What is it about?
Nash Sands is ridge of sands formed by strong tidal currents in the Bristol Channel, UK. Surveying over 19 years reveals how it started in 1991 as an almost linear ridge and became progressively more S-shaped in plan-view. Complete mapping of the bank with sonars (multibeam echo-sounders) from 2003 to 2010 revealed the migrations of dunes and how three areas accumulated sand. We use those data to evaluate the internal structures (stratigraphy) that would be produced within the bank. Changes in those structures are found to vary with wave conditions and storm surge in a complex way.
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Photo by Sumner Mahaffey on Unsplash
Why is it important?
The study had several important outcomes: Within one depositional area, dune migrations led to climbing cross-stratified beds with cross-sets almost as thick as the heights of originating dunes. This is normally considered impossible because sand deposited on lee slopes of dunes is usually re-eroded by the stoss slopes, leaving only thin beds. It does occur here however because of sand supplied laterally to the tidal current causing the dune migrations. Nash Sands was steadily creating a channel or swatchway through it. Dunes migrating through that channel indicate there was a current flowing in a direction not expected from tidal current models. We suggested the water level north of Nash Sands declines more rapidly than that south of it during the ebb part of the tidal cycle to create this current. We found some relationships between changes to the bank and wave conditions during the period of multibeam surveying. But over the longer term, other relationships occurred, so the dynamics of Nash Sands are complex. We found evidence of changes in total volume and volumes of different parts of Nash Sands that indicate that it is not a closed system. Rather it exchanges sand with other parts of the bed of the Bristol Channel.
Perspectives
I started working on Nash Sands in 2002 with Dr Thierry Schmitt, when he was working on it for his PhD thesis in Cardiff University. We collected some data over easterly Nash Sands. It felt like quite a lot of work to organise and carry out, though the datasets subsequently collected for the aggregates companies (then Hanson Aggregates Marine, Tarmac Marine Dredging and CEMEX UK Marine) are very large and must have taken an enormous amount of effort to collect. With the published article, we have only just begun to exploit the potential of this dataset.
Dr Neil C. Mitchell
University of Manchester
Read the Original
This page is a summary of: Dynamics and stratigraphy of a tidal sand ridge in the Bristol Channel (Nash Sands banner bank) from repeated high‐resolution multibeam echo‐sounder surveys, Sedimentology, August 2021, Wiley,
DOI: 10.1111/sed.12935.
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