A video taken by local resident and artist Carol Cameron shows a major land slip off a cliff face at Thorpeness. The interesting thing to note is that prior to the land slip, significant scouring, due to wave action, had occurred at the base of the cliff. The subsequent fall of material from the frontage of the cliff can be seen in the video.
Category: Erosion
A look at erosion processes, exploration of various definitions, and examples and analysis of contributory factors. Including meteorological, sea state impacts and physical geographical factors.
Sand and Shingle
Severe erosion continues at Thorpeness beach, particularly on the section of beach below North End Avenue. The most noticeable features include substantial damage to flood defences, a loss of material and retreat of the cliff face, scouring at the base of the cliffs and a significant loss of shingle.
Flood defences have been in place on the beach frontage, 250m south of Red House, since the 1970’s in an attempt to repel the destructive wave energy directed at this section of beach. At the beginning of the year, there was evidence of scouring around the edge of rock cages installed as flood defences and in some places, the cages had been pulled apart. However, recently with a loss of shingle in front of the defences, they have lost their solidity, with big gaps where there used to be firm continuous defences.
The effects of sculpting by wave action, leading to sand drifting from the cliff face, is a contributory factor to loss of material from the cliff frontage below Red House. In addition, land slips from the cliff face regularly occur. In the photo below, patches of turf, possibly from gardens above can be seen following a recent fall of cliff material.
The retreat of the cliff line reveals the increasingly perilous position of several significant properties positioned on North End Avenue, near the top of the cliffs.
Scouring at the base of the cliffs, is also quite extensive. This could weaken the cliffs further, potentially leading to further falls along the cliff frontage.
This vulnerability is exacerbated by the apparent stripping away of shingle from the beach. Thus removing any feature that could dissipate the energy of the waves before it hits the cliffs. A longer view down the beach from a ridge of shingle shows the erosion of the cliff frontage, the houses on North End Avenue and the recently installed rock revetment flood defence, with the Aldeburgh coastline in the background.
As was discussed in the post Elements of Erosion, the shifting of shingle, both off and around the beach, isn’t the only erosive process occurring at Thorpeness. But the wider question of sediment transport is a significant process driving the erosion of this section of coastline. The mechanisms and features driving this process iswill be considered in future discussions.
Elements of Erosion
This discussion will consider three particular elements to the on-going dramatic erosion at Thorpeness.
The first element is the flood defences installed in a series of attempts to hold back the sea. A Gabion defence, a stone filled mesh box was installed in 1976, following serious erosion with beach material added to cover the defence. However, storms in 2010 exposed this defence, so Geobags were installed at the bottom of the Gabion defence, but these in turn were seriously damaged in storms in 2013/14. Small rocks were then placed around damaged defences as a short-term measure in May 2021. In October of this year, a 35-metre rock revetment was installed below the fastest eroding section of cliff to protect property above.
The second element is a cairn made up of pieces of Coraline Crag that seems to symbolise what is solid and permanent and fragile and temporary at Thorpeness. A detailed report by Mott Macdonald outlines the geographical features that shape the beach morphology at Thorpeness. The Coraline Crag pieces have broken off a series of ridges that extend North-East from the end of North End Avenue. Historically, these ridges have been largely resistant to erosion. Another significant feature, the Sizewell Dunwich Bank Complex (SDBC) connects to a headland at Thorpeness, through the Coraline Crag ridges. The SDBC is sometimes extending towards Thorpeness with sediment from sources to the north accreting on the Ness, with beaches to the south starved of sediment. Sand may also move offshore from Thorpeness to the southern end of the SDBC.
This relationship is made more complex by observations of a shifting offshore bank which could focus wave energy onto North End Avenue, where erosion is particularly acute. Beach lowering, caused by sediment removal, could enable storm waves to reach the base of the cliff. This concurs with research by Earlie, Masselink and Russell from the Universities of Cardiff and Plymouth who compared contrasting beaches in Cornwall and found even though erosion was more constant at a beach, with a gentle foreshore, the beach with a steeper foreshore, saw massive erosion, when Mean High Water of a Spring Tide combined with a storm surge.
Wave energy can be reflected offshore by sand banks or defence structures and interact with an incident wave field. Constructive wave interference could see the crest or trough of a wave moving through or overlaying the crest or trough of another wave. This could heighten waves and drive sea floor currents and generate wave turbulence, which could drag additional sediment and hold it for longer, so even a weak current moving alongshore could remove beach sediment in larger quantities, than would be the case, were wave reflection to be absent. This lowers the beach further enabling larger waves to reach the shoreline driving further sediment loss. Notably, a future for Thorpeness state in the 6 months to August 2021, 10m of cliff were lost at the North end of the village in New Year storms.
