Sandbanks, Tides, Erosion

This discussion continues to look at severe erosion at the coastal community of Hemsby in Norfolk. Previous discussions considered Scarps and the effects of Lowering and Steepening on a Beach profile when the Sea Comes In. This latest discussion will consider how the tides, sandbanks and beach interact in a mobile, fluid environment on a fragile coastline.

The coast as a living feature, will also be explored using a process called Transgression. A mechanism whereby a coastline to compensate for an increase in sea levels due to beach lowering and reduction in width, reclaims land space by eroding into sand dunes to attain a new breadth and elevation.

The mechanism of Transgression is described in a report prepared in 2018 for Great Yarmouth Council, entitled Hemsby Coastal Management Report. It discussed possible solutions to erosion at Hemsby, following severe storms after the Beast from The East in 2018. The map below gives a rough indication of the location of the Sand Banks in this discussion. Although the paper from which it was sourced was written in 2007.

Key assertions in the report suggest a relationship between a feature known as Winterton Ness, and the area south of Hemsby Gap. With particular focus on the Caister Shoal Sand Bank which follows the line of the coast from Winterton down to California. Although the exact nature of the relationship is unclear, the presence of a deep-water near shore channel that runs between the Caister Shoal Sand Bank and the shoreline suggests a tidal flow pathway.

Particularly regarding alterations, leading up to 2011, when a swift sizable build-up of sand was observed on the sea ward side of the near-shore sand bank. This accumulation shrunk the cross-sectional area of the sandbank, causing it to move closer towards shoreline, reducing the width of near shore channel. The land facing side of Sand Bank is also the sea-facing side of the shoreline at Hemsby. Restriction in the width of near shore channel, led to cutting into the landward side of the Sand Bank, which focused higher tidal flows of deeper water further towards the shoreline.

This process restricted the area available for beach recovery, as increased erosion, caused beach retreat. To reclaim this restriction in space, the sea eroded into the dune system, to regain space to enable a more natural beach to develop at a higher level.

An abnormal tidal environment is also said to exist between Winterton to Caister. Normal Flood Tide flows North to South, and Ebb South to North. This creates a sizeable difference in tidal elevation and current speeds in both directions, forming a ‘Head’ of water between Winterton and Caister.

Regarding the tidal regime between Winterton and Caister, average tidal current velocities are thought to reach a maximum around 4 hours before High Tide at 1.75 m/s on Spring Tides and 0.98 m/s on Neap Tides. Regarding the Ebb Flow, average velocities reach a maximum 2 hours after High Tide, 1.45 m/s on Spring Tides, 0.82 m/s on Neap Tides.

Residents from Hemsby have observed that a Sandbank can be seen 50 yards out from the shore, this pinches the flow of tide which accelerates to this point. A recent video taken by Hemsby Beach Café, could indicate the location of a near shore channel on Hemsby Beach.

It has also been observed that it doesn’t take much to cause damage, an easterly wind, a reasonably high tide along with low pressure lifting up the sea. Recently on 11^th June 2023, a notice from Great Yarmouth Council warned of metal and concrete debris exposed on Hemsby Beach, in front of the Gap area, currently protected by large boulder blocks. This followed High Tide which covered the beach area at Hemsby Gap. In addition, only one line of dunes remains on The Marrams, with around 300 metres of sand estimated to have been lost since 2013.

One interesting point concerns the extent of erosion over time and whether a marked increase occurred prior to 2011. The time a noticeable accumulation of sand was observed on the seaward side of the Sand Bank. Residents assert that due to extreme winds early in the 1990’s, 12 foot of sand was stripped from the beach, and it hasn’t recovered since. It would be interesting to consider why sand lost in the 1990’s was not restored and retained on Hemsby beach.

Hemsby residents often say that they didn’t think a storm would be as bad as it turned out to be. This discussion considered living features on the coast, which could exacerbate the damaging effects of a storm. Regarding solutions, the report favours installation of a Rock Berm, planning permission has been granted, but funding is still to be sought. Nevertheless, a solution would seem to be needed. To enable both the living coast and the community with its life on the Marrams to coexist in this dynamic, living, breathing sea scape location.