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Gravel beach profile response allowing for bimodal sea-states
Get PDFThe south coast of the UK is identified as a location where significant wave swell components are present within the regional wave climate. During the winters of 2006 and 2014, several sites along the south coast of the UK were subject to significant damages where flood events were recorded. These sea–states were characterised by having a double–peaked wave spectra, observing a connection between wave spectrum shape and beach response. A 2D physical model study was carried out to investigate the effect of gravel beach profile response under wave spectra characterised by swell and wind wave periods in various combinations. The physical model results have shown the effect of bimodal wave spectrum on the beach crest erosion and have been compared with the parametric model of SHINGLE and the numerical model XBeach-G. Based on this 2D physical model study a new parametric model, Shingle–B, has been derived and an online tool has been developed and made available on the website for the National Network of Regional Coastal Monitoring Programmes of England. This new tool has been validated with two sites in the South of England where field data of both waves and profiles was available
Wave and overtopping predictions on reservoirs and inland waterways
Get PDFIn their 2010 paper, Allsop et al (2010) sought to bring together the latest methodologies for predicting waves and wave overtopping for the dams and reservoirs community and modernise the extant approach. In particular, they argued that the reservoir community should move away from the out-dated wave-run up and freeboard allowance approaches and embrace better-validated approaches that limit overtopping, as used by the coastal community. This followed publication of the EurOtop overtopping manual (Pullen et al, 2007), and included many of the methods discussed there.
Since the original publication of EurOtop, there have been several improvements to the principal prediction methods and also to methods to improve predictions in cases where the crest includes a promenade and/or a wall that may include a bull-nose. These updates are especially pertinent with regard to bank full reservoirs or where there is a parapet wall at the crest of the embankment. As such, this paper provides an update to Allsop et al (2010) to incorporate the new methods in EurOtop II (van der Meer et al, 2016)
Scour and seabed changes at cable protection rock berms – Field observations
No full textUnited Kingdom waters have morphologically dynamic bays and estuaries through which offshore windfarm export cables are laid. The export cables must cross existing cable and pipeline infrastructure before making landfall. Rock berms have traditionally been installed at such crossings and at locations of shallow cable burial for cable separation and protection. In some instances, the rock berms have caused scour and been affected by general seabed changes. Recent field observations of scour and seabed change at cable protection rock berms is presented.
In one location two separate rock berms are exposed to 1.0 m to 1.5 m general seabed lowering. One berm is aligned oblique to the current and has experienced excessive scour, while the other berm is aligned more parallel with the current flow direction and has experienced only minor scour.
A second location comprise a cable crossing with double rock berms aligned oblique to the current direction. The current speed was asymmetric being largest during flood and smaller during ebb tide. Excessive scour was observed in-between the two berms and on the in-shore side, while only limited scour was observed at the off-shore side of the berms. The asymmetry in scour is linked to the asymmetry in current speed indicating a strong scour dependence on the currents speed. In non-dimensional form, this dependence is linked to the so-called Shields parameter governing magnitude and mode of sediment transport
Distribution of wave by wave overtopping volumes at vertical seawalls
Get PDFThis paper reports results of small scale physical tests on vertical walls describing the probability distribution of wave by wave overtopping volumes at a plain vertical structure with a shingle foreshore slope. The present paper contributes to the existing knowledge on the distribution of overtopping volumes by analyzing the acquired Weibull shape parameter from the experiments consisted of a matrix of 180 test conditions (wave steepnesses, crest freeboards, water depths, shingle sizes) and, by comparing the test results with the empirical prediction. Alongside the outputs of this study, the measured Weibull b values under the VOWS project are also analyzed in the present paper. The results of this study showed that there is no apparent relationship between Weibull shape parameter and incident wave steepness, relative freeboard or, relative discharge
Evolutionary leap in large-scale flood risk assessment needed
