Search

The Met Office operational wave modelling system is an operational forecast system run four times daily at the Met Office to provide global and regional forecasts up to 7 days ahead. The underpinning model uses a recent development branch the spectral wave model WAVEWATCH III® (version 7.12) that includes a number of updates developed at the Met Office. Code contributions include the Spherical Multiple-Cell (SMC) grid, rotated pole grid formulation for mid latitudes, enhancements to OASIS coupling and updates to the netCDF postprocessing. Here we document and describe the technical details behind the Met Office operational system of WAVEWATCH III® configurations with a view to further development. These include a global forecast deterministic model (GS512L4EUK) and two regional models nested one-way covering the Northwest (NW) European shelf and UK waters (AMM15SL2) as well as an Atlantic wave ensemble (AS512L4EUK). GS512L4EUK and AS512L4EUK are based on a four-tier SMC 25-12-6-3km grid refinement where currents are not included. AMM15SL2 is run operationally both as a standalone forced model and as the wave component of a two-way ocean-wave coupled operational system FOAM-AMM15. The AMM15SL2 baseline configuration is based on a two-tier SMC grid that focuses on the shelf seas around the United Kingdom (3km resolution) where coastal cells have 1.5km resolution and wavecurrent interaction is included. Results from a 2-year hindcast demonstrate the ability of the baseline configurations to reproduce both in-situ and satellite wave observations. Model-observations correlation is above 0.94-0.96 with standard deviations of differences that correspond to maximum 13-25% of the observed mean bulk wave diagnostics, demonstrating quality and accuracy of the system. Evidence of resolution dependent differences in wave growth was observed, leading slightly overestimated significant wave heights when replicating coastal mid-range conditions by AMM15SL2, and better suited to replicate the extremes. Additionally, the inclusion of wave-current interaction in AMM15SL2 tends to larger spread the observation-model differences. Hence, although a positive impact of the surface currents is not always shown in the overall statistics of the significant wave height, the addition of currents helps to significantly improve the prediction of the wave direction and period near the coast (>20% improvement), which has implications in beach safety, risk to coastal overtopping and shoreline evolution. Future system developments such as the use of sea point wind forcing, the optimisation the models in line with model resolution, the utilisation of SMC multigrid and data assimilation are discussed. [ABSTRACT FROM AUTHOR]

Since the end of the last glaciation, the United Kingdom's land surface has been altered by isostatic rebound, rising in the north and sinking in the south. Numerous studies have been published documenting the impact of isostatic rebound on relative sea levels. However, due to the difficulties in acquiring evidence to prove former sea levels, locally, these data can be sparse or absent. In this work, we explored the suitability of the passive seismic survey (PSS) method to estimate the contemporaneous beach thickness in coastal environments where there is a high impedance contrast between the beach deposits and the underlying wave-cut platform. We conducted a three-day survey at Perran Beach, Cornwall, collected 149 measurements using PSS, and interpreted the observations supported by auxiliary topographical, geological, and independent geophysical observation in the study area. The study site is a contemporaneous beach mostly composed of sand underlain by a wave-cut platform composed of igneous and sedimentary rock, therefore high impedance contrast with the sandy beach is anticipated. The elevation of the bedrock relative to the topographical elevation suggests that the bedrock elevation is −15 m ± 5 m below the present day mean sea level, which is coherent with the observation of relative sea level rise along the region of the south-west. The present study contributes to our current limited understanding of land and sea level movements by providing further subsurface information to the coastal geological archive of south-west England, a region currently in need of more data to reconstruct land- and sea-level movements. [ABSTRACT FROM AUTHOR]

Bi-directional wave climates often drive beach rotation, increasing erosional risk at semi-sheltered locations. Identification of rotation and forcing mechanisms is vital to future coastal defence. In this study, regional investigation of modelled wave data revealed strong bi-directionality between dominant south-westerly and sub-dominant easterly waves for 14 offshore locations along the length of the south coast of England, U.K. South-westerly wave power was well correlated to positive phases of the West Europe Pressure Anomaly (WEPA), whilst easterly wave power was well correlated with negative phases of the North Atlantic Oscillation (NAO). Additionally, decadal records of beach morphological change and associated wave forcing, were investigated for 22 coastal sites across the same region. Significant rotational behaviour was identified at 11 sites, leading to the creation of a rotation index. Beach rotation was attributed to shoreline angle, with the strongest rotation occurring at south-east-facing beaches, with high obliquity to dominant south-westerly waves. The beach rotation index was well correlated with the normalized balance of wave power from opposing south-westerly and easterly directions. Direct correlations between beach rotation and WEPA at two sites showed that future forecasts of atmospheric indices may allow prediction of rotational beach state, at seasonal scales. [ABSTRACT FROM AUTHOR]

Sandbanks can occur downdrift of headlands across embayments as a result of tidal currents and wave forcing, and may present a potentially valuable resource of marine aggregate material. Large sandbanks can alter tidal flows and, through refraction and dissipation, increase or decrease wave exposure along the shoreline of an embayment. The impact of offshore sandbanks on shoreline morphodynamics at varying timescales has not been rigorously investigated. Here we show, for the first time, that removing, lowering or raising a headland-associated sandbank can have a significant impact on longshore sediment transport. A Delft3D model of Start Bay and Skerries Bank, UK, validated on hydrodynamic observations, was used to conduct the numerical experiments. Results indicate that removing or lowering the bank generally reduces dissipation and increases wave heights, increasing longshore flux in the lee of the bank. Raising the bank generally has the opposite effect, increasing dissipation and lowering flux. Under oblique waves, the bank may protect the adjacent shoreline by refracting waves away from the coast. Shoreline flux for the region directly onshore of the bank acts as a continuous function of bank height (R 2 = 0.7), but correlations for distal regions are increasingly non-linear or absent. A raised bank would reduce transport rates to near-zero across extensive sections of the bay, reversing the long-term net flux at points distal (>5 km from bank) from the bank location, causing currently eroding regions to accrete. Our results demonstrate that moderate (≤5 m) elevation changes to a sandbank will likely produce significant variations in wave power at the shoreline, which will: (i) modify the shoreline response for individual storms; and (ii) substantially change shoreline morphology at decadal timescales. From a broader perspective, this study can be used to inform planning decisions involving naturally varying banks and/or potential mining of sandbanks, aiding shoreline protection strategies for locations with similar embayment-sandbank configurations. Unlabelled Image • First robust study of bank control on inshore waves and longshore transport • 20% change to bank elevation (≤5 m) significantly impacts longshore flux. • Directly in lee of bank, shoreline flux is a continuous function of bank depth. • Bank protects shore, dissipating direct waves and refracting away oblique waves. • Raised bank would substantially accrete distal (>5 km) currently eroding shoreline. [ABSTRACT FROM AUTHOR]