Waves Across Shore Platforms

Lead Research Organisation: Bangor University
Department Name: Sch of Ocean Sciences

Abstract

Rocky coastlines are generally characterised by cliffs fronted by intertidal shore platforms and occur along 20% of the coastline of England and Wales. These shore platforms tend to be gently-sloping and they invariably represent hydrodynamically very rough surfaces. Cliffs and shore platforms are linked dynamically because the platform characteristics directly control the transformation processes of waves propagating across it, and thus the impact on the cliff and cliff erosion. For rocky shores this transformation process is virtually unstudied. The general aim of this project is to increase both understanding and modelling capability of wave transformation processes across rocky shore platforms. The research will not only benefit the coastal engineering community and contribute to better coastal management and planning, but will also benefit other coastal scientists, including geologists, geomorphologists and ecologists.

Our overarching hypothesis is that the transformation of the wave spectrum across shore platforms is primarily controlled by the elevation, gradient and width of the platform, and the roughness of its surface. We consider that it is feasible to model this wave transformation process, and thus energy delivery to the base of the cliff, using existing numerical wave models after appropriate parameterisation of the bed friction of the platform surface. We further propose that the bed friction of the platform surface can be parameterised based on the characteristics of the shore platform, namely its gradient and roughness (micro-topography).

Our intention is to conduct comprehensive and detailed field measurements of wave transformation across 6 different shore platforms under a range of wave/tide conditions and derive universally valid principles from our observations that better describe and enable the prediction of wave transformation processes across rocky shore platforms. Each of these 8-day experiments will involve deployment of a range of instruments, including pressure sensors to measure waves and water levels, acoustic current meters to record nearshore currents, digital video cameras for monitoring wave breaker patterns and wave runup, a laser scanner for measuring swash dynamics and a terrestrial LiDAR system for making high-resolution measurements of the shore platform topography.

The field data will be used to quantify wave energy dissipation by bed friction and wave breaking, and the dissipation rates will be used to back-calculate wave friction factors using linear wave theory. In turn, the obtained wave friction factors will be correlated to the roughness of the shore platform surface related to the overall morphology and micro-topography. The improved wave friction parameterisation will be implemented in the open-source XBeach numerical model and the model will be used for each of the 6 sites to evaluate the effect of changing sea level to the wave energy delivery to the cliff base to explore the potential effect of rising sea level on coastal cliff recession.

This project involves a multi-disciplinary research team from the Universities of Plymouth, Bangor and Auckland, and Deltares (Netherlands). The project will benefit from the complementary expertise of two oceanographers, two coastal engineers, two physical geographers and one geologist, all with proven track records in research areas that have a direct bearing on the current project: field experimentation, nearshore and surf zone dynamics, rocky coast processes and numerical modelling. The hosting institution also has an experimental infrastructure for studying shallow water oceanographic processes for fieldwork that is second to none in the UK, and is ideally suited to support the proposed research project. The combined strength in research infrastructure and researchers, as well as the relevance of the research topic, makes this a low-risk high-impact project.
 
Description Analysis of an extensive observational field data set linked to numerical modelling has demonstrated that frictional dissipation of wave energy across rock shore platforms is of secondary importance to dissipation by breaking waves. Frictional dissipation is only significant for very rough, flat platforms during small wave conditions outside the surf zone.
Exploitation Route The findings have implications for the design and implementation of coastal defence structures, indicating that attempts to reduce wave energy are probably best focused on causing waves to break further seaward of the coastline and then dissipating remaining wave energy over wide, flat and rough regions between the breakers and shoreline. These concepts are remarkably similar to coral reef morphology, whereby the waves break on the steep outer reef face, and then dissipation over the reef flat -- our current findings are therefore complementary to parallel research on coral reefs. In the concepts of rock platform morphology, our findings indicate that wave erosion of cliffs in strong tidal environments appears somewhat limited. Under modal conditions the sloping platforms dissipate the majority of wave energy before it impacts the cliff. It is suggested therefore, that cliff erosion is caused by infrequent periodic storm wave impact events, but that erosion of the platform by strong wave orbital velocities, modulated by large tidal ranges, act to maintain the platform gradient.
Sectors Aerospace, Defence and Marine,Energy,Environment,Leisure Activities, including Sports, Recreation and Tourism

 
Title WASP-fielddata 
Description Dataset of wave transformation across 6 UK field sites 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact Links with Deltares and TU Delft are directly based on this data, and impact in terms of modifications to models is expected 
 
Description Deltares XBeach links 
Organisation Deltares
Country Netherlands 
Sector Charity/Non Profit 
PI Contribution WASP field data is being used to further the development of the XBeach model, which originates from Deltares, into a new environmental setting.
Collaborator Contribution Deltares staff are actively engaged in the modelling process of the collected field data.
Impact Deltares have tied an MSc student from TU Delft to the project. The student will visit Bangor in 2016 to further the collaborations. Austin will act as external examiner to the MSc student at the end of their project.
Start Year 2015
 
Description NOC-L XBeach modelling collaboration 
Organisation National Oceanography Centre
Country United Kingdom 
Sector Academic/University 
PI Contribution Personal collaboration with Jenny Brown based on XBeach modelling. We're taking some ideas and findings developed under WASP into a new Master Research Project on gravel beach erosion, and potentially a NERC ENVISION DTP PhD studentship.
Collaborator Contribution Wider-scale modelling expertise of Jenny Brown at Liverpool extends the capabilities that have arisen from WASP. Particularly linking XBeach with larger-scale boundary forcing shelf-sea models.
Impact One internationally advertised DTP PhD studentship -- shortlisted student currently being assessed at DTP level.
Start Year 2016