Tuesday, 31 August 2021

Dunwich and climate change

David Sear, Professor of Physical Geography at the University of Southampton and a Trustee of Dunwich Museum, talked to Discover Dunwich editor Matt Salusbury about the impact of climate change on Dunwich throughout its history.

A much shorter version of this article (450 words) appears on issue 3 of Discover Dunwich, Dunwich Museum's newsletter for visitors, volunteers and supporters.

All Saint's Church, Dunwich, not long before it went over the cliff in 1919. From Dunwich Museum's Nicholson Collection of postcards, out of copyright

MS: I always believed that the erosion and storms that gradually destroyed Dunwich was just weather – that the events like the big storms weren't anything to do with climate change. Or were they? Were the big storms like the 1286 one and the two that followed later anything to do with climate change? Or are there other factors involved?

DS: This reminds me of the old adage "Weather is what you get, climate is what you expect" - Climate is the average state of the weather over time – so climate change is an alteration in that mean state.

So for example, right now we are in a world where the mean global temperatures are rising rapidly – so it's referred to as climate change. Back in the 13th century the northern hemisphere was in a period sometimes called the "medieval warm period", when average northern hemisphere temperatures were warmer. Later in the 16th through to the mid half of the 19th century average northern hemisphere temperatures were cooler – this period is the one referred to as the "Little Ice Age".

When the mean state of the climate changes, this results in changes in the larger scale climate systems such as the pattern, strength and extent of high or low pressure systems, the extent of sea ice, and temperature of the North Atlantic. Together these alter the path and strength of storms tracking across the UK.

We have seen this recently with the increased frequency of flooding in Cumbria, for example in 2009 and 2015. Individually these were storms (weather), but collectively we see the last two decades as a storm rich period for the UK and Northwest England in particular.

To go back to Dunwich, the storms of 1250, 1286/7 (beginning on New Year's Eve 1286 and lasting for several days) come during a period of increased storminess in the North sea, when Atlantic pressure systems favoured intense North sea storms with larger waves from the north east. Sediment transport (the movement of sediment caused by the action of the waves) would there have been south along the coastline at Dunwich, and based on what we now know about the conditions that lead to rapid cliff erosion at Dunwich, we can conclude that there was probably no protecting beach for the cliffed sections and that the land on which the town was built towards the northern areas, was probably lower, leading to inundation of the town in those areas.

Was this climate change? Well, by the definition of "change in mean state" of the climate, yes it was. But the difference to today is that 800 years ago, this was the result of natural variability – possibly driven by increased volcanic eruption activity (1258 was the largest eruption in the last 8000 years) and increasing solar activity. Similarly, the Little Ice Age period of increased storminess particularly around the late 17th into early 18th century was a period of natural climatic variability in which storm frequency at Dunwich increased, and cliff erosion rates accelerated resulting in the loss of St Peter's churchand the town goal and market place.

Current climate change is different, and is driven by increased warming resulting from the cumulative build up of carbon emissions in our atmosphere. The difference now is that the rate of change and the magnitude of change are now far faster and larger than those that drive the storminess of the 13th and 17th century - we are if you like in uncharted skies.

What we do know is that such change must alter the strength, path and frequency of storms tracking across the UK, and we must therefore expect changes in wave climate and shingle transport along the coast. What we cannot do with certainty is predict how all these processes will interact at Dunwich on a specific day or year. What past tells us though is that the current period of cliff stability will change at some point, and we do know the signs to look for. This means that there is a really important role for statutory bodies like the Environment Agency, East Suffolk Council and local residents to work together, to identify signs of change in the beaches and cliffs in Sole Bay, and to see things a bit more widely than just the beach and cliffs at Dunwich, because in the end Dunwich is one part of a vast connected system where climate, winds, waves and shingle movement interact with humans to change our exposure to risk. After all, if there was no Dunwich, would we be talking about cliff and beach erosion at all?

MS: I recall there there was a Little Ice Age in Britain’s history, when the port of Aberdeen was iced in and there were regular Frost Fairs on the Thames. Do we know what effect this Little Ice Age had on Dunwich, did it have any effect on the coastline at Dunwich?

