Dynamics and stratigraphy of a tidal sand ridge in the Bristol Channel (Nash Sands banner bank) from repeated high‐resolution multibeam echo‐sounder surveys

Abstract

Repeated multibeam echo-sounder surveys can provide information on developing stratigraphy over large areas and during periods when environmental conditions are known. In this study, analysis encompasses 13 timeseparated multibeam echo-sounder surveys between 2002 and 2010 of a tidal sand ridge in a macrotidal estuary: Nash Sands, a banner bank in the Bristol Channel (UK). Over the surveyed period, Nash Sands was S-shaped in planview, with two en echelon segments separated by a channel (swatchway). Migration of these inflections along the ridge led to deposition of clinoforms up to 5°to 7°steep and 12 to 14 m tall. The clinoforms downlapped onto their substrates or onto cross-sets formed by dunes migrating clockwise around the lower flanks of the ridge. Clinoform topsets were removed or truncated by repeated erosion and/or dune migrations over the ridge crest and replaced with packages of near-horizontal laterally discontinuous irregular beds. Flank dune crests were oriented obliquely to nearly perpendicularly to the clinoforms in all three sets, so the clinoforms developed by obliqueÀlateral accretion locally. Although one area had strongly eroded in the prior decade during elevated wave conditions, the 2002 to 2010 stratigraphic development revealed a different relationship with extreme wave heights; Nash Sands generally accumulated sand during times of more extreme waves and lost sand during more quiescent conditions. Using also single-beam survey data from 1991 to 2002 to study the ridge morphology over 19 years to 2010, the swatchway was absent in 1991 to 1992 and progressively developed as the ridge sinuosity became more accentuated. Dunes found migrating north-west through the swatchway are potential evidence of a current caused by tidal height differences across the ridge during ebb conditions. The study illustrates how repeated sonar measurements reveal the processes and timescales that lead to the deposition of stratigraphic units.

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

“…Contrastingly, in other regions, such as over cross-section L39 (Figure 15a,g), a decrease in the rate of bed level reduction is evident. This could be due to a temporary reversal of net current flow in this area or due to sediment resuspension during slack tide and sediment transport in the opposite direction during these slack tidal current conditions, as noted in previous studies in other environments [22][23][24]. This slows this east-west fluctuation behaviour.…”

“…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]. Mitchell et al [22] showed that surface waves enhance sand transport during slack tide by maintaining particles above the threshold of motion and providing a bed shear stress vector in the direction of wave propagation.…”

“…Furthermore, wave-induced flow has been shown to influence net sediment transport directions [20][21][22]. Mitchell et al [22] showed that surface waves enhance sand transport during slack tide by maintaining particles above the threshold of motion and providing a bed shear stress vector in the direction of wave propagation. Additionally, the wind-tidal current interaction is highly complex, whereby in areas of low tidal amplitudes, even moderate wind speeds can reverse residual tidal currents [23,24].…”

Modeling the Cross‐Sectional Dynamics of Tidal Sandbanks in Sediment‐Scarce Conditions

“…Detailed site‐specific modeling studies in combination with measurements can provide insight in the local hydrodynamics and sediment transport patterns. Examples include the studies into the Middelkerke Banks (Pan et al., 2007; Williams et al., 2000), Hinder Banks (Deleu et al., 2004), Great Yarmouth Banks (Horrillo‐Caraballo & Reeve, 2008), Kwinte Bank (Brière et al., 2010; Van den Eynde et al., 2010), and the Scarweather and Nash Sands banner banks (Fairley et al., 2016; Mitchell et al., 2021). While these studies provide valuable insights in the site‐specific processes, including the Hinder Banks and Kwinte Bank which experience sediment scarcity, they typically focus on dynamics on the timescale of a tidal cycle rather than the centennial timescale of sandbank evolution.…”

Evolution of marine gravel dunes on the open shelf under multi-directional currents conditions

“…The depth, morphology and orientation of seafloor bedforms can be captured with multibeam bathymetry, and this information is often used to infer submarine sediment transport processes (Lobo et al, 2000;Lamarche et al, 2011;Hughes-Clarke, 2012;Damen et al, 2018;Mitchell et al, 2021). However, morphological changes in bedforms cannot be observed in a single bathymetric dataset, and in the absence of direct observations, numerical or physical modelling experiments are needed to simulate bedform evolution and test potential driving mechanisms (Venditti et al, 2005;Doré et al, 2016;Wang et al, 2019).…”