In this paper, we investigate changes in the wave climate of the west-European shelf seas under g... more In this paper, we investigate changes in the wave climate of the west-European shelf seas under global warming scenarios. In particular, climate change wind fields corresponding to the present (control) time-slice 1961-2000 and the future (scenario) time-slice 2061-2100 are used to drive a wave generation model to produce equivalent control and scenario wave climate. Yearly and seasonal statistics of the scenario wave climates are compared individually to the corresponding control wave climate to identify relative changes of statistical significance between present and future extreme and prevailing wave heights. Using global, regional and linked globalregional wind forcing over a set of nested computational domains, this paper further demonstrates the sensitivity of the results to the resolution and coverage of the forcing. It suggests that the use of combined forcing from linked global and regional climate models of typical resolution and coverage is a good option for the investigation of relative wave changes in the region of interest of this study. Coarse resolution global forcing alone leads to very similar results over regions that are highly exposed to the Atlantic Ocean. In contrast, fine resolution regional forcing alone is shown to be insufficient for exploring wave climate changes over the western European waters because of its limited coverage. Results obtained with the combined globalregional wind forcing showed some consistency between scenarios. In general, it was shown that mean and extreme wave heights will increase in the future only in winter and only in the southwest of UK and west of France, north of about 44-45°N. Otherwise, wave heights are projected to decrease, especially in summer. Nevertheless, this decrease is dominated by local wind waves whilst swell is found to increase. Only in spring do both swell and local wind waves decrease in average height.
ICE2013 Coasts, Marine Structures and Breakwaters Conference Proceedings
The aim of this paper is to appraise the current knowledge on seawall performance and reliability... more The aim of this paper is to appraise the current knowledge on seawall performance and reliability, and to make the case for improved reliability assessments of vertical seawalls, which are used here as a representative for coastal flood defences. In order to achieve this aim, a brief introduction to flood risk management is first given. Then, vertical seawalls are introduced, and their most prominent failure modes are discussed. Reliability analysis is introduced within the context of flood risk management. More specifically, the fragility curve approach that is currently in use in industry is described, and its limitations are discussed. Finally, it is argued that recent advances in multivariate extreme value models would enable improvements to the approaches currently applied in practice. It is stressed that future risk assessment models of coastal flood defences ought to include multiple failure modes and their interactions, a thorough analysis of the model uncertainties, and potential computational costs, in view of providing practitioners with an improved and functional risk assessment tool.
The study reported in this paper focuses on the characterisation of wave energy resources of Guer... more The study reported in this paper focuses on the characterisation of wave energy resources of Guernsey's Western approach, within 12 nautical miles (NM) west of Guernsey Island, located within the English channel region, approximately 58 nautical miles south of the English coast and a few miles west of the Cherbourg peninsula (France). Wave conditions over 2 years (From January 2009 to December 2010) were produced using Delft3D-Wave (a version of SWAN embedded within the Delft3D modelling suite) at different locations around Guernsey. The model was forced with WaveWatchIII (WWIII) model data at a point with a latitude of 49.530, a longitude of -2.940, a depth of 77m, and at 3 hourly intervals for the 2-year study period. Model validations with data by the same WWIII model at a grid point with a latitude of 49.530, a longitude of -2.770, and a depth a 66m, at 3-hourly intervals and for the same 2-year period, showed an extremely good agreement between model predictions and WWIII data. The boundary conditions data and the model validation data were kindly provided by the Met Office for this particular project. A constant mean sea level was assumed. Computations of monthly and mean available power output throughout the study area for the 2-year period highlighted three regions, two near the 12NM limit, and one very near the Island itself, where maximum mean power outputs were of the order of 24 kW/m. This mean power output is similar to that at the WaveHub site, off the north coast of Cornwall, being developed as a Wave Energy test facility due to its excellent wave energy conversion potential.
Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly... more Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly profile surveys taken at two shore-perpendicular transects at Duck, North Carolina, USA. The data cover both the swash and surf zones. Singular Spectrum Analysis (SSA) and Multi-channel Singular Spectrum analysis (MSSA) methods are applied, on the shoreface, to three potential forcings: the monthly wave heights, the monthly mean water levels and the large scale atmospheric index known as the North Atlantic Oscillation. The patterns within these forcings are compared to the local bathymetric patterns; it is found that the patterns extracted using SSA and MSSA agree well with previous patterns identified using wavelets and confirm the highly nonstationary behaviour of beach levels at Duck. This is followed by analysis of potential correlations between the local bathymetry (at the two transects) and hydrodynamic and atmospheric patterns. The study is then extended to all measured bathymetric profiles, covering an area of 1100 m (alongshore) by 440 m (cross-shore). MSSA showed no collective inter-annual patterns of oscillations present in the bathymetry and the three potential forcings. Annual and semi-annual cycles within the bathymetry are found to be strongly correlated with the monthly wave height, in agreement with the SSA findings. Other collective intraannual cycles besides the semi-annual were identified; they were all correlated with the North Atlantic Oscillation.
This paper presents a novel data-driven methodology based on empirical orthogonal teleconnections... more This paper presents a novel data-driven methodology based on empirical orthogonal teleconnections (EOTs) to analyse and forecast the evolution of coastal navigational channels near the mouth of the Exe estuary, UK. This is the first time EOTs are used in coastal morphodynamics. Therefore, particular emphasis is placed on the comparison of EOTs with the well established empirical orthogonal functions (EOFs) method. EOTs and EOFs are used with a series of 14 surveys, taken approximately every 8 months, covering the period between January 2001 and February 2010. The skill of the methods in producing accurate bathymetric one-step forecasts for February 2010 is analyzed and compared with one-step forecasts based on the raw data. It is found that, provided the order of the autoregressive forecast method is chosen appropriately, EOTs and EOFs are better than the raw data and EOTs outperforms than EOFs. This is attributed to the fact that EOTs, without the orthonormality restriction for the temporal eigenfunctions required in EOFs, capture the temporal patterns within the data more accurately than EOFs.
This paper reports progress made in modifying and applying the X-Beach code to predict and explai... more This paper reports progress made in modifying and applying the X-Beach code to predict and explain the observed behaviour of coarse grained beaches. In a previous study a comparison of beach profile evolution measured during large scale experiments under constant water level with numerical model simulations was made. This placed particular emphasis on the tendency for onshore transport and profile steepening during calm conditions . The present paper extends that investigation to study the influence of the advection of surf processes induced by tidal water level variations effects, on gravel beach profile evolution. The parameter values and numerical model used in the simulation is similar to that presented previously. It is assumed that, to good approximation, the groundwater interface inside the beach follows the tidally modulated water level. The results obtained from the model shows that the model provides reasonable simulations of beach profile change in a tidal environment. In comparison with simulations under stationary water levels, a larger berm is produced in agreement with literature. Finally, good agreement is obtained between the model simulations and an example of field observations from a beach at Milford on Sea, UK. Further developments are outlined for future work.
Coarse-grained beaches are particularly prevalent in the UK, composed of accumulations of either ... more Coarse-grained beaches are particularly prevalent in the UK, composed of accumulations of either gravel, or mixed sand and gravel sediments. Understanding and predicting their morphological behaviour in response to short-term and long-term forcing has been the subject of recent research. Despite the focus on sandy beaches, it is important to understand that the balance of processes that govern different behaviour between sandy and gravel beaches. In this study we show how a public domain numerical model, XBeach, developed for sandy environments ) can be modified for use in predicting the cross-shore profile changes of gravel beaches. Improvements investigated here include: use of Lagrangian interpretation of velocity in place of Eulerian for driving sediment movement; incorporation of Packwood's (1983) pragmatic model of infiltration in the unsaturated area of the swash region; introducing of new morphological module based upon Soulsby's (1997) sediment transport equation for waves and currents. These changes are suggested in order to significantly improve the application of this model to gravel beaches, especially with regard to swash velocity asymmetry which is responsible for development of the steep accretionary phase steep berm above waterline. The results from the model agree well with the measured experimental data and improve upon the results presented by . Figure 1: Accretionary berm on coarse-grained beach (Milford-On-Sea, UK).
Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly... more Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly profile surveys taken at two shore-perpendicular transects at Duck, North Carolina, USA. The data cover both the swash and surf zones. Singular Spectrum Analysis (SSA) and Multi-channel Singular Spectrum analysis (MSSA) methods are applied, on the shoreface, to three potential forcings: the monthly wave heights, the monthly mean water levels and the large scale atmospheric index known as the North Atlantic Oscillation. The patterns within these forcings are compared to the local bathymetric patterns; it is found that the patterns extracted using SSA and MSSA agree well with previous patterns identified using wavelets and confirm the highly nonstationary behaviour of beach levels at Duck. This is followed by analysis of potential correlations between the local bathymetry (at the two transects) and hydrodynamic and atmospheric patterns. The study is then extended to all measured bathymetric profiles, covering an area of 1100 m (alongshore) by 440 m (cross-shore). MSSA showed no collective inter-annual patterns of oscillations present in the bathymetry and the three potential forcings. Annual and semi-annual cycles within the bathymetry are found to be strongly correlated with the monthly wave height, in agreement with the SSA findings. Other collective intraannual cycles besides the semi-annual were identified; they were all correlated with the North Atlantic Oscillation.
The chemical field around swimming plankton depends on the swimming style and speed of the organi... more The chemical field around swimming plankton depends on the swimming style and speed of the organism and the processes affecting uptake or exudation of chemicals by the organism. Here, we present a simple model for the flow field around a neutrally buoyant self-propelled organism at low Reynolds number and numerically calculate the chemical field around the organism. We show how the concentration field close to the organism and the mass-transfer rates vary with the swimming speed and style for Dirichlet (diffusion-limited transport) boundary conditions. We calculate how the length of the chemical wake, defined as being the distance at which the chemical field drops to 10% of the surface concentration of the organism when stationary, varies with the swimming speed and style for both Dirichlet and Neumann ( production limited) boundary conditions. For Dirichlet boundary conditions, the length of the chemical wake increases with increasing swimming speed, and the self-propelled organism displays a significantly longer wake than the towed-body model. For Neumann boundary conditions, the converse is true; because swimming enhances the transport of the chemical away from the organism, the surface concentration of chemical is reduced and thus the wake length is reduced.
In this paper a generic methodology is presented that allows the impacts of climate change on wav... more In this paper a generic methodology is presented that allows the impacts of climate change on wave energy generation from a wave energy converter (WEC) to be quantified. The methodology is illustrated by application to the Wave Hub site off the coast of Cornwall, UK. Control and future wave climates were derived using wind fields output from a set of climate change experiments. Control wave conditions were generated from wind data between 1961 and 2000. Future wave conditions were generated using two IPCC wind scenarios from 2061 to 2100, corresponding to intermediate and low greenhouse gas emissions (IPCC scenarios A1B and B1 respectively). The quantitative comparison between future scenarios and the control condition shows that the available wave power will increase by 2–3% in the A1B scenario. In contrast, the available wave power in the B1 scenario will decrease by 1–3%, suggesting, somewhat paradoxically, that efforts to reduce greenhouse gas emissions may reduce the wave energy resource. Meanwhile, the WEC energy will yield decrease by 2–3% in both A1B and B1 scenarios, which is mainly due to the relatively low efficiency of energy extraction from steeper waves by the specific WEC considered. Although those changes are relatively small compared to the natural variability, they may have significance when considered over the lifetime of a wave energy farm. Analysis of downtime under low and high thresholds suggests that the distribution of wave heights at the Wave Hub will have a wider spread due to the impacts of climate change, resulting in longer periods of generation loss. Conversely, the estimation of future changes in joint wave height-period distribution provides indications on how the response and power matrices of WECs could be modified in order to maintain or improve energy extraction in the future.► A generic methodology for assessing the impacts of climate change on wave energy generation is presented. ► The methodology is illustrated by application to the Wave Hub site off the coast of Cornwall, UK. ► Comparison of future scenarios and control conditions shows statistically significant changes. ► Changes are small in comparison to natural variability, but have significance over scheme lifetime. ► Analysis of downtime suggests that climate change will result in longer periods of generation loss.
A data-driven model has been developed to analyse the long-term evolution of a sandbank system an... more A data-driven model has been developed to analyse the long-term evolution of a sandbank system and to make ensemble predictions in a period of 8 years. The method uses a combination of empirical orthogonal function (EOF) analysis, (to define spatial and temporal patterns of variability), jack-knife resampling, (to generate an ensemble of EOFs), a causal auto-regression technique, (to extrapolate the temporal eigenfunctions), and straightforward statistical analysis of the resulting ensemble of predictions to determine a ‘forecast’ and associated uncertainty. The methodology has been applied to a very demanding site which includes a curved shoreline and a group of mobile nearshore sandbanks. The site is on the eastern coast of the UK and includes the Great Yarmouth sandbanks and neighbouring shoreline. A sequence of 33 high quality historical survey charts reaching back to 1848 have been used to analyse the patterns and to predict morphological evolution of the sandbank system. The forecasts demonstrate an improved skill relative to an assumption of persistence, but suffer in locations where there are propagating features in the morphology that are not well-described by EOFs.