The third element is the erosive forces themselves, in particular, Abrasion and Hydraulic action. Abrasion describes how bits of sand or rock in waves can grind down cliff surfaces like sandpaper, with boulders projected like strong hammers, pebbles like bullets and sand like abrasive powder.
Regarding Hydraulic Action, waves propel air in advance of them and squeeze it against the cliff face. When the air is freed, it inflates with considerable force, destabilising and tearing away part of the cliff. In a connected process, Desiccated Cracking, the combined attrition of the weather and action of waves, can weaken a cliff face. Alternate heating and cooling cause the body of the cliff to expand and contract, creating cracks in which rain and sea water seep into crevices. When water freezes it expands massively, loosening segments of cliff face, additionally, when a cliff surface, dries and hardens, cracks can widen and deepen. Hydraulic actions, dampening and drying can cause large slippages of whole sections of sand and clay in a process known as sheer stress.
When standing on Thorpness beach, it is possible to envisage how these elements interconnect to facilitate erosion on this dynamic, fragile stretch of coastline.
Hazlewood – intertidal fortunes
Hazlewood marshes provides unique biodiversity in an environment moulded by its fortunes as an intertidal reserve.
Sediment deposits are accreting at Hazelwood, noticeably in the former freshwater channels. The sediment is thought to consist of estuarine silt, which could concur with analysis that describes the Alde Ore estuary as flood-dominant, i.e that it has a tendency to drop silt.
As part of a report in 2014 by Kenneth Pye Associates, to investigate the implications of allowing Hazlewood to become intertidal, analysis was conducted into the sediment content of the Alde Ore estuary. It was found that sediment in the upper reaches of the Alde Ore estuary, consisted mainly of mud, but sand and gravel could be found in creek beds and in the toe of eroding marsh cliffs. Research has also found that erosion of the north Suffolk cliffs could supply 95% of the mud 89% of the sand and 62% of the gravel sediment found on the open coast and in the estuaries of Suffolk (Burningham, French, 2016).
It could be said, one fly in the ointment at Hazelwood, is that alongside the vegetation, bird, fish and sea life, the activities of the daily tides and the connection to the North Sea continue to shape the landscape.
The bird hide at the far end of the marshes which affords excellent views across Hazlewood and provides views to Ham Creek in the estuary, gets cut off by high spring tides.
The path to the hide is also being eroded across its width, possibly as a result of dessicated cracking.
The same process that contributed to the visible signs along the length of the remaining sea wall at Hazlewood, where evidence of failure of the entire landward facing wall can still be seen.
Sediment accretion and the development of islands, with saltmarsh are known to reduce the destructive energy of tidal waves. However, a report has found that rising sea levels could potentially reverse reductions in wave energy from offshore sand and shingle banks and saltmarsh (Burningham, French, 2016).
In its current form, Hazlewood continues to thrive, and potential exists for discovery of key fish species which are a conservation target for the Alde Ore estuary. But as the saying goes, time and tide wait for no man or living organism, in a dynamic, ever-changing intertidal regime.
Sudbourne Beach – Evolution of a shingle ridge.
In a continuation of discussions about Sudbourne beach, south of the Martello Tower in Aldeburgh, Suffolk, consideration will be given to the current position of Sudbourne beach.
Thanks to access granted by the Environment Agency, it has been possible to get a recent visual image of the dynamic interplay between the sea, the shingle ridge and the Alde and Ore Estuary.
Along this stretch of coast, there are three types of flood defence, which form structures within and on top of the shingle ridge, which stretches from the Martello Tower down to Orford Ness. The flood defences rather than form an impenetrable barrier, either flex in response to the action of the waves, or attempt to mould the shingle around boulders and concrete pillars, while the sea scours indents into the shingle ridge at the top of the flood defences.
The Environment Agency recycled the shingle ridge on Sudbourne beach in 2018, but this work was undone by winter storms in a matter of weeks. The purpose of this recycling and the use of flood defences seen on the edge of Sudbourne beach, and the use of Saltings between the shingle ridge and the estuary is to dissipate the energy of the waves when they hit the beach. Even though these defences, on their own will not ultimately stop a breach, as can be seen by the failure in the interlocking block revetment, which is beginning to outflank the end of the flood defence. They do attempt to hold together the natural structures of the beach. The situation at Aldeburgh offers a useful case study into types of flood defence, as either side of the boulder etc defences, the sea is pushing shingle ridges higher up towards either the estuary or the built-up areas of Aldeburgh.
