Nieves Garcia Valiente

Due partially to a changing global climate, there is an increasing need to develop a modeling approach capable of providing warnings of coastal evolution, including erosion at storm, interannual, and decadal timescales and in complex coastal settings. This research presents a reduced‐complexity model with a common approach to predicting shoreline change due to cross‐shore and longshore processes. The model architecture is designed to be flexible and data‐informed, incorporating an extended Kalman filter (eKF) for the assimilation of field measurements of shoreline change. Depending on the values of the optimized model‐free parameters resulting from the eKF assimilation, the longshore and cross‐shore shoreline model components can resemble either new, established, or hybrid versions of previously reported models. Thus, the model also provides new insight into the dynamics of shoreline change. The model is tested and demonstrated via a series of numerical tests and applied to field data collected at a complex coastal setting, showing good performance with R.M.S. shoreline prediction errors of <6 m. The eKF data assimilation suggests that a hybrid cross‐shore transport model, combining the characteristics of two established semiempirical models and a new disequilibrium longshore transport model, effectively describes the shoreline evolution at the swash‐aligned field site. The computational efficiency, stability, and versatility of the ForCE‐LX model provides a promising tool for the prediction of shoreline evolution on time‐scales of days to decades, in widespread coastal settings.

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 of 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 25-12-6-3 km 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 (3 km resolution) where coastal cells have 1.5 km resolution and wave-current 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 the quality and accuracy of the system. Evidence of resolution dependent differences in wave growth was observed, leading to 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 on 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 of the models in line with model resolution, the utilisation of SMC multigrid and data assimilation are discussed.

Extreme storms cause extensive beach-dune erosion and are typically considered to enhance coastal erosion due to sea-level rise. However, extreme storms can also have a positive contribution to the nearshore sediment budget by exchanging sediment between the lower and upper shoreface and/or between adjacent headlands, potentially mitigating some adverse sea-level rise impacts. Here we use three high-resolution morphological datasets of extreme storm-recovery sequences from Australia, the UK and Mexico to quantify the nearshore sediment budget and relate these episodic volume changes to long-term coastal projections. We show that sediment gains over the upper shoreface were large (59–140 m³/m) and sufficient to theoretically offset decades of projected shoreline retreat due to sea-level rise, even for a high-end greenhouse gas emissions scenario (SSP5-8.5). We conclude that increased confidence in shoreline projections relies fundamentally on a robust quantitative understanding of the sediment budget, including any major short-term sediment contribution by extreme storms.

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.

Extreme storms cause extensive beach-dune erosion and are universally considered to enhance coastal erosion due to sea-level rise (SLR). However, extreme storms can also have a positive contribution to the nearshore sediment budget by exchanging sediment between the lower and upper shoreface and/or between adjacent headlands, potentially mitigating adverse SLR impacts. Here we use three high-resolution morphological datasets of extreme storm-recovery sequences from Australia, the UK and Mexico to quantify the nearshore sediment budget and relate these episodic volume changes to long-term coastal forecasts. We show that sediment gains over the upper shoreface and beach were very significant (58-140 m³/m) and sufficient to offset decades of predicted shoreline retreat due to SLR, even for an upper SSP5-8.5 scenario. It is evident that increased confidence in shoreline predictions due to SLR relies fundamentally on robust quantitative understanding of the sediment budget, in particular any long-term contribution of sediment transport from outside the nearshore region.

Embayed beaches separated by irregular rocky headlands represent 50% of global shorelines. Quantification of inputs and outflows via headland bypassing is necessary for evaluating long‐term coastal change. Bypassing rates are predictable for idealized headland morphologies; however, it remains to test the predictability for realistic morphologies, and to quantify the influence of variable morphology, sediment availability, tides and waves‐tide interactions. Here we show that headland bypassing rates can be predicted for wave‐dominated conditions, and depend upon headland cross‐shore length normalised by surf zone width, headland toe depth and spatial sediment coverage. Numerically modeled bypassing rates are quantified for 29 headlands under variable wave, tide and sediment conditions along 75 km of macrotidal, embayed coast. Bypassing is predominantly wave‐driven and nearly ubiquitous under energetic waves. Tidal elevations modulate bypassing rates, with greatest impact at lower wave energies. Tidal currents mainly influence bypassing through wave‐current interactions, which can dominate bypassing in median wave conditions. Limited sand availability off the headland apex can reduce bypassing by an order of magnitude. Bypassing rates are minimal when cross‐shore length >5 surf zone widths. Headland toe depth is an important secondary control, moderating wave impacts off the headland apex. Parameterisations were tested against modeled bypassing rates, and new terms are proposed to include headland toe depth and sand coverage. Wave‐forced bypassing rates are predicted with mean absolute error of a factor 4.6. This work demonstrates wave‐dominated headland bypassing is amenable to parameterization and highlights the extent to which headland bypassing occurs with implications for embayed coasts worldwide.

Many human activities rely on accurate knowledge of the sea surface dynamics. This is especially true during storm events, when wave‐current interactions might represent a leading order process of the upper ocean. In this study, we assess and analyze the impact of including three wave‐dependent processes in the ocean momentum equation of the Met Office North West European Shelf ocean‐wave forecasting system on the accuracy of the simulated surface circulation. The analysis is conducted using ocean currents and Stokes drift data produced by different implementations of the coupled forecasting systems to simulate the trajectories of surface (iSphere) and 15 m drogued (SVP) drifters affected by four storms selected from winter 2016. Ocean and wave simulations differ only in the degree of coupling and the skills of the Lagrangian simulations are evaluated by comparing model results against the observed drifter tracks. Results show that, during extreme events, ocean‐wave coupling improves the accuracy of the surface dynamics by 4%. Improvements are larger for ocean currents on the shelf (8%) than in the open ocean (4%): this is thought to be due to the synergy between strong tidal currents and more mature decaying waves. We found that the Coriolis‐Stokes forcing is the dominant wave‐current interaction for both type of drifters; for iSpheres the secondary wave effect is the wave‐dependent sea surface roughness while for SVPs the wave‐modulated water‐side stress is more important. Our results indicate that coupled ocean‐wave systems may play a key role for improving the accuracy of particle transport simulations.

Prediction of severe natural hazards requires accurate forecasting systems. Recently, there has been a tendency towards more integrated solutions, where different components of the Earth system are coupled to explicitly represent the physical feedbacks between them. This study focuses on rapidly developing waves under extratropical storms to understand the impact of different wave source term parameterisations in the WAVEWATCH III (WWIII) model (ST4 and ST6) and coupling strategies (surface roughness closure versus surface stress closure) on the accuracy of the Met Office regional atmosphere-ocean-wave coupled research system for the north-west (NW) European shelf (UKC4). Results of a study focused on simulations during winter 2013/14 demonstrate that ST6 allows for a faster wave growth than the ST4 parameterisation but might degrade low to mid energy wave states. The difference between ST6 and ST4 in wave growth is larger for higher wind speeds and short fetches. The experiment with ST4 and roughness closure consistently under-predicts the wave growth in those locations where fetch dependence is an important factor (i.e., seas at the East (E) of Ireland and the UK for storms coming from the NW-WNW). The implementation in the wave model of ST6 physics with the stress closure coupling strategy appears to improve growth of young wind-seas, reducing bias in those locations where the storms are underestimated. The slower wave growth when using surface roughness closure seems to be related to an underestimation of the momentum transfer computed by the wave model when coupling the wind speeds. For very young to young wind seas, this can be overcome when the surface stress is computed by the atmospheric model and directly passed to the ocean.