Wave-induced, steep vortex ripples are ubiquitous features in shallow coastal seas and it is ther... more Wave-induced, steep vortex ripples are ubiquitous features in shallow coastal seas and it is therefore important to fully understand and model the sediment transport processes that occur over them. To this end, two two-dimensional vertical (2DV) models have been critically tested against detailed velocity and sediment concentration measurements above mobile ripples in regular asymmetric oscillatory flow. The two models are a k-o turbulence-closure model and a discrete-vortex, particle-tracking (DVPT) model, while the data are obtained in the Aberdeen oscillatory flow tunnel (AOFT). The models and the data demonstrate that the time-dependent velocity and suspended sediment concentration above the ripple are dominated by the generation of lee-side vortices and their subsequent ejection at flow reversal. The DVPT model predicts the positions and strengths of the vortices reasonably well, but tends to overpredict the velocity close to the ripple surface. The k-o model, on the other hand, underpredicts the height to which the vortices are lifted, but is better able to predict the velocity close to the bed. In terms of the cycle-and ripple-averaged horizontal velocity, both models are able to reproduce the observed offshore flow close to and below the ripple crest and the DVPT model is able to produce the onshore flow higher up. In the vicinity of the vortices, the DVPT model better represents the concentration (because of its better prediction of vorticity). The k-o model, on the other hand, better represents the concentration close to the ripple surface and higher up in the flow (because of the better representation of the near-bed flow and background turbulence). The measured and predicted cycle-and ripple-averaged suspended sediment concentrations are in reasonable agreement and demonstrate the expected region of exponential decay. The models are able to reproduce the observed offshore cycle-and ripple-averaged suspended sediment flux from the ripple troughs upwards, and as a result, produce net offshore suspended sediment transport rates that are in reasonable agreement. The net measured offshore suspended transport rate, based on the integration of fluxes, was found to be consistent with the total net offshore transport measured in the tunnel as a whole once the onshore transport resulting from ripple migration was taken into account, as would be expected. This demonstrates the importance of models being able to predict ripple-migration rates. However, at present neither of the models is able to do so. r
Objective: To formulate a mathematical model of odontogenic cyst growth and establish the dynamic... more Objective: To formulate a mathematical model of odontogenic cyst growth and establish the dynamics of cyst enlargement and the role of osmotic pressure forces throughout its growth.
We present results for the average mass transfer to a spherical squirmer, a model micro-organism ... more We present results for the average mass transfer to a spherical squirmer, a model micro-organism whose surface oscillates tangentially to itself. The surface motion drives a low-Reynolds-number flow which enables the squirmer either to swim relative to the fluid at infinity, at an average speed proportional to a streaming parameter, W , or to stir the fluid around it while remaining, on average, at rest (if W = 0), as represented by a hovering parameter, b. We assume that the amplitude of the time-periodic surface distortions is scaled by a dimensionless small parameter , and consider only high Péclet numbers P -a measure of convection versus diffusion -by setting P −1 = 2 γ , where γ is a parameter of O(1). It is shown that the average mass concentration distribution satisfies a steady convection-diffusion equation with an effective velocity field that is different from the actual mean velocity field. The model is used to calculate the mass transfer across the surface of the squirmer, measured by the mean Sherwood number Sh.
Quarterly Journal of Mechanics and Applied Mathematics, 2003
In this paper we study nutrient uptake by a very simple model of a swimming microorganism, a sphe... more In this paper we study nutrient uptake by a very simple model of a swimming microorganism, a sphere moving its surface tangentially to itself with constant concentration on the surface. The effect of its swimming motions on the concentration field and uptake is investigated. We find the relationship between the Sherwood number (Sh), a measure of the mass transfer across the surface, and the Péclet number (Pe), which indicates the relative effect of convection versus diffusion. Then we compare the results with those for a rigid sphere moving at the same speed under the action of an external force.
