UK salt marshes in retreat: anthropogenic versus
natural origin: Group J
Salt marshes:
• Range in size from a few mm’s to an entire estuary
• Found on the fringes of estuaries, lagoons and bays
• Generally found where sediment deposits are sheltered from wave
action
• Also above the levels of the high neap tide
• Formed on areas of mudflats where vegetation eventually takes hold
Salt marshes have important functions both as natural habitats and coastal
defence.
• Form important roosting areas and nesting sites for birds, including
species of international importance
• Act as natural buffers in terms of sea defence, absorbing the impact of
wave energy and reducing the probability of breach of defences
The principle natural threat to saltmarshes is erosion, which is caused by a
number of factors,
• Migration of estuarine channels
• Changes in coastal nearshore profile
• Reduction in sediment supply
• Increase in sea level
• Increase in storm wave activity
• Damage to the vegetation, caused by disease or environmental stress
The erosion of saltmarshes takes numerous forms, including
• Landward retreat of the marsh edge as a cliff, or a seaward dipping
ramp
• Internal dissecation of the marsh by widening, deepening and
headward extension of the tidal creeks
• Large scale death and removal of vegetation, lowering parts of the
marsh surface
Major salt marshes are found in southeast, northwest, eastern and
southern England and in southeast and mid west Wales.
Salt marsh erosion is not occurring in all of these areas. For example the
Dyfi Estuary in mid west Wales, is undergoing substantial salt marsh accretion
[1]. Most saltmarshes in northwest England have shown stability and lateral
accretion in the past two decades.
Net erosion over the same period has occurred on the southeast and
southern English coasts, the Severn Estuary and along the shores of the
Bristol Channel [2].
The retreat of saltmarshes has numerous causes, which can be divided
into different categories.
Biological causes
• Coastal wetland such as saltmarshes and mangrove forests are likely
to be heavily impacted by sea level rise if they are inundated with sea
water and can’t migrate inland sufficiently to compensate
• Temperature increases may disrupt peat accumulation: more peat will
be broken down under higher temperatures as microbial activity
increases (Stevenson et al. 1986).
This may however be offset by increased production of root and other
plant parts (and subsequently, peat) from increased CO2.
The Humber Estuary: An example of destabilising biological action
• The erosion threshold is reduced as algal benthic mats become
sodden and release sediment that acts as a binding force. Macoma
Balthica (clam) breaks up sediment leading to reduced accretion,
leading to increased erosion. This was seen in July 1996-97 when
reduced density of Macoma on the tidal limits led to increased
accretion.
Geological Causes
The present rate of mean sea level rise is estimated at 3.8mm/year
(conservative) to 20mm/year (high). This is due to
• Isostatically related upward movement of relative sea level in UK due
to positioning on the forebulge of the former Devonian ice mass to the
north [2]. This is shown in the Wash area of the eastern English coast,
which has a crustal subsidence rate of 0.91m/year [3].
• Warming of the atmosphere occurring naturally due to changes in solar
radiation - being further increased by ‘Greenhouse Effect’. Increased
global temperature leads to melting of polar ice caps, which leads to
increased mean sea levels.
It appears that the nature of the tides plays a very significant role in
determining whether marshes in any given area are eroding or accreting. In
many areas where the salt marshes are eroding (Essex and north Kent), there
is an asymmetry in the shallow water tidal curves so that the flood is longer
than the ebb. This leads to a higher current velocity on the ebb which means
that there is a net seaward transfer of sediment.
Natural Retreat
The concept of natural retreat as a coastal management method has many
advantages, and can work to minimise further detrimental anthropogenic
effects.
Case Studies
Some example of the success of natural retreat are given.
Bridge Creek, Dengie Peninsula, southeast Essex [4]
The rate of sea level rise is approximately 3mm/year, but the salt marsh
vegetation has shown no signs of stress due to increased submergence
indicating continued accretion of the marsh surface. It has been shown from
aerial photos that the marsh front has retreated over 40m since 1955 due to
erosion.
The area shows an uneven sedimentation rate, with erosion being
dominant at the seaward margin of the marsh and the marsh surface being
dominated by deposition, with deposition levels being higher at the seaward
edge of the marsh. Despite the uneven distribution all areas of the marsh
appear to be receiving sufficient sediment to maintain the elevation of the
marsh despite the continuing sea level rise.
