Classifying seabed sediment type using simulated tidal-induced bed shear stress

Sophie L Ward,Katrien J.J Van Landeghem,Simon P Neill,James D Scourse

doi:10.1016/j.margeo.2015.05.010

Abstract

Abstract An ability to estimate the large-scale spatial variability of seabed sediment type in the absence of extensive observational data is valuable for many applications. In some physical (e.g., morphodynamic) models, knowledge of seabed sediment type is important for inputting spatially-varying bed roughness, and in biological studies, an ability to estimate the distribution of seabed sediment benefits habitat mapping (e.g., scallop dredging). Although shelf sea sediment motion is complex, driven by a combination of tidal currents, waves, and wind-driven currents, in many tidally energetic seas, such as the Irish Sea, long-term seabed sediment transport is dominated by tidal currents. We compare observations of seabed sediment grain size from 242 Irish Sea seabed samples with simulated tidal-induced bed shear stress from a three-dimensional tidal model (ROMS) to quantitatively define the relationship between observed grain size and simulated bed shear stress. With focus on the median grain size of well-sorted seabed sediment samples, we present predictive maps of the distribution of seabed sediment classes in the Irish Sea, ranging from mud to gravel. When compared with the distribution of well-sorted sediment classifications (mud, sand and gravel) from the British Geological Survey digital seabed sediment map of Irish Sea sediments (DigSBS250), this ‘grain size tidal current proxy’ (GSTCP) correctly estimates the observed seabed sediment classification in over 73% of the area.

Highlights

The large-scale redistribution of sediments in shelf sea regions by hydrodynamical processes has direct implications for geological basin and coastal evolution
With focus on the median grain size of well-sorted seabed sediment samples, we present predictive maps of the distribution of seabed sediment classes in the Irish Sea, ranging from mud to gravel
In a tide-dominated shelf sea such as the Irish Sea, sediment transport in the nearshore zone can be dominated by wave action, whereas farther offshore the characteristics of seabed sediment distribution are more indicative of the tidal current conditions of a region

Summary

Introduction

The large-scale redistribution of sediments in shelf sea regions by hydrodynamical processes has direct implications for geological basin and coastal evolution. A number of studies have used the distribution of peak bed shear stress vectors from tidal models to infer sediment transport pathways and the location of bedload partings around the British Isles (Pingree and Griffiths, 1979; Austin, 1991; Harris and Collins, 1991; Aldridge, 1997; Hall and Davies, 2004; Neill and Scourse, 2009) as well as for the evolution of bathymetric features such as tidal sand ridges (e.g., Huthnance, 1982; Hulscher et al, 1993), in particular in the Celtic and Irish Seas (e.g., Belderson et al, 1986; Scourse et al, 2009; Van Landeghem et al, 2009a). They found that the direction of bedload transport correlates with the peak bottom bed shear stress vectors (M2 + M4), and most sand transport occurs in response to the peak current speed over a tidal cycle

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