In this paper, we investigate changes in the wave climate of the west-European shelf seas under g... more In this paper, we investigate changes in the wave climate of the west-European shelf seas under global warming scenarios. In particular, climate change wind fields corresponding to the present (control) time-slice 1961-2000 and the future (scenario) time-slice 2061-2100 are used to drive a wave generation model to produce equivalent control and scenario wave climate. Yearly and seasonal statistics of the scenario wave climates are compared individually to the corresponding control wave climate to identify relative changes of statistical significance between present and future extreme and prevailing wave heights. Using global, regional and linked globalregional wind forcing over a set of nested computational domains, this paper further demonstrates the sensitivity of the results to the resolution and coverage of the forcing. It suggests that the use of combined forcing from linked global and regional climate models of typical resolution and coverage is a good option for the investigation of relative wave changes in the region of interest of this study. Coarse resolution global forcing alone leads to very similar results over regions that are highly exposed to the Atlantic Ocean. In contrast, fine resolution regional forcing alone is shown to be insufficient for exploring wave climate changes over the western European waters because of its limited coverage. Results obtained with the combined globalregional wind forcing showed some consistency between scenarios. In general, it was shown that mean and extreme wave heights will increase in the future only in winter and only in the southwest of UK and west of France, north of about 44-45°N. Otherwise, wave heights are projected to decrease, especially in summer. Nevertheless, this decrease is dominated by local wind waves whilst swell is found to increase. Only in spring do both swell and local wind waves decrease in average height.
ICE2013 Coasts, Marine Structures and Breakwaters Conference Proceedings
The aim of this paper is to appraise the current knowledge on seawall performance and reliability... more The aim of this paper is to appraise the current knowledge on seawall performance and reliability, and to make the case for improved reliability assessments of vertical seawalls, which are used here as a representative for coastal flood defences. In order to achieve this aim, a brief introduction to flood risk management is first given. Then, vertical seawalls are introduced, and their most prominent failure modes are discussed. Reliability analysis is introduced within the context of flood risk management. More specifically, the fragility curve approach that is currently in use in industry is described, and its limitations are discussed. Finally, it is argued that recent advances in multivariate extreme value models would enable improvements to the approaches currently applied in practice. It is stressed that future risk assessment models of coastal flood defences ought to include multiple failure modes and their interactions, a thorough analysis of the model uncertainties, and potential computational costs, in view of providing practitioners with an improved and functional risk assessment tool.
The study reported in this paper focuses on the characterisation of wave energy resources of Guer... more The study reported in this paper focuses on the characterisation of wave energy resources of Guernsey's Western approach, within 12 nautical miles (NM) west of Guernsey Island, located within the English channel region, approximately 58 nautical miles south of the English coast and a few miles west of the Cherbourg peninsula (France). Wave conditions over 2 years (From January 2009 to December 2010) were produced using Delft3D-Wave (a version of SWAN embedded within the Delft3D modelling suite) at different locations around Guernsey. The model was forced with WaveWatchIII (WWIII) model data at a point with a latitude of 49.530, a longitude of -2.940, a depth of 77m, and at 3 hourly intervals for the 2-year study period. Model validations with data by the same WWIII model at a grid point with a latitude of 49.530, a longitude of -2.770, and a depth a 66m, at 3-hourly intervals and for the same 2-year period, showed an extremely good agreement between model predictions and WWIII data. The boundary conditions data and the model validation data were kindly provided by the Met Office for this particular project. A constant mean sea level was assumed. Computations of monthly and mean available power output throughout the study area for the 2-year period highlighted three regions, two near the 12NM limit, and one very near the Island itself, where maximum mean power outputs were of the order of 24 kW/m. This mean power output is similar to that at the WaveHub site, off the north coast of Cornwall, being developed as a Wave Energy test facility due to its excellent wave energy conversion potential.
Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly... more Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly profile surveys taken at two shore-perpendicular transects at Duck, North Carolina, USA. The data cover both the swash and surf zones. Singular Spectrum Analysis (SSA) and Multi-channel Singular Spectrum analysis (MSSA) methods are applied, on the shoreface, to three potential forcings: the monthly wave heights, the monthly mean water levels and the large scale atmospheric index known as the North Atlantic Oscillation. The patterns within these forcings are compared to the local bathymetric patterns; it is found that the patterns extracted using SSA and MSSA agree well with previous patterns identified using wavelets and confirm the highly nonstationary behaviour of beach levels at Duck. This is followed by analysis of potential correlations between the local bathymetry (at the two transects) and hydrodynamic and atmospheric patterns. The study is then extended to all measured bathymetric profiles, covering an area of 1100 m (alongshore) by 440 m (cross-shore). MSSA showed no collective inter-annual patterns of oscillations present in the bathymetry and the three potential forcings. Annual and semi-annual cycles within the bathymetry are found to be strongly correlated with the monthly wave height, in agreement with the SSA findings. Other collective intraannual cycles besides the semi-annual were identified; they were all correlated with the North Atlantic Oscillation.
This paper presents a novel data-driven methodology based on empirical orthogonal teleconnections... more This paper presents a novel data-driven methodology based on empirical orthogonal teleconnections (EOTs) to analyse and forecast the evolution of coastal navigational channels near the mouth of the Exe estuary, UK. This is the first time EOTs are used in coastal morphodynamics. Therefore, particular emphasis is placed on the comparison of EOTs with the well established empirical orthogonal functions (EOFs) method. EOTs and EOFs are used with a series of 14 surveys, taken approximately every 8 months, covering the period between January 2001 and February 2010. The skill of the methods in producing accurate bathymetric one-step forecasts for February 2010 is analyzed and compared with one-step forecasts based on the raw data. It is found that, provided the order of the autoregressive forecast method is chosen appropriately, EOTs and EOFs are better than the raw data and EOTs outperforms than EOFs. This is attributed to the fact that EOTs, without the orthonormality restriction for the temporal eigenfunctions required in EOFs, capture the temporal patterns within the data more accurately than EOFs.
This paper reports progress made in modifying and applying the X-Beach code to predict and explai... more This paper reports progress made in modifying and applying the X-Beach code to predict and explain the observed behaviour of coarse grained beaches. In a previous study a comparison of beach profile evolution measured during large scale experiments under constant water level with numerical model simulations was made. This placed particular emphasis on the tendency for onshore transport and profile steepening during calm conditions . The present paper extends that investigation to study the influence of the advection of surf processes induced by tidal water level variations effects, on gravel beach profile evolution. The parameter values and numerical model used in the simulation is similar to that presented previously. It is assumed that, to good approximation, the groundwater interface inside the beach follows the tidally modulated water level. The results obtained from the model shows that the model provides reasonable simulations of beach profile change in a tidal environment. In comparison with simulations under stationary water levels, a larger berm is produced in agreement with literature. Finally, good agreement is obtained between the model simulations and an example of field observations from a beach at Milford on Sea, UK. Further developments are outlined for future work.
Coarse-grained beaches are particularly prevalent in the UK, composed of accumulations of either ... more Coarse-grained beaches are particularly prevalent in the UK, composed of accumulations of either gravel, or mixed sand and gravel sediments. Understanding and predicting their morphological behaviour in response to short-term and long-term forcing has been the subject of recent research. Despite the focus on sandy beaches, it is important to understand that the balance of processes that govern different behaviour between sandy and gravel beaches. In this study we show how a public domain numerical model, XBeach, developed for sandy environments ) can be modified for use in predicting the cross-shore profile changes of gravel beaches. Improvements investigated here include: use of Lagrangian interpretation of velocity in place of Eulerian for driving sediment movement; incorporation of Packwood's (1983) pragmatic model of infiltration in the unsaturated area of the swash region; introducing of new morphological module based upon Soulsby's (1997) sediment transport equation for waves and currents. These changes are suggested in order to significantly improve the application of this model to gravel beaches, especially with regard to swash velocity asymmetry which is responsible for development of the steep accretionary phase steep berm above waterline. The results from the model agree well with the measured experimental data and improve upon the results presented by . Figure 1: Accretionary berm on coarse-grained beach (Milford-On-Sea, UK).
Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly... more Local bathymetric, quasi-periodic patterns of oscillation are identified from 26 years of monthly profile surveys taken at two shore-perpendicular transects at Duck, North Carolina, USA. The data cover both the swash and surf zones. Singular Spectrum Analysis (SSA) and Multi-channel Singular Spectrum analysis (MSSA) methods are applied, on the shoreface, to three potential forcings: the monthly wave heights, the monthly mean water levels and the large scale atmospheric index known as the North Atlantic Oscillation. The patterns within these forcings are compared to the local bathymetric patterns; it is found that the patterns extracted using SSA and MSSA agree well with previous patterns identified using wavelets and confirm the highly nonstationary behaviour of beach levels at Duck. This is followed by analysis of potential correlations between the local bathymetry (at the two transects) and hydrodynamic and atmospheric patterns. The study is then extended to all measured bathymetric profiles, covering an area of 1100 m (alongshore) by 440 m (cross-shore). MSSA showed no collective inter-annual patterns of oscillations present in the bathymetry and the three potential forcings. Annual and semi-annual cycles within the bathymetry are found to be strongly correlated with the monthly wave height, in agreement with the SSA findings. Other collective intraannual cycles besides the semi-annual were identified; they were all correlated with the North Atlantic Oscillation.
The chemical field around swimming plankton depends on the swimming style and speed of the organi... more The chemical field around swimming plankton depends on the swimming style and speed of the organism and the processes affecting uptake or exudation of chemicals by the organism. Here, we present a simple model for the flow field around a neutrally buoyant self-propelled organism at low Reynolds number and numerically calculate the chemical field around the organism. We show how the concentration field close to the organism and the mass-transfer rates vary with the swimming speed and style for Dirichlet (diffusion-limited transport) boundary conditions. We calculate how the length of the chemical wake, defined as being the distance at which the chemical field drops to 10% of the surface concentration of the organism when stationary, varies with the swimming speed and style for both Dirichlet and Neumann ( production limited) boundary conditions. For Dirichlet boundary conditions, the length of the chemical wake increases with increasing swimming speed, and the self-propelled organism displays a significantly longer wake than the towed-body model. For Neumann boundary conditions, the converse is true; because swimming enhances the transport of the chemical away from the organism, the surface concentration of chemical is reduced and thus the wake length is reduced.
In this paper a generic methodology is presented that allows the impacts of climate change on wav... more In this paper a generic methodology is presented that allows the impacts of climate change on wave energy generation from a wave energy converter (WEC) to be quantified. The methodology is illustrated by application to the Wave Hub site off the coast of Cornwall, UK. Control and future wave climates were derived using wind fields output from a set of climate change experiments. Control wave conditions were generated from wind data between 1961 and 2000. Future wave conditions were generated using two IPCC wind scenarios from 2061 to 2100, corresponding to intermediate and low greenhouse gas emissions (IPCC scenarios A1B and B1 respectively). The quantitative comparison between future scenarios and the control condition shows that the available wave power will increase by 2–3% in the A1B scenario. In contrast, the available wave power in the B1 scenario will decrease by 1–3%, suggesting, somewhat paradoxically, that efforts to reduce greenhouse gas emissions may reduce the wave energy resource. Meanwhile, the WEC energy will yield decrease by 2–3% in both A1B and B1 scenarios, which is mainly due to the relatively low efficiency of energy extraction from steeper waves by the specific WEC considered. Although those changes are relatively small compared to the natural variability, they may have significance when considered over the lifetime of a wave energy farm. Analysis of downtime under low and high thresholds suggests that the distribution of wave heights at the Wave Hub will have a wider spread due to the impacts of climate change, resulting in longer periods of generation loss. Conversely, the estimation of future changes in joint wave height-period distribution provides indications on how the response and power matrices of WECs could be modified in order to maintain or improve energy extraction in the future.► A generic methodology for assessing the impacts of climate change on wave energy generation is presented. ► The methodology is illustrated by application to the Wave Hub site off the coast of Cornwall, UK. ► Comparison of future scenarios and control conditions shows statistically significant changes. ► Changes are small in comparison to natural variability, but have significance over scheme lifetime. ► Analysis of downtime suggests that climate change will result in longer periods of generation loss.
A data-driven model has been developed to analyse the long-term evolution of a sandbank system an... more A data-driven model has been developed to analyse the long-term evolution of a sandbank system and to make ensemble predictions in a period of 8 years. The method uses a combination of empirical orthogonal function (EOF) analysis, (to define spatial and temporal patterns of variability), jack-knife resampling, (to generate an ensemble of EOFs), a causal auto-regression technique, (to extrapolate the temporal eigenfunctions), and straightforward statistical analysis of the resulting ensemble of predictions to determine a ‘forecast’ and associated uncertainty. The methodology has been applied to a very demanding site which includes a curved shoreline and a group of mobile nearshore sandbanks. The site is on the eastern coast of the UK and includes the Great Yarmouth sandbanks and neighbouring shoreline. A sequence of 33 high quality historical survey charts reaching back to 1848 have been used to analyse the patterns and to predict morphological evolution of the sandbank system. The forecasts demonstrate an improved skill relative to an assumption of persistence, but suffer in locations where there are propagating features in the morphology that are not well-described by EOFs.
