Davies, G.

Hashemi, Reza

Neill, Simon

English

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Conference Paper, Published VersionHashemi, Reza; Neill, Simon; Davies, G.A numerical study of wave and current fields aroundRamsey Island - tidal energy resource assessmentZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:TELEMAC-MASCARET Core GroupVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/104297Vorgeschlagene Zitierweise/Suggested citation:Hashemi, Reza; Neill, Simon; Davies, G. (2012): A numerical study of wave and currentfields around Ramsey Island - tidal energy resource assessment. In: Bourban, Sébastien;Durand, Noémie; Hervouet, Jean-Michel (Hg.): Proceedings of the XIXth TELEMAC-MASCARET UserConference 2012, 18 to 19 October 2012, St Hugh's College,Oxford. Oxfordshire: HR Wallingford. S. 135-141.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.  A numerical study of wave and current fields around Ramsey Island - tidal energy resource assessment  M. Reza Hashemi Centre for applied marine sciences School of Ocean Sciences, Bangor University Bangor, UK; Dept. of Water Engineering, Shiraz University r.hashemi@bangor.ac.uk  Simon P. Neill,  G. Davies Centre for applied marine sciences School of Ocean Sciences, Bangor University Bangor, UK    Abstract—In the Irish Sea, the best marine renewable energy sites are subjected to strong currents and exposed to relatively large waves (e.g. around Ramsey island and in the Bristol Channel).  The objective of this research is to present the potential of the TELEMAC modelling system in various aspects of marine renewable energy studies such as multi-scale modelling, and wave-tide interaction. Firstly, an idealised triangular domain was modelled to study the impact of tides on quantifying the wave power resource. The overall dimensions of this case study resemble those of the Bristol Channel.  The results of the idealised case study demonstrate that ignoring the tides when estimating wave power generates considerable errors, since wave power is related to significant wave height squared. Next, a multi-scale unstructured mesh model of the Irish Sea was developed using TELEMAC. Spatial and temporal variations of the currents around Ramsey Sound were captured using a relatively fine mesh (~40m).  Despite the strong current field and complex bathymetry of this region, the multi-scale tidal model led to convincing results. Some recommendations for research and priorities for data collection have been identified. I. INTRODUCTION A key step towards the successful deployment of marine energy devices is site selection. Assessment of the physical, environmental and socioeconomic constraints at a specific site is highly dependent on a proper understanding of the current (e.g. resource assessment for tidal energy devices) and wave (e.g. cost due to wave loads on tidal energy devices, or wave power estimation) fields.  Due to consistently high current speeds, Ramsey Sound in the southern Irish Sea has attracted both academic and business interest as a suitable site for tidal energy extraction.  Hence, companies are planning to install tidal energy devices here as pilot studies.  Further, the Bristol Channel is one of the most energetic sites in the Irish Sea for tidal and wave energy devices. Numerical simulations of the hydrodynamics of these areas is a challenging task, as a result of the highly irregular bathymetry, strong tides and relatively energetic wave climate. Additionally, in most studies relating to  marine renewable energy, waves and currents have been treated separately.  In the most recent version of TELEMAC (i.e. 6.1), it has been possible to develop current and wave models of a region based on the same mesh and boundary conditions, including one-way or two-way coupling  of those models. In the present research, the TELEMAC modelling system has been used to study tides, waves, and their interactions, in terms of estimating renewable energy resources.  An idealised triangular case along with a multi-scale Irish Sea model has been developed and discussed to address this aim. II. STUDY AREA The UK is located in a highly energetic environment in terms of marine renewable energy resources. In some areas like the Bristol Channel and around Ramsey Island, many studies have assessed the wave and tidal energy resources. The Bristol Channel is exposed to long fetch lengths from the Atlantic Ocean, potentially exceeding 1000 km.  