Fig1. Examples of net erosion/deposition measured at points along Bridge
Creek [4]
The findings at Bridge Creek can be used as template for the maintenance
of salt marshes in areas subjected to relative sea level rise.
Lymington and Keyhaven Saltmarshes, Hampshire
In common with the rest of the Solent maritime region, this site’s
geomorphological evolution since the late Pleistoene, from a coastal fluvial
valley to a coastal embayment, is continuing.
Most of the saltmarsh lies within the Hurst spit and Lymington estuary.
This shelter and reduced wave energy environment together with the shelter
from the Isle of Wight is primarily responsible for its formation and existence.
Acceleration of intertidal zone narrowing rates (as the edges of the marsh
retreat), coupled with salt marsh losses over large areas as a result of
spartina anglica decline is of significant local concern.
Reasons for the saltmarshes retreating are as follows,
• The tidal regime comprises and asymmetrical tidal curve with flood tide
duration being longer than the ebb tide duration. This results in greater
ebb current speeds, (approximately 1.1 to 1.5 times greater than the
flood tide)
• Wind/wave climate is controlled predominately by SW winds and
waves of local origin. Extreme wave conditions are possible as a result
of both easterly winds and a combination of strong winter winds with
strong ebb tides
• Sediment characteristics suggest relatively low suspended sediment
concentrations, but high concentrations in the water along the front
edge of the marsh
• Relative sea level rise is accelerating and is reflected in increasingly
frequent storm surge events
• Saltmarsh vegetation dominates the intertidal area at a height of 2.5 to
2.9m (above chart datum). Mean water depth over the salt marsh is
0.2 - 0.5m during spring tides, therefore the vegetation serves to
decrease current velocity and wave energy.
Fig2. Location of Lymington and Keyhaven saltmarshes
Severn Estuary, Bristol Channel [2]
The regression or retreat of a marine coastal environment depends on the
balance between the supply of sediment and the rate of change of relative
sea level.
It has been established that the Severn Estuary system has recently
reached maximum capacity and has been retreating inland.
Therefore its pattern of retreat can be explained by the continuing upward
trend of relative sea level, and is only a part of the continuing cycle of
deposition and erosion is it in.
Fig3. Model for the evolution and retreat of the Severn Estuary over postglacial time under the influence of a rising relative sea level [2]
Benacre Broad, Suffolk [5]
• Saline lagoon with associated reedbeds occupying a shallow glacial
outwash valley
• Broad is separated from the sea by a low shingle bar, reduced in width
and height as the coastline has rapidly receded in recent years,
leading to more frequent breaching
• Broad now reduced in area and becoming fully saline rather than
brackish, causing extensive die-back of reedbeds, and adverse effects
on lagoonal invertebrates
Since repair of the bar was deemed too expensive, a low cost retreat
scheme was created to allow continuation of the naturally occurring retreat,
with some preventative measure to ensure the continuing presence of the
lagoon invertebrates for which the site was noted.
Two low earth bunds were constructed to limit the frequency of breaches
of the shingle bar and so preserve conditions of comparatively low salinity,
with the seaward bun being lower and providing conditions intermediate
between the open lagoon and the reedbeds between the two structures.
Fig4. Example of managed retreat for flood defence [5]
Managed retreat is described as being ‘a policy of stepping back from
the presents show defence location and allowing the shore to develop the
wider profiles which it would adopt under a natural regime’, [5].
Managed retreat schemes are not a ‘ do nothing’ option. The proposed
retreat must be carefully planned, modelled and predicted using knowledge of
estuarine history, hydrology and sediment behaviour and feedback from
successful natural retreat examples, whilst taking into consideration the
specifications of the planned site such as sedimentation rates, species
present and local tidal processes.
The effects of the retreat on the soil chemistry, tidal regime and wave
climate need to be considered before the retreat can be implemented.
Natural and Managed retreat schemes both have examples of successes
and failures, they both have merits and flaws and all aspects of their
implementation should be investigated before they are employed.