As it is the sea that is dominant here. The ridges on the beach show the reach of the tides, the ridge nearest the shore is the low tide and the ‘cliff’ ridge at the top of Sudbourne beach signifying the reach of a high storm tide. This ridge has been gradually getting narrower with shingle being eroded from the sea-facing, front of the ridge. It is also noticeable that the shingle ridge at the top of Sudbourne beach, swerves back away from the sea, towards the estuary. The height of the ridge is maintained, but the top of the ridge becomes narrower and seems to reduce the distance between shingle ridge and the edge of the estuary.
There are visual signs that the sea has overtopped the section of shingle ridge south of the Martello Tower.
It is both the day to day, dynamic interplay between wave action and winds as well as occasional storm events that contribute to the instability of this stretch of coastline.
Landscapes of marshes and estuary
A selection of photos taken in February 2021 show the current landscape of Aldeburgh Marshes and Hazlewood marshes as well as the history of the scouring effects of the tidal surge in December 2013.
The first photo shows the lasting affects of the scouring of the trees at the back of Hazlewood marshes, alongside the raised islands created more recently by Suffolk Wildlife trust to attract Avocet’s and other birds.
The second photo shows reed beds which have survived at the back of Hazlewood marshes. The Reed beds would have been important habitats for Bitterns when the marshes were freshwater habitats before the tidal surge.
The third photo is taken from the top of the estuary flood defences, looking out to Aldeburgh marshes on the right and the Alde Ore Estuary on the left. The flood defences are built to reduce the effects of tidal surges risking damage to Aldeburgh marshes and town.
The fourth photo looks out on the Estuary at the shore of Iken Cliff, near Snape. Flood defences at Iken marshes, near Iken cliff were subject to partial and actual breaches due to the tidal surge in December 2013. The breaches to the freshwater reserve were repaired quickly, but the area continues to be vulnerable to future breaches due to tidal surges. Flood defences out in the estuary show signs of previous breaches.
Finally, the peace of the winter sun on Hazlewood marshes, shows a reserve that continues to thrive and is home to many native and migratory birds. But the area, like the rest of the estuary is subject to the forces of the North Sea and the ability of aging flood defences to withstand rising sea levels and consequent storm surges.
Sudbourne Beach – an update
This piece seeks to provide a brief update to a previous piece on Sudbourne Beach, which is a shingle spit south of Aldeburgh on the Suffolk coast.
A public consultation was held between 1 October – 30 November 2019 regarding the policy of the Coastal Partnerships East and East Suffolk Council Shoreline Management Plan (SMP) in regards to Sudbourne beach. The consultation sought views on whether the policy of the SMP should maintain an interim position of No Active Intervention, after the current policy to Hold the Line expires in 2025. The alternative up for consultation, was a change of policy to Managed Retreat, which would seek to actively manage the rapidly eroding shoreline at Sudbourne Beach.
As the result of the public consultation the East Coast Council has now formally adopted a permanent policy of Managed Retreat, which will mean Managed Resilience of the Shingle shore, extended to 2100.
Recent reports from the Environment Agency state that as a result of winter storms, the ridge on Sudbourne Beach is changing daily. A reminder that the future of this fragile spit is bound up in the actions of the shifting ferocious tides and the authorities and concerned residents attempting to prevent the spit submitting to the erosive forces that threaten its future.
Embankment Wall Breach
In field studies on the Essex and Kent coasts following the North Sea 1953 storm surge, academics, Cooling and Marsland listed four possible causes for flood embankment failure. Three of these causes are particularly useful to consider regarding the failure of river walls at Hazelwood marshes and Havergate Island. These are, a) erosion of sea-facing embankment wall by wave activity, b) erosion of land-facing embankment due to over-topping, c) slippage or slump of land-facing wall due to water dripping through the bank.
The three causes of failure described above can be said to be examples of two distinct processes, Scouring and Desiccating Cracking. Of the causes listed above, a and b, can be associated with scouring and c can be attributed to desiccated cracking. The elements of each process and how they contribute to flood embankment failure will be considered in more detail.
The process of scouring can occur when water overtops a flood defence and reaches the ground on the landward side of an embankment in a state of turbulence, therefore, it could be said, erosion begins the moment the wave reaches the border between soil and water. The force that moves the wave interacts at speed with soil at the base of the landward side of a flood embankment. At this point, two processes are said to be at work, the immediate movement of the water directed by the physical space it hits and the state of the soil when the wave meets the ground.