Hence, it is considered as one of the potential places for wave energy extraction in the UK. Further, with a mean spring tidal range of 12.2 m, and a mean neap range of 6.0 m at Avonmouth, the Bristol Channel has one of the highest tidal energy resources in the world.  Depths in the approaches to the Bristol Channel are around 50 m. Ramsey Island is about 1 km off the coast of the St David's peninsula in Pembrokeshire in southwest Wales. Tidal currents are very strong particularly in the channel located at the east side of the island (i.e., Ramsey Sound).  III. TELEMAC-2D - TOMAWAC MODELLING APPROACH TELEMAC-2D and TOMAWAC were used to model tides, waves and their interaction [1]. TELEMAC-2D is a well-known and popular model which solves the 2-D shallow water equations using FEM. One of the attractive features of this model is its unstructured discretization.  This makes it suitable for multi-scale problems without resort to nesting. As an example, to model the flow and waves around Ramsey Island a multi-scale Irish Sea model was developed in the present work. A mesh size of 7000 m was used around the southern boundary (Celtic Sea), while a much finer 40 m mesh was used to resolve the bathymetry and capture the flow field inside Ramsey Sound. 135XIXth TELEMAC-MASCARET User Conference Oxford, UK, October 18–19, 2012    TABLE I.  ESTIMATED ERROR IN THE WAVE POWER ESTIMATION DUE TO THE TIDE AT POINT A FOR FULLY COUPLED MODELS (FIG. 1)  Percentage error a*=2 a=3  Hmo Wave Power Hmo Wave Power Average 3.0 6.0 4.7 9.2 Max. 6.7 11.9 9.8 16.2 * The tidal amplitude at the left open boundary  2) Other considerations: Apart from estimating wave power, other parameters which have a primary role in the design of marine renewable energy schemes can be affected by wave-tidal interactions.  Accordingly, the extreme wave loads have traditionally been used for the structural design of nearshore or offshore structures.  Hence, underestimating wave height due to ignoring wave-tidal effects can increase the risk of damage to a tidal energy device. As a final discussion point for this idealised case, the effect of waves on tidal currents is briefly demonstrated.  The predicted current fields with and without inclusion of the wave forces have been subtracted and plotted in Fig. 8 to represent a typical time snapshot. Also, the corresponding wave forces are presented in Fig. 9. As expected, the wave-generated currents are more noticeable in the nearshore zone. This can lead to 0.40 m/s error, which is about 20% in    Figure 8.  The difference of current speed with and without the inclusion of wave forces at t=14h00.  Figure 9.  Wave forces at t=14h00. the prediction of tidal currents. There is a clear association between this difference and wave generated forces. B. Irish Sea/Ramsey Sound Model The FEM mesh of the Irish Sea model is presented in Fig. 10. The mesh size is about 7 km near the southern boundary in the Celtic Sea, and gradually approaches 40 m around Ramsey Sound (Figs. 11 and 12).   Figure 10.  Bathymetry and FEM mesh of the Irish sea TELEMAC model. The model was forced by tidal water elevations at the open boundaries. The tidal constituents at the open boundaries were interpolated from an analysis of POLCOMS shelf scale model output.  Model results at different tidal gauge locations in the Bristol Channel and Liverpool bay compared well with observed data (not presented here). ADCP data collected during the spring tide of May-2011 (14-20 May 2011) were used to evaluate the simulated currents around Ramsey Island.  As an example, Fig. 12 and Fig. 13 demonstrate the flow pattern on 15-May-2011 at 11:30, along with measured ADCP data. As can be seen, the spatial variations of the currents in this region are extensive, and the model and observations have a convincing agreement.  The current speed approaches 3.5 m/s inside Ramsey sound during this period. A TOMAWAC wave model of the same area was developed.  Since the majority of waves in the Irish Sea are generated by winds emanating from the southwest [7], the 139XIXth TELEMAC-MASCARET User Conference Oxford, UK, October 18–19, 2012    distribution of the significant wave height for a typical southwesterly wind speed of 20 m/s is plotted in Fig. 14.   Figure 11.  A closer view of the variable mesh density with very fine mesh around Ramsey Island.  Figure 12.  