References
[1]
Shi Z (1993) Recent saltmarsh accretion and sea level fluctuations in
the Dyfi Estuary, central Cardigan Bay, Wales, UK. Geo-Marine
Letters, 13: 182-188
[2]
Allen JRL (1990) The Severn Estuary in southwest Britain: its retreat
under marine transgression, and fine-sediment regime. Sedimentary
Geology, 66: 13-28
[3]
Tooley MJ, Jelgersma S (1992) Impacts of Sea Level rise on European
Coastlines. pp 83, 76
[4]
Reed DJ (1988) Sediment dynamics and deposition in a Retreating
Coastal Salt Marsh. Estuarine, Coastal and Shelf Science, 26: 67-79
[5]
Pethwick J, Burd F (1999) Coastal defence and the environment: a
guide to good practice. London: MAFF
[6]
French PW (1999) Managed retreat: a natural analogue from the
Medway estuary, UK. Ocean and Coastal Management, 42: 49-62
[7]
Nicholls RJ, Klein RJT (1999) Adaptation Frameworks for sea-level
rise impacts
www References
1
http://www.ipieca.org/publications/oilspillsummaries/saltmarshes.html.
'Saltmarshes' - a summary of the IPIECA report
2
http://www.solentforum.hants.org.uk/othercoast/western_solent_smp.htm
#saltmarsh. Western Solent Shoreline Management Plan
3
http://www.whoi.edu/seagrant/education/focalpoints/shoreline.html. WHOI
Sea grant education and outreach.
4
http://chomsky.arts.adelaide.edu.au/Geogenvst/adams/abcostal.htm Coastal
Environments Adelaide university
The Debate
Introduction
Hannah Uttley
Hello, my name is Hannah Uttley and we are group J. Our title for discussion is UK
Saltmarshes in retreat and we are arguing the side of the natural origin.
Coastal salt marshes are environments high in the intertidal zone, which are covered
by halophytic vegetation. The lower limit of pioneer marsh vegetation corresponds
approximately with the level of mean high water of neap tides, while the potential upper limit is
slightly above the level of mean high water spring tides. In many areas algal-covered mud or
sand flats occur to seaward of the salt marsh zone, while the landward margin is formed by a
transition zone to brackish and freshwater communities, or by an artificial embankment.
According to A. Pye and his work in 1992, active salt marshes play an important role
both as natural habitats and as coastal defences. In ecological terms, marshes are
particularly important as roosting areas and nesting sites for birds, including a number of
species of international importance. In sea defence terms, Brampton 1992, states that salt
marshes which are located in front of earth embankments act as a natural buffer which
absorbs the impact of wave energy, thereby reducing the probability of overtopping and
breaching. During storms, erosion of the saltmarsh edge serves to resupply the fronting
mudflat with sediment, thereby reducing the rate of lowering of the intertidal profile and
encouraging the establishment of a new equilibrium with the indicant waves (Pethick, 1992a,
1992b).
The principal natural threat to saltmarshes is erosion, which may be induced by a
number of factors including the migration of estuarine channels, changes in coastal near
shore profile, a reduction in sediment supply, an increase in sea level, or an increase in storm
wave activity.
Coastal wetland such as saltmarshes and mangrove forests are likely to be heavily
impacted by sea level rise if they are inundated with sea water and cannot migrate sufficiently
inland to compensate. The will be possible destabilisation of biological action. For example in
the Humber estuary the erosion threshold is reduced as algal benthic mats become sodden
and release sediment that acts as a binding force. Balthica breaks up sediment leading to
reduced accretion leading to increased erosion. This was seen in July of 1996-97 when
reduced density of Macoma on the tidal limits lead to increased accretion.
The erosion status of saltmarshes around the coast of England has indicated that the
majority of erosion in salt marshes has generally been confined to areas where there have
been recent shifts in channel position). A high proportion of marshes in Suffolk, Essex and
north Kent, and on the south coast in West Sussex, Hampshire and east Dorset, have
suffered net erosion in the last 20 years (e.g. Gray & Pearson, 1984). Lateral accretion (e.g.
at Sheliness, Isle of Sheppey), has been the exception, rather than the rule in these areas. In
the southwest most of the small estuarine Marshes in Cornwall and Devon have remained
stable, but active marshes along the shores of the Bristol Channel and Severn estuary have
suffered widespread erosion of the marsh edge.
It appears that the nature of tides plays a very significant role in determining whether
marshes in any given area are eroding. In many areas where the salt marshes are eroding
such as Essex, of N, Kent for example, there is an asymmetry in the shallow water tidal
curves so that the flood is longer than the neap. This leads to a higher velocity on the ebbs,
which therefore leads to a net resource in the transfer of sediment.
