Journal of Maps Student Edition, 2007, 14-22
Wave-Sediment Interactions on an High Energy Beach System EMILIA GUISADO PINTADO Area of Physical Geography, University of Pablo de Olavide, Ctra. Utrera, Km.1, 41013, Seville, SPAIN;
[email protected] (Received 18th February 2007; Accepted 28th May 2007)
Abstract: The North coast of Northern Ireland is exposed to high-energy swells and frequent storm events and therefore beach systems found along this coast tend largely to be dissipative in character. Bedrock-framed coastlines formed by basalt cliffs, shore platforms and smaller sandy embayments dominate this study area. West Strand beach at Portrush (located 55◦ 12’N/ 6◦ 40’W) is a high-energy system shaped by a headland-embayment with a sandy beach to the north and boulder beach to the south. It is characterised by a microtidal regime and predominant local winds are west and south westerly. The research work aims to examine this high-energy beach under multiple storm scenarios and to investigate hydrodynamic and morphodynamic behaviour at the site. For the analysis, a wave propagation model was used to compare wave energy dissipation from storm wave events as well as examining currents and sediment transport through the analysis of the spatial distribution of radiation stress. Real data from deep-water wave parameters (significant wave height, period and direction), wind direction and speed, and recent high-resolution bathymetry data were used as an input to the propagation model. The methodology was applied to West Strand beach (9 km2 ) at a scale of 1:250.
14 ISSN 1755-2958 http://www.journalofmaps.com
Journal of Maps Student Edition, 2007, 14-22
1.
Pintado, E.G.
Introduction
Beaches along the north coast of Ireland are located in headland-embayment settings (Carter, 1991) and contain a finite sediment volume derived from reworking of glacial sediments during the Holocene (Cooper et al., 2002). There is no contemporary terrigenous sediment supply. In the absence of human interference, these beaches typically show relative stability in shoreline position or slow rates of erosion over the historical period related to slow leakage of sediments from the sedimentary cells (Carter, 1991). The beach at Portrush West Strand is somewhat anomalous in this context because historically it was consistently backed by a prominent eroded dune scarp within which thick and extensive peat horizons are exposed. A seawall was constructed in the 1960s to defend the eroding dunes and since that time the beach has been reduced in width and elevation (Figure 1). This paper examines the reasons for this regionally anomalous behaviour throught the study of the effects of storm events along the shoreline.
Figure 1. Evolution of west Strand Beach. (CARTER, R.W.G. (1991) Shifting sands. A study of the coast of Northern Ireland from Magilligan to Larne. HMSO, Belfast) Photographs show the evolution of the beach from 1900 to 1990. (A) The beach was wide with a back dune. (B) In the 60s the seawall was constructed and beach decreased in width, the back dune was reduced in height and width. (C) The 1990 photograph show the current situation characterise by a narrow beach and a back promenade.
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Pintado, E.G.
Evironmental Setting
The North West coast of Ireland is exposed to the North Atlantic Ocean at latitudes between 52◦ and 55◦ North. Much of the coast is formed of late-Mesozoic (65 to 130 m.a) and Tertiary (25 to 65 m.a) rocks, overlain by a veneer of more recent (last 40.000 years) glacial deposits (Carter, 1991). West Strand Beach is a bay located at 55◦ 12’N/ 6◦ 40’W on the coast of Portrush, County Antrim. A bedrock-framed coastline formed by basalt cliffs, shore platforms and smaller sandy embayments dominate the study area. The orientation of the platforms is mainly NNW co-inciding with maximum wave approach and energy levels, which has helped to develop the local shore platform. The Bay can be categorised as a high-energy system dominated, along with the rest of the west coast, by long-period swells and storm waves. Swell waves from the Atlantic have a modal approach direction of around 280◦ when travelling through the Malin Sea. As a consequence the beach presents a dissipativeintermediate profile (Wright and Short, 1984). The tidal range is micro-tidal with a spring tidal range about of 1.7m. Local wind directions affecting the wave climate are typically from the west and the southwest with an average wave height of 1.22m. The coastline platform comprises a sandy bay confined between a rocky headland on the north side of the bay and basalt cliffs along the southern side of the beach.
2.