During the Little Ice Age (c. 1450-1850) - a period of cooler average northern hemisphere temperatures, there were indeed frost fairs on the Thames. During this period intense storm activity resulted in major cliff erosion at Dunwich – with the loss of St John's church 1540s, St Peter's in 1695-1702, Cock and Hen Hills (the great storm of 1740). Within this period there were also quieter phases when cliff erosion rates were lower, and Dunwich still sent ships to fish the waters of Iceland. There would almost certainly have been impacts on fishing and other parts of the economy due to cooler winters and stormier conditions – in fact a project might be for historians to look through the records for the area during these times to see how the regional and local economy responded.

MS: I frequently show visitors to the Museum the series of photos of All Saints going over the cliff in the space of about 20 years, ending with just that last buttress left in 1919. And the extract from the Domesday Book in the Museum tells how some landowners has lost half their land in the time since "Kind Edward" (the Confessor) so in the space of less than 20 years between the end of Edward's reign and the Domesday survey. This compares to today, when there is much less erosion of the beach and cliffs at Dunwich. Do we know what factors have slowed down erosion in recent times?

Bits of stonework from Dunwich churches hauled up during the Dunwich Dives. These are thought be be fragments of St Peter's Church, All Saints and the Templar Church. Photo: Matt Salusbury

The slow down in erosion has been marked in the last 20 years but is part of a pattern of slowing down that started around the 1920s, and may be related to the growth of the off shore Dunwich banks. Wave energy and direction at a coast is largely driven by wind strength and fetch – the distance over which the wind can interact with the sea surface to create waves. As waves approach the coastline, shallowing depths increase the friction on the water and this slows down the movement of the water, causing the waves to increase in steepness until unable to support themselves, they break, releasing the energy stored in the wave and driving sand and shingle transport (movement of sand and shingle due to the action of the waves). Off shore bars do the same although not always to the point of wave breaking.

The Dunwich bank (formerly two banks), grew and coalesced into a single bank sometime between 1867 and 1922. At the same time the depth over the banks shallowed by two meters. Wave modelling has demonstrated that the energy of wavs at the coastline at Dunwich is reduced by the Dunwich bank so one part of the story must relate to this natural protection. In the last 20 years the bar depths have deepened from a peak in 1980s as the Dunwich bank has flattened and migrated towards the coastline.

Dunwich cliffs and beach with the remains of All Saints Church still visible, at the turn of the 20th century. Dunwich Museum's Nicholson Collection of postcards, out of copyright.

The other main cause of slow down in cliff erosion is the sustained presence of a protecting beach. To get this you need a supply of sediment to offset the removal of beach shingle by wave action. The slow down since the 1920’s may in part be a result of the period of high rates of cliff erosion, coupled with a reducing wave energy that tipped the balance in favour of net accumulation of sediment at the tow of the cliffs at Dunwich. Subsequently, revegetation of the cliff face, and continual sediment supply from updrift areas has maintained a buffer between the sea and the tow of the cliffs. Recent growth of sea cabbage onto the single at the back of the beach at Dunwich points to the stability of the beach in this area. Cliff erosion over the last 10 years has been mainly caused by saturation and flows resulting from long periods of wet weather and intense rainstorms, coupled with animal burrowing and tree fall. Since these processes are very localised, so too is the cliff collapse.

The challenge in all this is that while the cliffs at Dunwich (by which I mean from the Flora Tearooms behind the beach to the end of Greyfriars monastery wall) are relatively stable now, the accumulation of beach material inevitably means a lack of material to beaches down drift unless there is sufficient supply and transport to supply them. What will be interesting to look out for is a change in the activity of the cliffs from Greyfriars to Minsmere naoture reserve, and to see how the beach elevations change along this section.

MS: At the recent Dunwich Museum talk by Graham Scott of Wessex Archaeology on the Dunwich Bank Wreck earlier this year, he said the wreck was being buried, with a lot of it buried since its discovery in the 1990s by Stuart Bacon. You said in the chat during the talk that there was the prospect of the wreck being uncovered again by some natural processes. (Did I get that right?) Can you explain that?

