Development and Dynamics of Sediment Waves in a Complex Morphological and Tidal Dominant System: Southern Irish Sea

Abstract

With the recent push for a transition towards a climate-resilient economy, the demand on marine resources is accelerating. For many economic exploits, a comprehensive understanding of environmental parameters underpinning seabed morphodynamics in tidally-dominated shelf seas, and the relationship between local and regional scale sediment transport regimes as an entire system, is imperative. In this paper, high-resolution, time-lapse bathymetry datasets, hydrodynamic numerical modelling outputs and various theoretical parameters are used to describe the morphological characteristics of sediment waves and their spatio-temporal evolution in a hydrodynamically and morphodynamically complex region of the Irish Sea. Analysis reveals sediment waves in a range of sizes (height = 0.1 to 25.7 m, and wavelength = 17 to 983 m), occurring in water depths of 8.2 to 83 mLAT, and migrating at a rate of 1.1 to 79 m/yr. Combined with numerical modelling outputs, a strong divergence of sediment transport pathways from the previously understood predominantly southward flow in the south Irish Sea is revealed, both at offshore sand banks and independent sediment wave assemblages. This evidence supports the presence of a semi-closed circulatory hydrodynamic and sediment transport system at Arklow Bank (an open-shelf linear sand bank). Contrastingly, the Lucifer–Blackwater Bank complex and associated sediment waves are heavily influenced by the interaction between a dominant southward flow and a residual headland eddy, which also exerts a strong influence on the adjacent banner bank. Furthermore, a new sediment transfer system is defined for offshore independent sediment wave assemblages, whereby each sediment wave field is supported by circulatory residual current cells originating from offshore sand banks. These new data and results improve knowledge of seabed morphodynamics in tidally-dominated shelf seas, which has direct implications for offshore renewable developments and long-term marine spatial planning.

Morphological Modelling to Investigate the Role of External Sediment Sources and Wind and Wave-Induced Flow on Sand Bank Sustainability: An Arklow Bank Case Study

“…Residual transport vectors, derived from modelled accumulative total sediment transport and tidal current, show that on the eastern side of the bank, sink pathways 3, 5, and 6 facilitate sediment transport out of the local transport system. From here, partial sediment load returns via pathways 2, 4, and 7, while partial load is transported towards an independent offshore sand wave field approximately 8 km east of Arklow Bank (sand wave field 1 in Figure 12) [15]. According to Creane et al [15], sediment then either (i) returns back to the offshore sand bank, in this case via source transport pathways 7 and 8, or (ii) the offshore independent sand wave field acts as an intermediary stepping stone to independent sand wave fields further offshore (sand wave field 2 in Figure 12).…”

“…From here, partial sediment load returns via pathways 2, 4, and 7, while partial load is transported towards an independent offshore sand wave field approximately 8 km east of Arklow Bank (sand wave field 1 in Figure 12) [15]. According to Creane et al [15], sediment then either (i) returns back to the offshore sand bank, in this case via source transport pathways 7 and 8, or (ii) the offshore independent sand wave field acts as an intermediary stepping stone to independent sand wave fields further offshore (sand wave field 2 in Figure 12). Creane et al [15] use a combination of environmental variables and theoretical factors to determine the mechanism of sediment transport in this circulatory residual transport pathway.…”

“…This is reflected in bed load transport, whereby sand waves with a mean height and wave length of 3 m and 140 m, respectively, migrate southwards at a mean rate of 23 m/year on the south-eastern side of the bank. Contrastingly, sand waves on the south-western side of the bank display a mean height and wave length of 2.3 m and 123.5 m, respectively, and migrate northwards at a rate of 32.7 m/year [15] (Figure 1). Within this sediment transport cell, Arklow Bank displays a highly mobile upper surficial sediment layer [16,17].…”

“…Evidence of a semi-open circulatory residual sediment transport regime over Arklow Bank is presented by Creane et al [15], who identify multiple residual transport pathways that facilitate the recycling of sediment material between offshore sand banks and offshore independent sand deposits. In this way, sediment passes in and out of Arklow Bank's local sediment transport cell, implying an influence of external sediment sources on Arklow Bank's sustainability and longevity.…”

“…Although the dominant seabed disturbance process is tidal currents, areas of the bank display an MFI of up to 20% under pure wave-induced bed shear stress [16]. Sediment stirring caused by this increased erosion is shown to modify the shape of sand bank-associated sand waves and influence sand wave migration rate and direction [15]. Furthermore, wave-induced flow has been shown to influence net sediment transport directions [20][21][22].…”