Neil Trickett
Conclusion
Alan Watson
As we have already heard in the introduction, the principal natural threat to
saltmarshes is erosion. However, as stated on the Woods Hole Sea Grant project website,
without erosion, many of the Commonwealth's biologically productive bays such as estuaries,
saltmarshes, and tidal flats would not exist. Therefore not only do natural processes erode the
coastline they also seem to battle their own processes by providing a natural buffer to their
own actions
Currently a number of councils in the South are working alongside the Environment
agency to formulate the Western Solent and Southampton Water Shoreline Management
Plan, the SMP. Saltmarshes are an important part of the Shoreline management plan
because they are abound in this local area.
Recent studies have shown that the saltmarshes of the Western Solent and Southampton
Water are eroding. Throughout the SMP it is recommended that the saltmarshes be
monitored and, as far as possible, maintained, for both their nature conservation and coastal
defence importance. An overall Saltmarsh management strategy to preserve these important
natural landforms is also presented within the Shoreline Management Plan.
The Solent forum is working alongside the Sea Level Rise workshop at the University of
Portsmouth. One of the three main research priorities of the workshop is to look into the
managed retreat of saltmarshes. This obviously indicates how important the participating
councils and the environment agency believe saltmarshes to be as a means of defence
against coastal erosion.
Earlier in our presentation Neil briefly mentioned that the BBC News web page
recently featured an article by Dr David Whitehouse. In this article Dr. Whitehouse addressed
research by the Rutherford Appleton laboratory in Oxford, which provided evidence for an
increasingly energetic Sun. The importance of this is that solar magnetism is closely linked
with sunspot activity and the strength of sunlight reaching Earth. This increase could have
produced warming in the global climate, a theory that is supported by Professor Eugene
Parker of the laboratory for Astrophysics and Space Research at the University of Chicago.
With the principal threats to saltmarshes including rising sea levels and the changes in
coastal nearshore profiles then the increasingly energetic Sun perfectly illustrates just one
example of the natural origin of retreating saltmarshes.
Around 15,000 years ago, ice sheets started melting (as has happened hundreds of
times over the past couple of million years), and sea level rose rapidly around the world.
Sea levels 'plateaued' about 6,000-4,000 years ago, at perhaps a metre or two higher than
today's sea level. What has happened since then is that the ocean basins have slowly dipped
under the extra weight of the postglacial meltwater, this has dragged the sea level back down
a bit.
This 'mid-Holocene optimum' was also a slightly warmer world, global ice volume was
slightly less, and given the warmer temperatures the ocean water was slightly thermally
expanded. So the higher mid-Holocene sea level was the effect of a combination of natural
events.
These higher sea levels obviously destroyed many saltmarshes, however as we can
see by the abundance of saltmarshes today, they have excellent powers of recovery.
Therefore, unless our ancestors of 15,000 years had ago ran big cars and had large
factories, which fossil records seem to suggest they didn’t, then we, Group J can accurately
state that saltmarshes have been forced to retreat before by natural processes and it is the
same natural processes that are forcing them to retreat again.
Question 1
Ruth Pratt
[Group H]
A conservative estimate from Tooly and Jelgersma in 1992 stated that sea level is
rising by 3.8 mm per year. This increase in sea level is due to an increased global
temperature, which causes thermal expansion of sea water and the melting of polar ice
sheets. Ice cores show that temperature patterns are closely related to the level of carbon
dioxide in the air. Due to the burning of fossil fuels, the concentration of carbon dioxide in the
atmosphere has augmented over the years, an increase mirrored by global temperature.
Humans also release many other greenhouse gases, such as methane, CFCs, nitrous oxide
and sulphur dioxide into the atmosphere.
Is it your opinion that the anthropogenic inputs that contribute to global warming are
outweighed by natural causes?
Answer 1
Andrew Teuma
The Wash, an area of about 70 miles along the East coast of England from Skegness
to Hunstanton, has a crustal subsistence rate of 0.91m per year. This is due to the continual
down tilting of S.E. Britain caused by the former Devonian ice sheet fore bulge, which once
covered and forced down Northern Britain. This rapid, isostatically related sea level rise rate,
along with the increase erosion rate due to rise in storm frequency has overtaken the marsh’s
aggradation rate. This has lead to a reduction rate of salt marsh area of 4% per annum along
the East Coast.
