Hydrodynamic modelling of sediment transport and bedform formation on the NW Irish shelf

Abstract

Much of our knowledge of sediment transport on the northwest Irish shelf was hypothesised in the 1970's with limited resources and without the benefit of modern seafloor mapping equipment and techniques (Kenyon and Stride, 1970). Utilising existing and newly generated data sets, this research focuses on modern sediments located on the north Irish shelf between 54° - 56°N the aim of which is to establish connections between bedform morphology, sediment distribution and modelled hydrodynamics in order to describe sediment transport processes from a shelf to bedform scale.

Interrogation of multibeam echosounder data has enabled classification of 8 depositional and 2 erosional bedform types (Evans et al., 2015). Many of these bedforms overlap those glacial formations identified in previous research (Benetti et al., 2010b, Dunlop et all., 2011, O'Cofaigh et al., 2012).

Classification of back scatter data has identified 5 sedimentary classes across the shelf, with gravel dominating the inshore areas and large soft sediment deposits concentrated in proximity to moraine features and areas of bathymetric lows.

These mapping techniques applied alongside outputs from a sediment transport specific, hydrodynamic model created as part of this research, have led to a number of novel findings and challenges to existing research.

The location of bedload parting zones presented by (Belderson et al., 1982) are challenged and a new location suggested based on bedform distribution, bedform geometry and modelled current data.

It is also proposed that bedforms south of the Malin Deep in 130 m water depth, previously described as ‘active’ by (Dunlop et al., 2011) , are relict in nature due to insufficient flow velocities in the area to mobilise the sediments of which they are comprised (Terwindt and Brouwer, 1986).

It is hypothesised that a change in orientation of identified sediment lineations is a sedimentary record of the point of incursion onto the Irish shelf by the European Slope Current (ESC), a longstanding oceanographic feature (Fernand et al., 2006, White and Bowyer, 1997). This change of orientation occurs ~55º30’N and correlates with literature on the ESC, model and oceanographic data (Souza et al., 2001).

On a bedform scale, repeat bathymetric surveys have revealed a wide range of sediment wave migration rates. Utilising a new ESRI Arc GIS based technique, these rates were accurately measured across the shelf with highest rates of ~ 8 m yr-1 displayed by giant sandwaves 14 km north of Malin Head. Through analysis of repeat survey data, these same waves display a rotary migration pattern. Located on an offshore tidal sandbank (Caston, 1972), sandwaves rotate around a central point, driven by currents connected to a nearby tidal sound (Sanay et al., 2007). It is hypothesised that these currents are the primary means of soft sediment retention in an otherwise gravel dominated area of shelf (Williams et al., 2000, Zhu and Chang, 2000).

The multidisciplinary approach adopted by this research, including bathymetric, backscatter, sedimentary, seismic and hydrodynamic model data, reveals new approaches to examining and insights into the sediment transport processes on the northwest Irish shelf.

Date of Award	May 2018
Original language	English
Sponsors	Department of Education and Learning
Supervisor	Derek Jackson (Supervisor) & Sara Benetti (Supervisor)