Report on the Salt Marshes at Marshside,...
Transcript of Report on the Salt Marshes at Marshside,...
Coastal Defence
Report on the Salt Marshes at
Marshside, Southport.
Version 1.1
March 2008
Prepared by: Vanessa Holden Natural, Geographical & Applied Sciences Edge Hill University St Helens Road Ormskirk Lancashire L39 4QP Email:[email protected]
Sefton Council Technical Services
Balliol House Balliol Road
Bootle L20 3NJ
Tel: +44 (0)151 934 4238 Fax: +44 (0)151 934 4559
www.sefton.gov.uk
Title Report on the Salt Marshes at Marshside, Southport Creator/Author/ Originator/ Publisher Sefton Council Date of publication March 2008 Contact name or title of Location Coastal Defence, Sefton Council
Subject - Keyword Keyword – Free text Southport, salt marsh Description/Abstract The natural processes occurring within the salt
marsh at Marshside are discussed, along with the evolution of the salt marsh system over time, and the nature of the current situation. Potential future changes are identified, notably with regard to sea level rise and the implications to the Marshside salt marsh.
Identifier Coverage - Spatial Southport, Merseyside, Coverage - Temporal 1800’s to 2006 Format/ Presentation type Document Digital Type Report Subject - Category Coast Subject - Project Language English Rights - Copyright O/S maps reproduced under licence number LA
076317 by Sefton Metropolitan Council from the Ordnance Survey’s 1:50,000 map with the permission of the controller of Her Majesty’s Stationary Office Crown Copyright reserved
Rights - EIR disclosability indicator
Rights - EIR exemption Rights - FOIA disclosability indicator
Rights - FOIA exemption Postal address of location Ainsdale Discovery Centre, The Promenade, Shore
Road, Ainsdale-on-Sea, Southport Postcode of location PR8 2QB Telephone number of location
+44 (0)151 934 2960 Fax number of location +44(0)1704 575628 Email address of location [email protected] Online resource www.sefton.gov.uk Date of metadata update
This report should be referenced as: Holden, V.J.C. (2008) Report on the Salt Marshes at Marshside, Southport. Sefton Council, Bootle. Unless otherwise stated all photographs © Vanessa Holden
Document History Date Release Notes
March 2008 1.1 Final Draft
Prepared _____V. Holden_______________ Approved ____________________________ Authorised ____________________________ Copyright Sefton Council Sefton Council accepts no liability for the use by third parties of results or methods presented in this report. Sefton Council also stresses that various sections of this report rely on data supplied by or drawn from third party sources. Sefton Council accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data.
Contents Section Page 1.0 Introduction 1 2.0 Background to the Sefton Coast 1 3.0 Introduction to Salt Marshes 3 3.1 Morphology 4 3.2 Sedimentology 9 3.3 Ecology 10 4.0 Historic Evolution of Marshside Salt Marshes 4.1 Pre-1970s 13 4.2 1970s to Present 16 5.0 Current Situation 24 6.0 The Future 6.1 Potential Future Changes 29 6.2 Management 32 7.0 Summary 34 8.0 References 35
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1. 0 Introduction This report is part of a series of reports designed to give a detailed account of a
particular feature on the Sefton Coast as part of the process of updating the Sefton
Coast Database. Updating of the database includes analysis and interpretation of
existing materials held by the Council which will identify gaps in knowledge and future
works that could be undertaken to improve our understanding of the geomorphology of
the Sefton Coast. This particular report discusses the natural processes occurring within
the salt marsh at Marshside, the evolution of the salt marsh system over time, and the
nature of the current situation.
2.0 Background to the Sefton Coast
The Sefton Coast, which extends over 34 kilometres (21 miles), is comprised of soft
and granular deposits of sand, silt, clay and peat. There are no outcrops of rock on the
shoreline. Hence, the forces of nature readily mould it, so the shoreline is constantly
changing in response to the fluctuating influence of wind and water and as a result of
human activity. Its overall shape derives from two major river estuaries, the Mersey
and the Ribble. The River Alt and the Crossens Channel, each have important local
zones of secondary influence.