No full textCurrent approaches for assessing large-scale flood risks contravene the fundamental principles of the flood risk system functioning because they largely ignore basic interactions and feedbacks between atmosphere, catchments, river-floodplain systems, and socioeconomic processes. As a consequence, risk analyses are uncertain and might be biased. However, reliable risk estimates are required for prioritizing national investments in flood risk mitigation or for appraisal and management of insurance portfolios. We review several examples of process interactions and highlight their importance in shaping spatiotemporal risk patterns. We call for a fundamental redesign of the approaches used for large-scale flood risk assessment. They need to be capable to form a basis for large-scale flood risk management and insurance policies worldwide facing the challenge of increasing risks due to climate and global change. In particular, implementation of the European Flood Directive needs to be adjusted for the next round of flood risk mapping and development of flood risk management plans focusing on methods accounting for more process interactions in flood risk systems
Advanced computational methods for dam protections against overtopping
Get PDFThe paper presents an overview on the computational tools developed in CIMNE for the analysis of the behavior of rockfill dams in overtopping scenarios, as well as for the design of protections systems against overtopping. In this latter case two different applications are presented: the design tool for a wedge shaped blocks (WSB) shoulder protection and the computational models for the analysis of the hydraulic performance of highly convergent spillways. Advanced finite element techniques are combined with particle techniques and artificial neural networks in the framework of the Kratos Multiphysics for reaching the optimal solution procedure for any of the above mentioned problems. An extensive validation is carried out using experimental data provided by the SERPA group of the Technical University of Madrid
Coastal Surges (Chapter 12)
No full textThe risk of coastal flooding is increasing as a result of population growth, an increase in infrastructure located in coastal zones, sea level rise, and in some cases subsidence, caused by pumping of groundwater in deltas, which can lead to ground levels falling by 1–4 cm/year. Average global coastal flood losses in the 136 largest coastal cities in the world have been estimated to be approximatively US$6 billion. The deadliest storm surge on record was the Bhola cyclone in 1970. It resulted in around 225,000 deaths in Bangladesh alone and approximately 500,000 fatalities in the countries surrounding the Bay of Bengal. In Europe, in 1953 a coastal surge killed 300 people in the east of England and several thousand in the Netherlands and Belgium. In the past decade, despite improvements in early warning systems, there have been a number of significant storm surge events both in Europe and worldwide that have had deadly consequences. Shows an area of New Orleans in the United States flooded as a result of the storm surge caused by Hurricane Katrina in 2005
The effect of grain size distribution and bimodal sea-states on coarse beach sediment dynamics
Get PDFIn this work we investigated the effect of gravel beach profile response under wave spectra characterised by swell and wind wave periods in various combinations. This was done by running an extensive series of 2D physical model tests. It was found that even a small percentage of wave energy within the low frequency range triggers a significant landward displacement of the beach crest. Based on this 2D physical model study, a new parametric model, Shingle-B, was derived and an online tool was developed and made available
CFD simulation of clear water scour at complex foundations
No full textOffshore gravity base foundations (GBFs) are often designed with complex geometries. Such structures interact with local hydrodynamics, creating an adverse pressure gradient that is responsible for flow and scour phenomena, including the bed shear stress amplification and scour. At present, physical modelling and simple prediction equations have been the only practical engineering tool for evaluating scour around these support structures. However, with the increasing computation power of modern computers and the development of new Computational Fluid Dynamics (CFD) solvers, scour prediction around foundations has become possible.
In the present work three-dimensional (3D) numerical modelling has been applied to reproduce local scour around a complex cylindrical base structure under the forcing of a unidirectional current in clear water scour conditions. The simulations are carried out using a state of the art three-dimensional Euler-Lagrange scour model based on the open source CFD software OpenFOAM. The fluid phase is resolved by solving modified Navier-Stokes equations, which take into consideration the influence of the solid phase, i.e., the soil particles. The solid phase is solved using multi-phase particle-in-cell (MP-PIC) approach, a method which takes into account the sediment-sediment interaction, while the particles follow Newton’s Law of Motion.
The present paper also presents physical modelling results for scour around the same type of structure which were conducted for the same hydrodynamic forcing conditions as in the CFD model. The results of the experimental campaign are used to evaluate the ability of the CFD model to predict the: Time evolution of scour; Equilibrium scour depth; 3D characteristics of the scour hole