Anthropogenic causes, including increased green house gas emission leading seawater
thermal expansion and water displacement due to glacial rebound together with terrestrial
water redistribution, have increased global sea levels. However it is near impossible to put a
figure to the rate of increase due to human causes. The present maximum rate quoted is
20mm per year.
Even this maximum rate of increase is relatively small when compared to the local
sea level rise due the crustal subsistence on the East Coast. Some sources doubt that human
activity has a significant influence on global sea level. Current research into global magnetic
flux, which is directly related to sunspot activity and therefore global insulation, could prove
that the increase in sea level rise rate in the last century could be related purely to sun spot
activity.
With these facts is it possible to claim anthropogenic factors dominate in causing salt
marsh regression along the East Coast of England?
Question 2
Phillip Parker [Group H]
Saltmarsh retreat in the last 50 years in SE England has been 40% since 1973.
Average loss over the whole of the UK has been 22% between 1978-88, equalling 6 500ha in
15 years (Pye 1995).
In relation to Benacre Broad, Suffolk ‘Coastal Defence and environment: a guide to
good practise F Burd and J Perthwick’ [Case study 5 of your essay], can you justify that a lack
of coastal management to a rapidly receding coastline around Suffolk will have any beneficial
consequence to the prevention and overall long-term conservation of the Broad, in respect to
the receding shingle bar. As a result can you prove that without anthropogenic intervention
the Broad is likely to survive in the long-term as a brackish water environment, causing the
die back of reed beds and lagoonal invertebrates, thus reducing biodiversity of the area?
Answer 2
Richard Stokes
The case study of Benacre Broad is not an example of a lack of coastal management.
A managed retreat scheme was put in place, based on experiences at other sites of
successful natural retreat in the Broads area.
The initial cause of the managed retreat scheme mentioned was cost – it was
deemed too expensive to provide hard defences to that area of coastline, and so the opening
of a channel in the existing deteriorating defences was made, to reopen existing creeks and
allow the gradual
Question 3
Ruth Pratt
[Group H]
The present position of many of the coastal defences on the southeast coast of
Britain are not sustainable due to sea level rise. In areas of low urban and industrialisation, an
economically viable option is that of managed retreat. Managed retreat is basically
abandoning the area and letting natural processes occur. This scheme will cause the loss of
many habitats, which has wide economic and environmental implications. If the sea level
continues to rise, do you think managed retreat is the best option, as erosion will continue
until it reaches some solid, non-erodable feature?
Answer 3
Vicky Stedall
Managed retreat is currently the most natural process being used as a coastal
defence system. By creating saltmarshes we provide a situation that erodes slowly as it is in
virtual equilibrium with the sea. Wave action is also reduced due to the natural slope of the
marsh.
Currently sea walls around the UK are expensive to maintain and only divert the
erosion to another area. They may be worthwhile for saving current human habitats from
erosion but only those in immediate danger require such drastic protection.
If we use saltmarshes to protect our coastlines we benefit in a number of ways.
1. They are cheaper to install and they maintain themselves.
2. They are attractive.
3. They provide good natural reserves for wildlife
The general worry is that allowing the land to be converted into salt marshes forces the
country to lose agricultural land. In reality a large number of agricultural land is laid to rest
each year because the country produces too much farming produce. It is clear that some of
this land could be used for managed retreat providing farm owners with compensation.
In February 2000, the government brought a 750-acre farm called Abbots Hill on the Essex
coastline. The plan is to destroy the 500-year-old seawall, which is in great need of repair,
allowing over 200 acres of the farm to become saltmarsh. They believe that this will slow the
rate of erosion while creating a good wildlife habitat.
So in answer to your question, in my opinion Managed retreat is the best option
currently available to us. Admittedly without a sea wall erosion will continue to occur possibly
until it hits a hard substrate, however this would have occurred naturally and with the use of
managed retreat we can slow this process dramatically. Finally, I believe that it is surely
better to slow down erosion than to prevent it in some areas only for it to damage others
areas more.
OC2.07
Group J
8
UK salt marshes in retreat: anthropogenic versus natural origin
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