Figure 2.1: Generalised Sefton Coast landscape.
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The coast is a long wide arc of sand (Figure 2.1) with a hindshore dune system, which
at one time would have stretched from the Mersey Estuary to the Ribble Estuary.
Human use of the dune system over several centuries has created a dune landscape of
great variety. To the north of the Sefton Coast is an extensive area of Saltmarsh
extending into the Ribble estuary; other smaller areas of Saltmarsh also occur at the
River Alt and Smiths Slack (located on the foreshore between Birkdale and Ainsdale).
Several towns have developed along the coast; at Crosby, to the south, and Southport,
to the north, artificial defences have been put in place. In-between these areas towns
such as Formby rely upon the sand dunes to provide protection from the sea.
The sand dunes, beaches and marshes of the Sefton Coast are one of the most
important areas for nature conservation in Europe. The entire Coast is designated as
either Special Protection Area (SPA) to the north of the pier at Southport or Special
Area of Conservation (SAC) to the south of the pier, notable species include Sand
Lizards and Natterjack Toads with the estuarine area being very important for birds.
The Sefton Coast is also an important visitor destination with popular bathing beaches,
open countryside, and the seaside resort of Southport.
According to JNCC (2006), 75% of the wetland of the Sefton Coast is tidal flat, with
16% being salt marsh. 8% is sandy shore, with 1 % being freshwater marsh. The
intertidal flats and salt marshes that comprise the Marshside salt marshes are located
on the southern edge of the southern shore of the Ribble Estuary, to the north of the
town of Southport (Figure 2.2).
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Figure 2.2: Aerial photograph of the Marshside Salt Marshes. Aerial photography courtesy of
Sefton Metropolitan Borough Council © Cities Revealed. Orthorectification based on the
Ordnance Survey map © Crown Copyright Sefton MBC licence No. LA076317.
3.0 Introduction to Salt Marshes
The intertidal zone is the area of coastline that is covered by the sea during tidal
inundations (sometimes only during very high tides), but is exposed to the air during
low tide. Salt marshes are areas within this zone that are sufficiently high up in the
intertidal zone to be exposed to the air for long enough periods of time to become
vegetated by specialised plant species.
Southport Pier
Ribble Navigation Channel
Bog Hole Channel
Marshside Marshes
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3.1 Salt Marsh Morphology
There are three different environments or ‘zones’ that can commonly be seen within
salt marshes (Plate 3.1). These are generally determined on the basis of the vegetation
present, and can be divided up as:
• ‘Low’ or ‘pioneer’ salt marsh. This area is generally nearest to the sea, and at
lower elevations, and receives the greatest number of tidal inundations
compared to other salt marsh zones. Large areas of bare mud are usually still
visible. The vegetation in this zone is often limited to a very small number of
species.
• ‘Mid’ salt marsh. Vegetation in this zone remains generally very short, but
covers a much greater percentage of the ground, with a higher number of
different species, leaving less bare mud visible. Characteristic salt marsh
features such as creeks become more apparent.
• ‘High’ or ‘mature’ salt marsh. Plants are much more terrestrial in their nature,
being taller, and often flowering in summer. There is often complete coverage
of vegetation with no bare mud visible. The elevation of the marsh surface is
usually higher than in the previous two zones, so tidal inundations in this zone
can be relatively rare.
These zones are not static, and over time, given the right conditions for salt marsh
growth, the low marsh can gradually become mid marsh then eventually high
marsh. Similarly, if the conditions are unfavourable for marsh growth, the
zonations can revert from high or mid marsh back to low marsh. These conditions
are determined by a number of factors, including mean local sea level, sediment
supply and the management strategies being undertaken, such as realignment. On
a large scale, salt marshes appear to be flat expanses of land sloping towards the
sea, but close-up, there are many features that make the surface far from flat and
uniform.
5
Plate 3.1: The three commonly differentiated zones within a salt marsh: a) low or pioneer zone; b) mid marsh; c) high or mature marsh.
