and applied in the National Flood Risk Assessment – State of the Nation project
Spatial analysis and simulation of extreme coastal flooding scenarios for national-scale emergency planning
No full textThe UK has a long history of coastal flooding, driven by large-scale low-pressure weather systems which can result in flooding over large spatial areas. Traditional coastal flood risk analysis is, however, often undertaken at local scales and hence does not consider the likelihood of simultaneous flooding over larger areas. The flooding within the UK over the Winter of 2013/2014 was notable both for its long duration, lasting over two months, and its spatial extent, affecting many different areas of England and Wales. It is thus apparent that to plan and prepare for these types of extreme event it is necessary to consider the likelihood of flood events arising at different locations simultaneously (i.e. to consider the spatial dependence of extreme flood events). This paper describes the application of a state-of-the-art multivariate extreme value methodology to extreme sea levels and wave conditions around the coast of England and Wales. The output of the analysis comprises a synthetic set of extreme but plausible events that explicitly captures the dependence between sea conditions at different spatial locations around the coast. These simulated extreme events can be used for emergency management and advanced flood risk analysis
A flood risk analysis model with topographical inundation and life-loss
No full textQuantified models of flood risk that incorporate an explicit representation of the performance of flood defence infrastructure are becoming an increasingly important component of efforts to manage flood risk. The models have been applied to support a wide range of decisions, including long-term strategic planning, assessment of investment needs and shorter-term asset management. The model currently applied in practice by the Environment Agency in England and Wales makes a number of simplifying assumptions to achieve computationally practical run-times. One of these simplifications relates to the volume-based flood-spreading algorithm that is applied. This paper describes the implementation of a new time-based inundation model within the existing risk analysis modelling approach. The new inundation model offers a significant increase in the representation of the physical processes within the flood simulation component of the risk analysis model. It also offers new outputs in terms of estimates of maximum depths and flood velocities. Velocities are known to be an important factor in flood-related fatalities. A simplified method to estimate life-loss has been combined with new output from the improved inundation model to illustrate how estimates of risk to life could be provided
Pevensey Bay coastal defences, East Sussex: The use and validation of sea level and wave forecasts in a novel approach to allocation of flood risk
No full textPevensey is on the East Sussex coast, between Eastbourne and Bexhill (see Figure 1). Its coastal defence is in the form of a shingle embankment facing approximately south-east into the English Channel. It is the subject of a 25-year contractual arrangement, unique at its inception, between the private consortium Pevensey Coastal Defence Limited (PCDL) and the Environment Agency (EA). After each storm, any necessary coastal defence repair is carried out. At the same time, a return period for the storm is estimated based on the predicted tidal level and the forecast surge and waves, with reference to look-up tables for the joint return period of sea level and wave height
The pervasive role of biological cohesion in bedform development
No full textSediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS
USACE adaptation approach for future coastal climate conditions
No full textThe US Army Corps of Engineers (USACE) is currently looking at variable temporal and geographic scales for total water level and event loading projections including storm description and characterisation relevant to project design and performance. USACE projects and event description must transition from engineering to planning to economics and project management. Capturing and articulating the appropriate level of uncertainty is important to a realistic projection of resultant risk. Close collaboration with national and international experts is an essential component in USACE's process of developing practical, nationally consistent, and cost-effective measures to reduce potential vulnerabilities resulting from global changes. The USACE's approach to developing guidance for evaluating and adapting to sea level change and total water level assessment are good examples of this collaboration. A primary focus at this time is the examination of methods and tools available at graduated levels of a project study. For project-level use, the USACE is defining specific assessments of components of total water level in addition to their varying impacts on project stability and performance. The required planning and risk assessment products for each performance type will be explained. The goal is to project adequately and cost-effectively future climate contributors that can result in various levels of project non-performance in a manner that will support effective long-term planning and project expenditures
Improving decadal coastal geomorphic predictions: an overview of the iCOASST project
No full textCoastal areas are already at high risk from a range of geohazards. The cumulative effect of human intervention on soft coastlines has frequently left them far from equilibrium under today’s conditions, especially in densely populated areas. Future changes in marine forcing due to climate change reinforce the need to understand and predict processes of change in shoreline position and configuration at management (decadal) scales. The UK-based iCOASST project is developing new and improved methods to predict decadal geomorphic evolution, linked to coastal erosion and flood risk management. This is based on a framework that links several components to develop a system-level understanding of this change. The framework includes: (1) new methods for system-level analysis and mapping of coast, estuary and inner shelf landform behaviour; (2) well validated ‘bottom-up’ hydrodynamic and sediment transport shelf models which can be applied at shelf scales to investigate inner shelf-coastal interactions; and (3) model compositions formed of existing or new ‘reduced complexity models’ of selected coastal landforms and processes that are suitable for multiple decadal length simulations. This will ultimately allow multiple simulations of coastal evolution which can explore uncertainties in future decadal-scale coastal response, including the effects of climate change and management choices. This paper outlines the current state of progress in the iCOASST Project