A typical flow pattern in the vicinity of Ramsey Island at 15-May-2011 11:30 PM compared with ADCP data (measured data were collected by Cardiff University). It is clear that Ramsey Sound is sheltered from this typical southwesterly wind event.  However, the wave heights to the west of Ramsey Island reach up to 7 m for this event.  This suggests that running a coupled wave current model is more essential to the west of the island, while the wave energy inside Ramsey sound is not that significant. Further, the model results could be improved, particularly by improving the bathymetry near the coastline and intertidal zone. To the best of our knowledge, no wave buoy data are available around Ramsey Island. However, wave data exist for Scarweather WaveNet Site, which is located at the middle of the Bristol Channel. The effect of the inclusion of tides on estimated wave heights during an event in October-2005 is depicted in Fig. 15. Fluctuations of the wave height due to the tide are present in both the measurement  and the coupled wave current model. However, a pure wave model cannot capture these fluctuations. Further work is underway to improve the wave-current model and assessment of wave energy resources using this coupled model. V. CONCLUSIONS AND RECOMMENDATIONS Our study of wave-tidal interactions in an idealised triangular domain suggest that assessment of the wave energy resource, without considering the effect of tides, could lead to errors of up to 10% in high tidal range areas like the Bristol Channel. Further research is underway to investigate this for the actual bathymetry of the Irish Sea.  The errors associated with estimating wave power are almost twice those errors in simulating the significant wave height.  Additionally, for the safe design of offshore or nearshore tidal energy devices, the effect of tides on modulating the wave heights should be considered.  Figure 13.  A closer view of flow around Ramsey Island (see also Fig. 12). 140XIXth TELEMAC-MASCARET User Conference Oxford, UK, October 18–19, 2012     Figure 14.  The distribution of the significant wave height around Ramsey Island for a typical southwesterly wind of 20 m/s simulated by TOMAWAC.  Figure 15.  Effect of including the tide on wave prediction at a Scarweather WaveNet Site, Bristol Channel (51°26'.00N and 003°55'.99W), 17-24 October 2005. The primary hydrodynamic model of Ramsey Sound demonstrated the suitability of TELEMAC for multi-scale modelling of waves and currents in highly energetic environments with complex bathymetry. The resolution of the bathymetry, especially near coasts and in the intertidal zone, is the key factor for improving the model results. Ramsey Sound is sheltered from southwesterly wind waves, while the interaction of waves and tides to the west and southwest of Ramsey Island is important, and can be simulated by models like TELEMAC. ACKNOWLEDGEMENT Thanks to Marine Digimap for supplying the bathymetry data, and LCRI-Marine (Cardiff) for ADCP data. Wave data were supplied by the British Oceanographic Data Centre (BODC). This work was undertaken as part of the SEACAMS project, which is part-funded by the EU’s Convergence European Regional Development Fund through the Welsh Government. REFERENCES [1] The TELEMAC Modelling System, www.opentelemac.org [2] J.M. Brown and A.G. Davies, “Methods for medium-term prediction of the net sediment transport by waves and currents in complex coastal regions”, CONT. SHELF RES., vol. 29, pp 1502-1514,  2009.  [3]  N. Guillou and G. Chapalain, “Modeling the tide-induced modulation  of wave height in the outer Seine estuary”, J COASTAL RES, vol. 28, pp. 213-223,  2012 [4] C C Mei, The Applied Dynamics Of Ocean Surface Waves, Advanced series on Ocean Engineering, vol 1, World Scientific, 2003.  [5] US Army Corps of Engineers, Coastal Engineering Manual - Part II,  2008. [6] A.S. Bahaj, “Generating electricity from the oceans”. RENEW. SUST. ENERG.  REV., vol. 15, pp. 3399-3416,   2011. [7] S.P. Neill, J.D. Scourse, G.R. Bigg  and K. Uehara “ Changes in wave climate over the northwest European shelf seas during the last 12,000 years”, J. GEOPHYS. RES., vol. 114, C06015, 2009.     141

A numerical study of wave and current fields around Ramsey Island - tidal energy resource assessment

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A numerical study of wave and current fields around Ramsey Island - tidal energy resource assessment

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