© V Holden © V Holden © V Holden
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Characteristic features of salt marshes, particularly of the mid and high
marsh zones are creeks (Plate 3.2). These are effectively channels that
cut across the marsh surface that fill up at high tide and drain at low
water. They are similar to river drainage channels, forming dendritic,
meandering patterns across the marsh, but with the water flowing through
them in two directions depending on the state of the tide, flowing inland
during an incoming tide, and flowing out to sea with an outgoing tide.
Creeks can range from just a few centimetres to many metres across and
be over a metre deep. In the low marsh zone, the drainage is more
commonly via wide, shallow gullies (Plate 3.3).
Running along the edges of creeks are features called ‘levees’. These are
small mounds of sediment that have built up slightly higher than the
surrounding flat plains between the creeks. They form by the deposition
of sediment that is held in suspension in the water flowing up the creek.
When the creek becomes full and the water floods over the adjacent flat
marsh plain, it instantly loses energy, so the sediment that was held in
suspension in the faster flowing water of the creek is deposited on the
levee. The coarser sediment of the levees results in them having a higher
percentage of air space within the sediment, and draining more quickly
than the adjacent plains (Long and Mason, 1983).
Plate 3.2: A creek at low tide at Marshside.
© V Holden
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Plate 3.3: The dendritic pattern of drainage gullies beginning to form
across the pioneer zone of the salt marsh at Marshside.
A second feature of salt marshes are ‘pans’. These can be classified as
either ‘primary pans’ or ‘channel pans’. The former are roughly circular
depressions on the surface of the marsh. There are a number of theories
as to how and why they form, potentially due to either debris swirling
around during tidal inundation preventing vegetation from growing, or due
to a localised build-up of salt. Channel pans, as their name suggests, are
formed from creek channels that have become cut-off from the creek
system, keeping their elongated shape but effectively becoming an
isolated feature (Plate 3.4 and 3.5). They can become waterlogged
following inundation by the tide, or during periods of high rainfall or high
groundwater, but can dry out in intervening periods.
© V Holden
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Plate 3.4: Two flooded adjacent channel pans at Marshside that had
previously been part of the creek system, but which have become blocked
by vegetation.
Plate 3.5: A channel pan at Marshside.
© V Holden
© V Holden
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3.2 Salt Marsh Sedimentology
Salt marshes have a characteristic sedimentology. The mean grain size of
salt marsh sediments generally decreases landwards, with higher
concentrations of silt and clay size particles with increasing distance from
the sea (Postma, 1961; Evans, 1965; Allen, 1996). This is due to the
velocity of the incoming tide decreasing as the tide advances up the
intertidal zone, so the heavier sand particles being deposited in the higher
energy conditions further down the intertidal zone, with the finer sediment
being able to be held in suspension for longer, so being able to be
transported further up the intertidal zone before being deposited.
In addition, the concentration of sediment held in suspension decreases
landwards, as sediment is deposited as the tide comes in, so there is
gradually less held in suspension as the tide moves further up the
intertidal zone. This results in the low marsh zones building up
(‘accreting’) at a faster rate than the high marsh zones that receive less
tidal inundations and hence less sediment deposition.
3.3 Salt Marsh Ecology
Plants that colonise salt marshes are termed ‘halophytes’, meaning that
they are tolerant of the salt conditions. Two species in particular are
associated with the initial colonisation of the low marsh zone, Salicornia
europaea (Glasswort or Samphire) and Spartina anglica (Common Cord
Grass) (Plate 3.6). The zonation within the salt marsh is strongly reflected
in the vegetation present. In mid and high marsh the number of species
increases, and as the conditions become more terrestrial in nature, so this
is reflected in the nature of the plants. Within individual zones, some
species of vegetation favours specific habitat, for example, Halimione
portulacoides (Sea Purslane) can often be found on the slightly higher
creek banks rather than on the lower flatter marsh plains.
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Plate 3.6: Two characteristic colonising species of salt marsh vegetation.