The immediate area the wave hits is said to contribute to scour due to water meeting an obstruction, presumably this could be a rock or the edge of the base of the river embankment. Meeting this obstruction can interrupt flow and decrease its space and redirect surge water. As this alteration is very sudden and occurs at speed it can multiply the rapidity of the energy directing the water which can cause eddies to form.
The state of the soil the wave meets when it hits the ground, contributes to what is known as shear stress. Britannica.com define shear stress as an energy whose impact can distort a substance causing sliding along a horizontal surface alongside the source of the stress. The shear that occurs correlates to the descending progress of earth impacted by this process. The extent that shear stress causes a deep scour hole is related to the make-up of the soil at the base of the embankment, depending on soil makeup, sheer stress can lead to an eventual lifting of sediments particles causing scour.
A photo that was taken after the storm surge of December 2013 showing evidence of a shallow slippage, caused by scouring after wave overtopping.
The second process that can cause an embankment to breach is Desiccated Cracking. This is particularly said to occur in alluvial clay, a material used in some flood defences in the Alde Ore estuary.
Desiccated Cracking or fissuring relates to the formation of an intersected web of internal vertical and horizontal fissures, about 60 cm deep within the surface layers of a flood embankment. It is thought repeated wetting and drying of estuary embankments can contribute to desiccated fissuring.
In a flood surge, large amounts of water drip through desiccated fissures, in extreme conditions, this can cause hydraulic fracture, when the flow transmits through fissures to the landward side of a river wall. Rather than a wave overtopping, water flows through fissures below the crest, into the embankment. This can cause the lifting of blocks of material, leading to gradual slope failure and the eventual breach of a river embankment.
Photo from: https://eprints.hrwallingford.com/1291/
Regarding, the process of water seepage that causes failure on the land-facing embankment, a member of the Alde Or Association visited Hazelwood marshes during the storm surge at its peak on December 6th 2013. It was reported that the water level reached the top of the embankment with minor overtopping at low points. However, the observation of real interest was amount of free water flowing through desiccation cracking issuing from the landward bank. This is indeed the puzzling aspect that strikes an outside observer of photos of the after-effects of the storm surge at Hazelwood, that they all seem to show water flowing outwards from the land-facing side of the embankment.
Photo from https://eprints.hrwallingford.com/1291/
The two processes of Scouring and Desiccated Cracking are separate yet are linked in that they interact and are influenced by the physical space and soil make-up of the flood embankment they detrimentally affect. Scouring interprets then shapes the space and the particles it interacts with, the scouring out of the base of a flood embankment, being the physical result of this interpretation. Whilst desiccated cracking, develops over time within the structure of the embankment, with the fissures functioning as vehicles for the rapid movement of water and its mechanisms of erosion.
The tendency of recent storms to become ever more powerful and unpredictable alongside rising sea levels, make the complex processes of scouring and desiccated cracking increasingly useful to understand, so the effects of storm surges can be assessed to reduce the likelihood of flood embankment failures.
Havergate Island
Havergate Island is situated near the Suffolk village of Orford, at the meeting of the Ore and the Butley rivers. The Ore divides in two around the two-mile long Havergate Island with flows splitting through what is known as the Gull on the north side of island and the Narrows on the south of island. Coriolis forces, roughly defined, state a force influenced by the earth’s rotation tends to deflect moving elements to the right in northern hemisphere and to the left in southern hemisphere. Such forces would suggest flood tidal flow would run through the Narrows and ebb flow through the Gull. Both flows are similar in width, but the Gull is much deeper.
A video, kindly provided by the RSPB taken by a warden at Havergate in 2019, shows a breach caused by a sea surge. The breach was at the narrowest part of the island, on the side facing the Gull, on Havergate Island.
A large sea surge in December 2013 breached the sea walls at Havergate. In order to limit damage from any future surges, the RSPB, introduced a scheme to allow certain areas (known as ‘sills) to overtop. The purpose is to allow the sills to flood in a relatively controlled manner to manage water inundation and enable areas recover relatively quickly.
This approach seeks to address the problem of storm surge sea water once it enters a reserve scouring (eroding) the insides of sea walls fundamentally weakening them. When this occurred during the December 2013 storm surge, the RSPB at Havergate built up damaged walls, but are aware the walls may never be as strong as they were.
