Waves Across Shore Platforms

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.

The proposed research will contribute directly to a better understanding and improved modelling capability of wave transformation processes across rocky shore platforms and the benefits will be observed by end users engaged in coastal engineering and management (i.e., consultants and local/national authorities). Wide and narrow shore platforms occur along 28% of the coastline of England and Wales. With government agencies as one of the principal investors in coastal management and infrastructure (44% of the England and Wales coastline is defended) and the projection that current spending will double by 2080 (from £358 million in 2007), the need for accurate inshore wave modelling over rocky substrates is clear. It is important, however, that the benefits of the proposed research are appropriately and sufficiently wide disseminated.

In addition to journal papers and conference presentations, we will disseminate the knowledge gained and the tools produced from this project to end users in a number of ways. Firstly, throughout the project's lifetime, and 5 years beyond, we will maintain a detailed project website which will log the progress and outputs of the research. On completion of the project, the main datasets comprising of measurements of platform topography and surf zone hydrodynamics will be made freely available to national and international users in NetCDF format from a dedicated server for further collaboration and engagement with interested parties (end users, scientists and engineers). Secondly, the improved capability for modelling transformation of waves across shore platforms will be incorporated into the existing XBeach model. This model is open-source and the improved algorithms will be available for downloading both from the XBeach website, as well as from the project website. Thirdly, a one-day conference at the end of the project will organised to which we will invite coastal engineers from leading national consulting agencies (e.g., HR Wallingford, ABPMER, Halcrow, Haskoning, Fugro), as well as other organisations with an interest in coastal processes and protection (e.g., Environment Agency, Natural Resources Wales, Natural England, Coastal Observatories). During the conference, we will report the results of the research and demonstrate the improved modelling capabilities.

PI Masselink, CI Austin and PDRF Poate, have a strong record in applied research and end user engagement and two examples are detailed here. Firstly, the NERC-funded project Dynamics of Rip currents and Implications for Beach Safety (DRIBS; NE/H004262/1) was a partnership with the RNLI and was followed up with an extension part-funded by the RNLI and Met Office. A large number of end-user tools were delivered by the end of the project ranging from the production of a web-based rip manual for the training of RNLI lifeguards to a daily rip-risk forecasting tool hosted on the Met Office web site. Secondly, the EPSRC-funded project New Understanding and Predicting Storm Impacts on Gravel beaches (NUPSIG; EP/H040056/1) is current and involves a number of end-user partners, including HR Wallingford, Environment Agency and Channel Coastal Observatory. The key deliverable of this project is a Graphical User Interface (GUI) based on the XBeach model that is specifically designed to be used by coastal engineers and manager to predict the response of a gravel barrier to extreme storms.

At a total cost of £7k, the implementation of the impact plan, as detailed above, represents extremely good value for money. The workshop will ensure that the benefits of the proposed research reach the end users, whereas making the full data set available on the web in Net-CDF format will facilitate the use of the field data by other scientists beyond the duration of the project.