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

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 winddriven 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 wellsorted 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.

The Tides They Are A‐Changin': A Comprehensive Review of Past and Future Nonastronomical Changes in Tides, Their Driving Mechanisms, and Future Implications

“…Tidal laminations (rythmites), which are formed by varying amounts and types of sediment deposited on the flood and the ebb tide (e.g., Dalrymple, ), can be used to infer tidal components and longer‐term variations but are highly localized in time and space and can give a range of potential tidal amplitudes (e.g., Tanavsuu‐Milkeviciene & Plink‐Bjorklund, ; Williams, ). Attempts have also been made to reconstruct tidal dynamics by linking sea bed sediment types from sediments cores to bed shear stress in order to infer tidal current strength (Ward et al, ) but were hampered by limitations of the sedimentary records. Studies reconstructing changes in stratification in paleo–shelf seas from plankton assemblages (e.g., Scourse et al, ; Woods et al, ) can provide an indication of the strength of tidal currents by tracking tidal mixing fronts but again only provide an indication of regional tidal conditions.…”

The Impact of Waves and Tides on Residual Sand Transport on a Sediment‐Poor, Energetic, and Macrotidal Continental Shelf

“…Physical sediment characteristics are more important than biological factors for determining bed stability in the Celtic Sea (Thompson et al, ). Ward et al () show regional bed shear stress patterns at 1.1‐km resolution. This and aforementioned modeling studies focus on tide‐induced bed shear stress, neglecting surface waves.…”

A synthetic map of the north-west European Shelf sedimentary environment for applications in marine science

“…Tidal and wave energy levels at the seabed should strongly influence mud, sand and gravel percentages. Large grain sizes require more energy to dislodge from the seabed, and therefore high bed shear stress is associated with increases in average grain size and reductions in mud content (Ward et al, 2015;Heath et al, 2016). There is scarce evidence to determined if seabed composition is influenced by year-round bed shear stress or individual high-energy events.…”

“…However, this is unlikely to be true everywhere. Ward et al (2015) have argued that the coarser sediments found south-east of Ireland were inherited from prior stress regimes. Furthermore, the Irish Sea has linear tidal sand ridges, which are likely relics from an earlier more energetic stress regime (Uehara et al, 2006;Scourse et al, 2009).…”

“…Recently, Stephens and Diesing (2015) demonstrated that the mud, sand and gravel percentages of the seabed in British territorial waters and a large part of the North Sea can be predicted using random forest models (Liaw and Wiener, 2002) which have environmental conditions at the seabed as predictors. Further, other work has shown clear relationships between the sediment composition of the sea floor and the energetic regime at the sea floor (Porter-Smith et al, 2004;Ward et al, 2015;Heath et al, 2016).…”