Plants have to tolerate very specific and harsh environmental conditions
on salt marshes. These include:
• Growing in unstable sediment, particularly during tidal
inundations;
• Being covered by sea water, potentially for hours at a time in
the low marsh;
• Being disturbed by the action of the tide, with water and
sediment moving around the plants. Most plants have
adaptations to cope with such conditions, for example Salicornia
is a succulent with fleshy rounded leaves, which is believed to
help against mechanical damage.
Spartina
Salicornia
© V Holden
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• Waterlogging during and after tidal inundations (Popham, 1966;
Packham and Willis, 1997). Even after the tide has receded, it
can take a long time for water to drain away from the muddy
sediment.
• High salt concentrations within the sediment. Levels are not
only high during inundation, but can also concentrate even
higher as the water dries out of the sediment leaving the salt
behind. Halophytes require a salt concentration in excess of
100-200 mM sodium chloride to survive (Flowers et al., 1986;
Packham and Willis, 1997).
• Anaerobic conditions where there is very little or no oxygen
present within the sediment. These conditions often occur
within only a few millimetres of the surface, and are associated
with black or grey colouration to the sediment as a result of iron
sulphides present (van Straaten, 1952) (Plate 3.7).
The vegetation is a key factor in the development and continuance of salt
marsh. Its presence reduces the energy of the waves during an incoming
tide, resulting in sediment being deposited which allows the salt marsh to
develop further. The presence of the leaves and stems also physically
traps sediment, both from suspension during inundation and wind blown
sediment (Stumpf, 1983; Packham and Willis, 1997) (Plate 3.8). The
roots of vegetation also help to bind sediment together to give it greater
stability during tidal action (French, 1997). When leaves, stems and roots
die, they are also incorporated into the sediment or surface of the marsh
which adds organic matter.
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Plate 3.7: Black or grey discolouration just below the surface of the salt
marsh at Marshside due to anaerobic conditions.
Plate 3.8: The sediment trapping ability of vegetation.
© V Holden
© V Holden
Report on the Salt Marshes at Marshside, Southport
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The fauna of the salt marsh is also key to its survival. Intertidal flats and
salt marshes have been identified as some of the most biologically
productive environments, with primary production being in the region of
500 g m2 a-1 of organic matter in an average estuary (Nelson-Smith,
1977). It is because of this high productivity that such large numbers of
wading birds migrate to salt marshes, as they are rich in invertebrate food
sources. Each tidal inundation brings a replenished supply of sediment
and food for invertebrates, which are predominantly made up of burrowing
species. Lower down the intertidal zone only one or two species may be
predominant, such as Arenicola marina (Lugworm) and Hydrobia ulvae (a
mollusc), with species diversity increasing up the intertidal zone as the
sediment becomes finer (Yates et al., 1993). The invertebrates, along
with microscopic organisms such as algae and diatoms that exist within
the sediment, also secrete slimes and films that are important to bind fine
sediment together (van Straaten and Kuenen, 1957; Coles, 1979; Frostick
and McCave, 1979; Adam, 1990; French, 2007).
4.0 Historic Evolution of the Marshside Salt Marshes
4.1 Pre-1970s
The area around the Marshside salt marshes has experienced notable
human influence since the development of the town of Southport in the
1800s. Developments have included urbanisation with associated
infrastructure development; land reclamation, predominantly for
agriculture; port development with related estuarine channel training and
dredging associated with the Port of Preston to the east; tourism and
recreation; and increases in industry and commerce directly impacting
upon the coastal zone.
Such developments within the Ribble Estuary have had implications for the
evolution of the salt marshes. Particularly, the tendency of the estuarine
environment to ‘infill’ with sediment has resulted in numerous reclamation
schemes along the coastline. A series of reclamations were made
throughout the 19th century (Figure 4.1). Gresswell (1953) recorded the
scale of the reclamations, when he calculated that in the 50 years between
1830 and 1880, over a mile in width of salt marsh had been reclaimed. In
1932, to further encourage sedimentation, ‘saltings’ or evenly spaced
clumps of Spartina townsendii were planted to promote sedimentation at
Marshside (Gresswell, 1953; Berry, 1967) (Plate 4.1). Reclamations
continued throughout the 20th century, with the most recent being an
enclosure just north of Marshside at Hesketh Out Marsh in 1980 (Smith,
1982), which is however, due to be breached as part of a realignment
scheme in the near future.
