Geophysical and Geological Assessment of Offshore Sediment Banks in the South Western Irish Sea

To date, Ireland’s only operational offshore wind farm project is located on Arklow Bank; a sediment bank situated nearshore in shallow (up to 2mbsl) waters. Such bank structures are preferable for wind farm development due to their competent sediment composition, shallow waters and proximity to shore for cable routing. However, as proven at Arklow Bank, issues of scour and sediment mobility around fixed foundations and cabling can compromise infrastructure stability. These issues highlight the need for adequate ground model assessments of such banks to aid planning, design, construction and maintenance phases of wind farm development.

The southern Irish Sea is characterised by a series of such NNE-SSW trending sediment banks, many of which are earmarked for further wind farm developments. These coast-parallel bedforms are located approximately 10km off the south-east coast of Ireland, in circa 20-40m water depth and rise to only a couple of metres below sea level. They exert a strong control on the tidal flow pathways along the coast and offer coastal protection [1]. The banks themselves are quasi-stable in their own environment, influencing local hydro- and morphodynamics in terms of sediment waves. For offshore sediment banks to develop two fundamental conditions must be present: (1) an adequate source of sediment and; (2) a hydrodynamic regime capable of moving sediment [2]. European continental shelf sediment bank origins generally fall into two broad categories [2]; a) those formed under present day hydrodynamic and sediment conditions, or b) relict features created during post LGM transgression during periods of rapid sea level rise and stronger tidal current velocities. Paleo-tidal models have been used to reconstruct post-glacial hydrodynamic conditions in support of the proposed view of a glaciomarine environment genesis of these large bedforms in the Irish Sea [3].

This study will use previously collected high resolution multi-beam echo sounder (MBES) data from the Integrated Mapping for the Sustainable Development of Ireland’s Marine Resource (INFOMAR) project in conjunction with newly obtained MBES, sub-bottom profiler, grab sample and vibro-core data to characterise; a) the stratigraphy of this highly dynamic and geologically complex region and b) sediment mobility on and around the sediment banks including their stability in the current hydrodynamic regime. Furthermore, this study will use this data to attempt to elucidate the origin of these sediment banks and their evolution through geological time. Data processed and analysed from this study will be used as an input and as validation for a coupled hydrodynamic, spectral wave and sediment transport 2D numerical model developed using MIKE 21 software.

 

References

1. Williams, J.J., MacDonald, N.J., O’Connor, B.A., Pan, S., 2000. Offshore sand bank dynamics. Journal of Marine Systems, 24, 153-173.
2. Dyer, K.R., Huntley, D.A., 1999. The origin, classification and modelling of sandbanks and ridges. Continental Shelf Research, 19, 1285–1330.
3. Uehara., K., Scourse, J.D., Horsburgh, K.J, Lambeck, K., Purcell, A.P., 2006. Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present. Journal of Geophysical Research, 111(9).