Scarp cut into sand behind rocks at Hemsby Gap after storm on 20th April 2023
The precarious position of homes on the Marrams in Hemsby has been highlighted in a BBC documentary Life on The Edge. The vulnerability of the community on this coastline was exposed again when a storm on 20th of April saw waves cut into the sand behind rocks newly placed to try and stem the erosion. On the night of 20th April, waves were observed at 3 m high, with a lot of damage done before the peak of high tide. The BBC reported that around 4 metres of the sand ramp intended for use by Hemsby Independent Lifeboat was washed away.
The Scarp created was around 10 ft, but this erosion event was simply the latest in a series of severe episodes in the 10 years since 2013, when 20 ft waves scoured a considerable amount of beach and dune material with the loss of seven properties on the Marrams. In fact, in the BBC documentary, Life On The Edge, retired fisherman from Hemsby, Kenny Chaney says that the combined effect of two hurricanes in 1991 and 1993, lowered the beach surface by 12 ft, and the beach has never recovered. Therefore, this discussion will consider the likelihood of increased erosion on beaches considerably lowered by years of wave action.
Three photos taken in 1993, 2013 and 2023 show how much of the beach at Hemsby Gap has been eroded. In 1993 a large sandy beach is shown, with the shoreline around 40 metres from Hemsby Gap. In a photo taken in 2013, it is clear how much sand was scoured from the beach, interestingly a ‘cliff’ or Scarp also appears to be starting to form across the Gap. The final photo in 2023 shows Hemsby Gap after the erosion in February 2023 with the 10 ft Scarp across the Gap. Photos credit of Great Yarmouth Mercury and photographer Mike Page.
To consider the effect of sustained attrition by wave action, the discussion, will consider two points. Firstly, how the profile of the beach has changed, and once waves frequently cover a beach how waves interact with beach structures and the toe of the dunes, contributing to severe erosion.
To begin, it might be useful to consider two metaphors. Firstly, imagine a beach like the tread on a tank rolling over itself in a landward direction. Destructive high-energy waves reach further up the beach, scouring sediment from the dunes. Sand is scoured offshore, causing the cliffs to retreat, removing sand from the beach. This steepens and lowers the beach surface, leading wave action to drag more sediment from the beach, driving the dunes ever landward as cliff retreat increases.
Secondly, beaches with damp sand, are more likely to experience erosion. As sand that is wet, perhaps because lowering of the beach surface means it is covered in sea water most of the time, will not drift and accrete onto the dunes. Fine, dry sand is better able to drift and blow onto dunes, resulting in accretion, of the dunes and beach surface.
It is thought there is a link between beach condition and erosion. Once the Beach Wedge Area, (an interpretation for its width and thickness) reaches 10 m2 per metre of beach length, resistance to further erosion is much reduced. A survey of the beach profile of Hemsby undertaken as part of Anglian Coastal Monitoring by the Environment Agency from 2012 up to 2016, found areas experiencing continuous erosion with others some accretion.
Sections of the beach are separated into individual transects. In the transect next to Hemsby Gap, Profile: HW413, the graph shows Elevation Change derived from LIDAR data, (a method of remote sensing) acquired in the period November 2012 to October 2016. Data in the information below, is taken from November 2012 – November 2015. The graph shows Elevation and Chainage, the distance in metres, from the chosen landward starting point of the profile being measured, to the most seaward point on a beach, with both points remain stationary and fixed over time. Chainage Point 0, is the most landward point. In November 2012 and November 2015, at Hemsby Beach profile HW413, at Chainage Point 0, elevation was just under 7 metres. In November 2012, Chainage at most seaward point, was just over 145 metres, Elevation at 0. metres. In November 2015, Chainage at seaward point was 140 metres, elevation around -1.6 metres.
The survey concludes the north section of Hemsby beach showed signs of persistent erosion in the foreshore, and like other areas of the beach, there was a clear movement of the dunes in a landward direction. Data showed a build-up of sediment in the south at Hemsby beach and at Transect HW430, in November 2012 and November 2015, at Chainage point 0, the most landward point, elevation was around 14 metres. In November 2012, at the seaward point, Chainage was around 130 metres, with elevation at 0 metres. In November 2015, Chainage at the seaward point, was around 140 metres, with Elevation at 0.4 metres.
Storms in 2018, 2021 and 2022 each saw 20 ft of sand dune washed away, with drops or Scarps cut into the ramp used by Hemsby Lifeboat. This moves the discussion onto once waves frequently cover a beach, how they interact with beach structures, and dunes contributing to severe erosion.
Hard Structures on a beach can cause what is known as wave reflection, spray is driven upwards, as the force of the wave meets a solid element. It is said roughly an equal amount of energy is also driven down into the surface of the beach. This force can scour sand from the surface and causing the beach level to lower, revealing more of the hard defence to salt water. Over time this can reduce the width of the beach. It could also be useful to consider a process in which waves can be reflected offshore by sandbanks, clashing with the crest or trough of a wave, driving turbulence and increasing damaging sediment transport.
In a video of the storm on the 10th March in 2023 at Hemsby, wavesappear to clash with spray driven upwards, as they reach the base of the dunes. Equally, on 20th April 2023, when waves hit rocks installed to try and protect Hemsby Gap, spray from the waves, surged upwards, and arguably this force transmitted through the rocks, causing sea water to scour into the sand behind the rocks.
This discussion has considered the effect of successive periods of high-energy waves, over a number of years, progressively lowering a beach profile, scouring sediment and driving the dune ever landward. Once the sea comes in, its erosive properties strip away the surface and essence of the beach. Placing in jeopardy shoreline, dunes and communities on this vulnerable stretch of coastline.
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