Internal scouring of sea walls was one of the main causes of damage following the 2013 surge. As the reserve contains saline lagoons the flooding of salt water wasn’t as damaging as it was to the freshwater marshland at Hazelwood marshes. Wildlife has also recovered following the 2013 surge with the hare population recovering and uncommon yellow vetch growing again.
As the RSPB have lowered sections of the sea wall, they are able to utilise sluice gates to allow flood water to leave lagoons without surges causing lasting damage. The process for creating the lower sea walls in addition to reducing wall height involves laying wire netting on the lowered sea wall and sowing grass seed. Once the grass has grown, combined with the wall lowering and the net laying, it should enable defences to withstand several hours of overtopping. Such schemes are favoured as they enable constructive management of overtopping. Which often proves cheaper than dealing with breaches in sea walls which are seen as more damaging and costly to fix.
Regarding the future of Havergate Island, it has been predicted by site manager Aaron Howe that the reserve could have another 20-40 years before it succumbs to coastal erosion. Increasing storm surges and rising sea levels are developing alongside another process, known as Isostatic Rebalancing. This refers to the post-glacial rebound of northern Great Britain which is said to be happening at a rate of around 10cm per century. This is causing a corresponding downward movement of the southern half of Great Britain, which is developing at around 5cm per century. This could increase the risk of floods in southern England.
The sea is nibbling away at Havergate, and the RSPB believe tidal surges are increasing, but the wardens and site managers work hard to adapt and evolve. As do the many bird and wildlife species the island is famous for. In this way, Havergate, placed as it is, in the middle of a tidal estuary shaped by the North Sea, serves as a barometer of the evolution of tidal reserves and species, and the size and nature of sea surges. Like the boat October Storm that takes visitors, to and from the island, Havergate is pushing into its tidal fortunes, to innovate as currents and surges shape its environment.
Sudbourne Beach
Sudbourne Beach situated south of the Martello Tower near Aldeburgh on the Suffolk coast is a shingle spit that separates the open coast from the Alde/Ore estuary. Recently, the sea has eroded the shingle spit at a greater rate than has previously been experienced.
Coastal Partnerships East and East Suffolk Council have a Shoreline Management Plan (SMP) which maintains a policy to Hold the Line up to 2025. This approach seeks to uphold or if necessary, add to and improve existing sea defences. These bodies also hold an interim position of No Active Intervention in the medium to long term, which would see no active investment in sea defences. However, concern about increased erosion has prompted these bodies to launch a public consultation to change the SMP position from No Active Intervention to Managed Realignment. A policy of Managed Realignment as defined by the Department for Environment, Food and Rural Affairs (DEFRA) means ‘allowing the shoreline to move backwards or forwards, with management to control or limit movement’. It is hoped this policy will enable maintenance of the ridge to prevent a permanent breach.
A Draft report, the SCF-130619-SMP7-Policy-Review-Slaughden undertaken by Jacobs Consultancy commissioned by East Suffolk council in partnership with a Client Steering Group, identifies the area of Sudbourne beach and Slaughden ridge as ORF15.1. This area begins at the termination of the concrete walls at the Martello Tower and extends southwards to Lantern Marshes. The open sea and the Alde/Ore Estuary exist as two distinct but linked elements, but the ORF15.1 shingle spit is very narrow, prompting fears of a breach at a future date.
As an indication of just how the state of ORF15.1 is constantly shifting, three photos show how quickly the sea can alter the beach and shingle bank. The first two photos are from the Draft SCF-130619-SMP7-Policy-Review-Slaughden. The first photo taken in July 2017 shows a substantial wide shingle mass on top of Slaughden Ridge.
The second photo taken in December 2018 shows a big reduction in the mass of shingle with a cliff formed of shingle on top of Slaughden ridge.
This later photo is similar to photos used again as a comparison by the Alde and Ore Association in their Spring 2019 newsletter. The first photo they use taken from above in December 2018 shows the steep shingle cliff with a wide section of shingle on the beach.
The second photo taken in January 2019, shows the sea has moved the shingle to the back of the beach raising it up to a much higher level. The shingle cliff looks much diminished from the December 2018 photo.
It is also worth considering from the point of view of wider consideration of coastal erosion in the Alde Ore Estuary that each feature involves distinct but connected alteration processes. Hazelwood marshes became an intertidal marshland following a storm surge that breached sea walls protecting the freshwater marsh.
Sudbourne beach and Slaughden ridge whilst processes are clearly not helped by storm surges, are subject to erosion caused by the regular attrition of changes in wave behaviour.
The next feature, Havergate Island could be said to be subject to both these processes, it has been struck by storm surges and regularly manages changing, escalating sea activity.
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