DS: One thing we know from Stuart Bacon's descriptions of diving at Dunwich is that the sea bed – by which I mean the sand banks, are highly changeable. He writes of ruins visible in one dive, being no longer there by the next time he dived. Overlaying historic bathymetric charts of the sea bed confirms this dynamism over the last 200 years (See Fig 43, pg 106 Sear et al 2012 Report to English Heritage) – documenting the movement of millions of tons of sand and shingle by tides, storms and waves. Over the time since I worked on the Dunwich surveys starting in 2008, the Dunwich bank has moved closer towards and coast and flattened, partly burying the sites of St Peter's church, St Nicholas's church and St Katharine's chapel. At the same time, reducing sand depths in the eastern part of the site may reveal some of the ruins of earlier churches like St John's and St Martin's.

Similarly, shifting sand bank around the Dunwich bank will bury and re-expose the site many times. Since we do not monitor these changes very often – it is only when divers or surveyors work on them that we discover what the conditions are – I've often though it would be fun to have a small buoy anchored over the ruins of St Peter's church, the largest area of ruins so far discovered, measuring the turbidity (the cloudiness or haziness) of the water and taking webcam images during periods when the water was clear during daytime. These could be relayed by an internet link into a screen in the Museum.

MS: Do we know whether the story of Dunwich being lost to the sea is one of constant erosion, or were there periods in history when the erosion reduced or stopped, or even the coastline recovering?

The cruel North Sea has claimed the city of Dunwich over the years. Photo: Matt Salusbury

DS: We do indeed, thanks to extensive research and analysis published in Sear et al. (2012) Dunwich Project Report 5883: Mapping and assessing the inundated medieval town – free to access on the Dunwich - the Search for Britian's Atlantis website or from Historic England Historic England.

Cliff erosion at Dunwich and indeed the whole Suffolk coastline is driven by two processes; the presence or absence of a protective beach at the toe of a cliff which significantly influences the rate of erosion and the height and frequency of large waves relative to beach height. Cliff erosion by the sea can only happen if the toe of the cliff is reached by waves of sufficient power to remove cliff materials. Conversely, the height of a beach is the result of the balance between the volume of sediment being supplied to the beach and the rate at which that sediment is being transported away from that point. Thus, the dynamics of the Suffolk coastline are strongly linked to the processes that generate cliff erosion and the transport of sediment.

Over the last one thousand years, the principal influence on these processes has been storm surges and storm waves, whereas the rise in relative sea level has had less effect locally. (Burningham and French 2017, 84; Sear et al. 2008; 2012, 14; Hamilton et al. 2019, 155; Shennan et al., 2018, 150.) This is because the direction and the magnitude of waves — their power and height — is the main influence on the volume and direction of sediment transport along this coast. The passage of very low pressure systems into the North Sea drives storm surges and creates high sea levels independent of wave height which flood low lying areas, breach shingle barriers, and erode beaches and cliffs: such as occurred in 1953 or the more recent 2013 event.

Intervening periods of lower wave energy tend to reconstruct breaches in barriers and to elevate beaches after normal winter storms. Another feature that can also influence the transport of sediment is the presence or absence, and the growth and decline, of off shore banks, because these alter the amount and direction of wave energy at a given point along the coastline. For example, the northern extension of an off shore bank at Dunwich during the early twentieth century appears to have reduced wave energy at Dunwich cliffs, contributing to the rebuilding of the beach and the reduction in the speed of cliff erosion.

The direction and size of waves, and the elevation and direction of storm surges, are ultimately controlled by the strength and direction of storm tracks from the north Atlantic Ocean. Storms that track over the north of England and Scotland are formed of low pressure systems, which create storm surges that travel south down the North Sea, and which are then followed by gales from the south and south east as the low pressure system heads east. These gales generate large waves from the south east that transport sediment north along the Suffolk coast.