Wave-induced, steep vortex ripples are ubiquitous features in shallow coastal seas and it is ther... more Wave-induced, steep vortex ripples are ubiquitous features in shallow coastal seas and it is therefore important to fully understand and model the sediment transport processes that occur over them. To this end, two two-dimensional vertical (2DV) models have been critically tested against detailed velocity and sediment concentration measurements above mobile ripples in regular asymmetric oscillatory flow. The two models are a k-o turbulence-closure model and a discrete-vortex, particle-tracking (DVPT) model, while the data are obtained in the Aberdeen oscillatory flow tunnel (AOFT). The models and the data demonstrate that the time-dependent velocity and suspended sediment concentration above the ripple are dominated by the generation of lee-side vortices and their subsequent ejection at flow reversal. The DVPT model predicts the positions and strengths of the vortices reasonably well, but tends to overpredict the velocity close to the ripple surface. The k-o model, on the other hand, underpredicts the height to which the vortices are lifted, but is better able to predict the velocity close to the bed. In terms of the cycle-and ripple-averaged horizontal velocity, both models are able to reproduce the observed offshore flow close to and below the ripple crest and the DVPT model is able to produce the onshore flow higher up. In the vicinity of the vortices, the DVPT model better represents the concentration (because of its better prediction of vorticity). The k-o model, on the other hand, better represents the concentration close to the ripple surface and higher up in the flow (because of the better representation of the near-bed flow and background turbulence). The measured and predicted cycle-and ripple-averaged suspended sediment concentrations are in reasonable agreement and demonstrate the expected region of exponential decay. The models are able to reproduce the observed offshore cycle-and ripple-averaged suspended sediment flux from the ripple troughs upwards, and as a result, produce net offshore suspended sediment transport rates that are in reasonable agreement. The net measured offshore suspended transport rate, based on the integration of fluxes, was found to be consistent with the total net offshore transport measured in the tunnel as a whole once the onshore transport resulting from ripple migration was taken into account, as would be expected. This demonstrates the importance of models being able to predict ripple-migration rates. However, at present neither of the models is able to do so. r
Objective: To formulate a mathematical model of odontogenic cyst growth and establish the dynamic... more Objective: To formulate a mathematical model of odontogenic cyst growth and establish the dynamics of cyst enlargement and the role of osmotic pressure forces throughout its growth.
We present results for the average mass transfer to a spherical squirmer, a model micro-organism ... more We present results for the average mass transfer to a spherical squirmer, a model micro-organism whose surface oscillates tangentially to itself. The surface motion drives a low-Reynolds-number flow which enables the squirmer either to swim relative to the fluid at infinity, at an average speed proportional to a streaming parameter, W , or to stir the fluid around it while remaining, on average, at rest (if W = 0), as represented by a hovering parameter, b. We assume that the amplitude of the time-periodic surface distortions is scaled by a dimensionless small parameter , and consider only high Péclet numbers P -a measure of convection versus diffusion -by setting P −1 = 2 γ , where γ is a parameter of O(1). It is shown that the average mass concentration distribution satisfies a steady convection-diffusion equation with an effective velocity field that is different from the actual mean velocity field. The model is used to calculate the mass transfer across the surface of the squirmer, measured by the mean Sherwood number Sh.
Quarterly Journal of Mechanics and Applied Mathematics, 2003
In this paper we study nutrient uptake by a very simple model of a swimming microorganism, a sphe... more In this paper we study nutrient uptake by a very simple model of a swimming microorganism, a sphere moving its surface tangentially to itself with constant concentration on the surface. The effect of its swimming motions on the concentration field and uptake is investigated. We find the relationship between the Sherwood number (Sh), a measure of the mass transfer across the surface, and the Péclet number (Pe), which indicates the relative effect of convection versus diffusion. Then we compare the results with those for a rigid sphere moving at the same speed under the action of an external force.
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