Methods
2.1
Numerical wave model
In order to investigate the contemporary morphodynamics of the beach, a series of wave simulations were run as an aid to interpreting beach behaviour. The numerical wave model, Simulating Waves Nearshore (SWAN) (Booij et al., 1996) was used to simulate the generation and propagation of wave conditions into shallow water. Input data for the numerical wave propagation model was topography and bathymetry of West Strand Bay, initial wave geometry (deepwater waves), and wind speed and directions. Wave and wind parameters were all sourced from the United Kingdom Meteorological Office wave model, as indicated below. 16
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The wave propagation model was used for the general understanding of the currents and sediment transport due to wave radiation stress as defined by Longuet-Higgins and Stewart (1964) and Longuet-Higgins (1970). Radiation stress vectors measure the excess flux of momentum due to the presence of the waves and is responsible for longshore and nearshore current generation (Komar, 1976). The analysis of the wave-induced stress provides a general approximation to long-shore drift, onshore currents and potential sediment transport in the bay.
2.2
Bathymetry
Bathymetry surveys were carried out in West Strand Bay using a MIDAS Surveyor GPS Echo-sounder. The system is supplied with a 210 kHz transducer (single beam) with an integral Differential Global Position System (DGPS) receiver that allows recording and displaying of position and depth information in WGS84 and local grids (Irish Grid). The MIDAS surveyor was used to survey eight profiles from 1m to almost 20 m depths across the bay. These surveys were developed to achieve a broad bathymetric grid of West Strand Beach (Figure 2). The surveys were completed during March 2005. All the bathymetric measurements were tidally corrected from Ordnance Datum Belfast (O.D.B) using the Portrush tide data maintained by the British Oceanographic Data Centre. All coordinate points were then normalized and referenced to the Irish National Grid. Offshore bathymetry, were digitized using a published Admiralty bathymetry Chart, number 2499, of the area published in 1986.
2.3
DGPS topographic profiles along the beach
A topographic survey was undertaken out using a Trimble 4400 Ssi DGPS achieving relative sub-centimetres accuracy. Topographic values were measured using ten profiles along the beach with the aim of surveying the inter-tidal zone of the Bay (from 4m height to 0.8m depth). All the co-ordinates were subsequently referenced to Irish National Grid and elevations were reduced to meters using O.D.B. The DGPS data was appended to the nearshore bathymetry data (after correction) from the echo sounder and the digitized bathymetry data from Admiralty Charts. 17
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Figure 2. Bathymetry map of West Stand Beach, Portrush, Northern Ireland. Shoreline and land is represented with grey colour. Depths are represented by a palette of blue and green colours. Note that coordinates are Irish Grid.
All data was compiled and organised in a grid for further use in the wave propagation model SWAN using a kriging interpolation method.
2.4
Wave and wind data analysis
The climate conditions were taken from the United Kingdom Meteorological Office wave model. The UK waters model covers from 12◦ W, between 48◦ N and 63◦ N at a resolution of 1/9 longitude by 1/6 latitude. This wave model collects data from ships, buoys, platforms and land stations about surface wind, surface pressure, wind speed and direction. Sea condition parameters such as wind speed (knots) and direction (degrees), resultant wave height (meters), resultant wave period (seconds) and resultant wave direction (degrees) were provided for a 5 year period. The analysis identified seven major storms from the following directions: East, Northeast, North, North-North-West, Northwest, West-North-West and West. For comparison purposes, modal conditions for the site were also defined (Table 1).
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CHARACTERISTICS OF THE MAIN STORM EVENTS Storm Direction Modal E NE N NNW NW WNW W
Wind Speed (m/s) 14 31 28.3 26 28.3 33.2 32.6 33
Wind Direction (degrees) 170 90 57 12 350 315 330 270
Hs (metres) 1.35 3 2.9 3.1 3.6 4 5.3 5.2
Wave Period (seconds) 5.44 6 6.5 6.85 7 7.6 8.4 8.35
Wave Direction (degrees) 10 85 65 9 360-0 337 360 350
Event Duration (hours) – 39 15 30 30 12 42 45
Table 1. Analysis and definition of main storm events in West Strand beach. Direction (degrees), speed (m/s), significant wave height (m), wave period (second) and duration of the storm event (hours) is presented fro each storm event defined.
3.
