Dave Simmonds

Separation of incident and reflected waves over sloping bathymetry

Abstract An existing 2D method for separating incident and reflected waves over a horizontal bed [Frigaard, P., Brorsen, M., 1995. A time domain method for separating incident and reflected irregular waves. Coastal Eng., 24, 205–215.] is modified to account for normally incident linear waves propagating over a bed with arbitrary 2D bathymetry. Linear shoaling is used to determine the amplitude and phase change between two measurement positions; thereafter the existing technique can be applied. Comparisons between the existing and modified methods are made using numerically simulated data. Errors in the reflection coefficient are found to be small for large reflection coefficients, but may become large if reflection is low. However, if an accurate assessment of the amplitude of the incident and reflected wave trains is required, the bathymetry must be accounted for in order to avoid significant errors (up to 90% for cases considered).

Numerical and physical modeling of extreme waves at Wave Hub

ABSTRACT Ransley, E., Hann, M., Greaves, D., Raby, A. and Simmonds, D., 2013. Numerical and physical modelling of extreme waves at Wave Hub With a history of international failures, the survivability of coupled systems of wave energy devices and their moorings, particularly those to be installed at development sites like Wave Hub, is surrounded by uncertainty. Potential design solutions require a better understanding of the hydrodynamics and structural loading experienced during extreme events, like rogue wave impact, in order to mitigate the risk of device and mooring failure. Rogue waves are waves with amplitudes far greater than those expected, given the surrounding sea conditions. Intense study into these events stems from their potential for catastrophic impact on ocean engineering structures. However, little is known about their physical origins and, currently, there is no consensus on their definition or explanation of the mechanism which drives them. This paper concerns the numerical modeling and experimental validation of extreme rogue wave examples at the Wave Hub site. Using hindcast data, the 100 year extreme wave at the Wave Hub site is determined. This extreme wave is replicated in Plymouth Universitys new COAST Lab using a NewWave, dispersive focusing input. To simulate and analyse these events, we duplicate these conditions in a numerical wave tank (NWT), solving the fully nonlinear Navier-Stokes equations, with a free surface, using the Volume of Fluid (VoF) method and open source CFD library OpenFOAM®. The comparison shows that the CFD software is capable of simulating focused waves similar to those produced in the physical tank but tends to overestimate the crest heights. It is also noted that nonlinear effects are important when considering the shape and location of focused wave events.

The evolution of an equilibrium bay

This paper reports on laboratory morphological studies of a shore-parallel porous breakwater system. A physical model study of the Elmer breakwater scheme, West Sussex, was conducted in the UK Coastal Research Facility (UKCRF) at HR Wallingford as part of an EPSRC-funded composite model evaluation (LUPY project). Mobile bed experiments, using both sand and anthracite as model sediments are described and discussed. Subsequent analysis showed that the evolution of equilibrium bays and their final width depend not only on the breakwater length and gap width, but also on the properties of the chosen model sediment and the permeability of the structure. It was also found that the 3D morphological changes influence the hydrodynamics, which in turn influences the evolution of the equilibrium morphological features.

A Bayesian network model for assessments of coastal inundation pathways and probabilities

Coastal flood assessments often require the analysis of a complex system of flood sources, pathways and receptors. This can be challenging for traditional numerical modelling approaches. In this paper we use a Bayesian networks approach to assess coastal floodplains as networks of inter-linked elements. A Bayesian network (Bn) model is built to describe flood pathways and estimate flood extents for different extreme events. The network of the Bn model is constructed from a quasi-2D Source – Pathway – Receptor (SPR) systems diagram. The Bn model is applied in Teignmouth in the UK, a coastal floodplain of typical complexity. It identifies two key flood pathways and assesses their sensitivity to changes in sea levels, beach widths and coastal defences. The advantages, utility and limitations of the Teignmouth Bn model are discussed. The process of 2D SPR and Bn model construction helps identify gaps in floodplain understanding and description. The Bn model quantifies inundation probabilities and facilitates the rapid identification of critical pathways and elements, before committing resources to further detailed analysis. The approach is transferable and can be readily applied in local-scale coastal floodplains to obtain a systems-level understanding and inform numerical modelling assumptions.

Exploring the Potential for Multivariate Fragility Representations to Alter Flood Risk Estimates: Potential for Multivariate Fragility Representations to Alter Flood Risk Estimates

In flood risk analysis, limitations in the multivariate statistical models adopted to model the hydraulic load have restricted the probability of a defense suffering structural failure to be expressed conditionally on a single hydraulic loading variable. This is an issue at the coastal level where multiple loadings act on defenses with the exact combination of loadings dictating their failure probabilities. Recently, a methodology containing a multivariate statistical model with the flexibility to robustly capture the dependence structure between the individual loadings was used to derive extreme nearshore loading conditions. Its adoption will permit the incorporation of more precise representations of a structures vulnerability in future analyses. In this article, a fragility representation of a shingle beach, where the failure probability is expressed over a three-dimensional loading parameter space-water level, wave height, and period-is derived at two localities. Within the approach, a Gaussian copula is used to capture any dependencies between the simplified geometric parameters of a beachs shape. Beach profiles are simulated from the copula and the failure probability, given the hydraulic load, determined by the reformulated Bradbury barrier inertia parameter model. At one site, substantial differences in the annual failure probability distribution are observed between the new and existing approaches. At the other, the beach only becomes vulnerable after a significant reduction of the crest height with its mean annual failure probability close to that presently predicted. It is concluded that further application of multivariate approaches is likely to yield more effective flood risk management.