Conversely, storm tracks passing along the English Channel or from the south create large storm waves from the northeast, driving drift south along the coast. The dominance of northerly or southerly storm tracks over the British Isles is caused by variations in the jet stream position over the north Atlantic, and by the spatial patterns and magnitudes of northern Atlantic ocean temperatures. In combination, these create low pressure systems that determine their route of travel over Britain. A measure of the pressure gradients in the North Atlantic, and by extension a measure of whether northern or southern storm tracks are dominant in the North Sea, is the North Atlantic Oscillation or NAO. During periods of positive NAO, the dominant storm tracks are generally from the north and followed by south to south easterly gales, which transport shingle north along the Suffolk coast. During negative NAO, storm tracks from the south generate north easterly gales with large waves, which transport shingle south along the Suffolk coast.

In addition to these natural forces, human interventions also alter the process of sediment transport and the patterns of drift and accretion along the coastline. Attempts to protect beaches and prevent cliff erosion by constructing groynes to alter the rate of longshore drift are well known.

So to answer your question directly, during the late 19th century into the early 20th century, beach levels at Dunwich cliffs were very low, and so it did not take large wave heights to reach the tow of the cliff. The frequency of erosion was higher which, coupled to a period of positive NAO and large storms combined to increase cliff erosion rates to their highest in the last 150 years – for the period 1894 – 1906 cliff retreat at Dunwich average 8.8m per year. Between 1930 and 1970 this fell to less than 0.5 metres a year, before rising to 2.8 m per year in the early to mid 1990’s before again falling to less than 1m a year since 2007. Rates of cliff retreat between 1695 – 1765 when the Church of St Peter’s was lost, were around 2-3m per year.

If we look at these periods of high cliff erosion, we see that they coincide with periods of high storm frequency and severity along the East coast and southern North sea basin, with positive winter NAO, and where we have evidence, these also coincide with periods when beach height were lower. In short, the cliffs at Dunwich are both a source of sediment to down drift beaches, but also a part of w wider natural system involving off shore bar growth and climate that together result in periods of stability and erosion.

MS: I remember being told when my family moved to Dunwich at around the turn of the century that Greyfriars had about 40-50 years left before it was lost to the sea. Do we have any estimate or projection of how long what's left of Dunwich will remain before we lose it to the sea?

Greyfriars monastery has an estimated 50-80 years before it's lost to the sea. Photo: Matt Salusbury

DS: Yes, in the Sear et al (2012) report, Fig 49, pg 114 there is a map showing the projected cliff line in 2050 and 2100. The good news is that based on extrapolations over the past century of cliff retreat, Greyfriars ruins will still be standing in the main although closer to the cliff top of course. The Pales Dyke and south east perimeter wall is likely to disappear over the cliff in the next 50-80 years. There is considerable uncertainty in these predictions and as past analysis shows, there are times when cliff retreat is much more rapid (the later 19th century for example – see above) – it is therefore highly dependent on two connected processes – rate of sediment movement from the beach in front of the cliffs and frequency and magnitude of storms. Currently the beach level if relatively stable at the toe of the cliff and cliff retreat is slow and largely determined by rainfall and slope processes.

Loss of the beach however will expose the toe of the cliff to much more frequent erosion by the sea during storms, and like the Francis Frith photos of the aftermath of the October storm 1911, drastic removal of the beach can expose the cliffs to erosion at high tides. For now, we can be reasonably relaxed about the future of Greyfriars, but residents can help by keeping a careful watch on changes in the beach and cliffs, which alongside regular surveys by the Environment Agency will provide the best early warning that the system is changing.

MS: Are there any questions I’ve forgotten to ask, or anything else you'd like to add?

DS: Enough I think, although a controversial question is the role of local coastal defences like the mesh bags of shingle – the evidence is that the cliffs and beach are stabilised naturally, so the effectiveness of the coastal protection works is uncertain. What they will do – if they are working is reduce the supply of shingle to downdrift areas, which means that these may get starved of sediment and the balance could tip towards beach loss in those areas. If this results in beach lowering then the larger storm waves might start to access the toe of the cliffs in those down drift areas, creating the condition for cliff erosion. In other words – local interference with the natural system has implications elsewhere and knowing that helps people make better informed decisions, one of which might be to accept it, but perhaps do local monitoring to check the health not just of Dunwich cliff beaches where they are protected but also downdrift.

The top a mesh bag containing shingle protrudes from Dunwich Beach - it's not clear whether these coastal defences have had any effect in stabilising the cliffs. Photo: Matt Salusbury

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