Results
For the analysis a TIN was created with the bathymetry data and outputs from SWAN were converted to vectors and overlaid to the 3D model. Results of the simulations were radiation stress values for the beach under West-north-westerly conditions, and these results were compared with modal conditions for the studied beach. The radiation stress gradients inside the embayment, particularly in the surf zone, generate nearshore currents. A gradual change in gradient is indicative of potential sediment transport; however, if the change in gradient is radical, potential erosion can be expected. The results of the simulations run on the beach show (Map 1) that for all conditions simulated, the net wave-induced stress in the bay is directed offshore, with a maximum value of 6 N/m2 . Only under high magnitude, north-westerly waves were the vectors directed onshore with a maximum value of 10 N/m2 . Under storm conditions, a potential longshore current with offshore direction is observed inside the embayment. This current has a south to north directional component and is located in the northern part of the embayment (284500 Easting/ 441300 Northing), characterised by an increment in magnitude of the vectors under WNW, NW and W storm conditions; it appears under all storm directions. Under modal condition, however, all the vectors are oriented offshore and grouped around the northeast part of the headland.
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Pintado, E.G.
Conclusions and Discussion
Results suggest that the embayment has been under erosion during the last century due to the construction of the seawall. Modal conditions, south and easterly winds represent the main wave/wind climate affecting this coastal environment. Under these conditions an offshore current develops. This suggests that the sediment is transported outside the embayment causing erosion along the nearshore. Under westerly and northerly storm conditions, when a nearshore current is generated, this leads to onshore sediment transport, and possible restoration of the embayment. Offshore currents and offshore sediment transport is interpreted under modal conditions and this may be one of the causes of the erosion that has been occurring during the last century. Concluding, the bay suffers erosion under modal and easterly conditions due to offshore sand movement. Recovery of the sand due to onshore movement of the sediments occurs under westerly and northerly storm conditions. Results achieved differ from observations elsewhere, where low energy swell is associated with onshore transport and storms are frequently associated with offshore sediment transfers.
Software The numerical computer model SWAN (Booij et al., 1996; SWAN, 2007) was used for modelling storm wave effects at West Strand beach. This model is based on the creation of different wave energy scenarios with the aim of analysing the spatial distribution of wave related hydrodynamic parameters (Malvarez et al., 2004). For the plotting and comparison of the results the surface mapping system, Surfer 8.0 (Golden Software) was used.
Acknowledgements I would like to thank my supervisors Dr. Derek Jackson, Dr. Andrew Cooper and Dr. Gonzalo Malvarez for their advice and guidance 20
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throughout the year. Gratitude is extended to Euan Dawson (CCMR, University of Ulster) for technically assistance and all the staff and students of the Centre for Coastal and Marine Research This research is part of a Master of Research Project and was undertaken with funding from the European Social Fund in collaboration with the University of Ulster.
References ADMIRALTY CHARTS (1986) Approaches to Londonderry and Coleraine, Ireland, North Coast, 2499. Scale 1:40 0000. BOOIJ, N., HOLTHUIJSEN, L. and RIS, R. (1996) The SWAN wave model for shallow water, In Proceedings of the 25th International Conference on Coastal Engineering, Orlando, USA, pp. 668–676. CARTER, R. W. G. (1991) Shifting sands. A study of the coast of Northern Ireland from Magilligan to Larne, HMSO, Belfast. COOPER, J. A. G., KELLEY, J. T., BELKNAP, D. F., QUINN, R. and MCKENNA, J. (2002) Inner shelf seismic stratigraphy off the north coast of Northern Ireland: new data on the depth of the Holocene lowstand, Marine Geology, 186, 369–387. KOMAR, P. D. (1976) Beach processes and sedimentation, Englewood Cliffs, New Jersey: Prentice Hall. LONGUET-HIGGINS, M. S. (1970) Longshore currents generated by obliquely incident sea waves, Journal of Geophysical Research, 75, 6778–680. LONGUET-HIGGINS, M. S. and STEWART, R. W. (1964) Radiation stress in water waves, a physical discussion with implications, Deep-Sea Research, 75, 6790–680. MALVAREZ, G., NAVAS, F. and JACKSON, D. W. T. (2004) Investigations on the morphodynamics of sandy tidal flats: a modelling application, Coastal Engineering, 51, 731–74. SWAN (2007) SWAN User Manual, versio 40.51, Department of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands. 21
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WRIGHT, L. D. and SHORT, A. D. (1984) Morphodynamic variability of surf zones and beaches: a synthesis, Marine Geology, 56, 93–118.
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