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The reclaimed land has been used predominantly for agriculture (arable
and grazing land), for urban development, and for leisure activities (Pye
and French, 1993b). The reclamations that have been most significant for
the Marshside marshes were those to enclose the Marine Lake and
construction of the Coast Road during the 1960s and 1970s. The latter
enclosed marshes at Marshside, that are now brackish marshes and are an
important RSPB reserve. This enclosure, however, now provides an
engineered sea wall to the landward side of the active salt marsh and
intertidal flats. This therefore potentially has implications for allowing the
process of ‘coastal squeeze’ to occur (French, 1997; Haslett, 2000). This
would result in the restricted opportunity for landward migration of the
salt marsh if local sea level rise is occurring at a rate with which the salt
marsh cannot keep pace.
Accretion in the Ribble estuary has long been of interest to the decision
makers of Southport. As far back as 1936, A.E. Jackson, the Borough
Engineer of Southport, reported that:
“The Ribble Estuary…on the south side of which Southport is
situated, has been the subject of much interest and inquiry as to
the source of the great accretion which is taking place there,
probably unequalled anywhere in the British Isles except the
Wash”. [A.E. Jackson, 1936; from a paper given at the Health
Congress of the Royal Sanitary Institute].
It is this continued accretion that has permitted the large scale of
reclamations along the coastline. It is calculated that there has been
2,320 hectares of marshland reclaimed since 1800, with the active marsh
area in 1993 being 2,134 hectares, (Pye and French, 1993b). Evidence
from aerial photographs and previous studies (e.g. Pye and French,
1993b; Ribble Shoreline Management Partnership, 1997; Coastal
Engineering UK Ltd, 2005) indicate the marshes are continuing to accrete
laterally and subsequently vertically.
However, changing attitudes towards the positive sedimentation along the
coastline have been indicated by the accretion of muddy sediments in a
southerly direction towards Southport Pier being considered a major
aesthetic issue by the local community. French (1997) remarked that
Southport was experiencing problems with Spartina encroachment and
sediment accretion. The Ribble Estuary Shoreline Management Plan
(2002) also identified sea-level rise and salt marsh spread as two major
issues along the coast. However, the presence of the reclamations
indicates that this negative approach to the presence of the salt marsh
was not always necessarily present.
Figure 4.1: Map showing embankments and probable dates of reclamations on the
southern shore of the estuary. Image
produced by SMBC. (Barron, 1938;
Gresswell, 1953).
Report on the Salt Marshes at Marshside, Southport
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Plate 4.1: Plate of the ‘saltings’ at Marshside, early twentieth century. (Source: Gresswell, 1953, facing p.72).
4.2 Evolution of the salt marsh, 1970s – Present
Figures 4.2 and 4.3, and Plate 4.2 demonstrate the changes in the location
of the marsh edge at the study location over time. In 1976 the southerly
marsh edge was restricted to a small area concentrated around the sand-
winning plant, with a reasonable assumption being that it was the
presence of the plant boundary that provided an agreeable environment
for this initial extension of the marsh. It should be borne in mind that the
final Coast Road extension was only completed in 1974, two years prior, so
the marsh may still have been adjusting to the presence of the sea wall.
Subsequent expansion of the marsh has continued to such an extent that
any further influence from the plant itself is highly unlikely. The track
adjoining the plant, however, appears to continue to exert an influence
over the positive marsh growth.
In a report by R.J. Holliday of the University of Liverpool in 1976, the
‘problem’ of the spread of Spartina townsendii towards the Southport
foreshore is considered. In which, reference is made to previous attempts
to control the Spartina by spraying with Dalapon (a herbicide), but which
17
Figure 4.2: Changes in
marsh edge over time
taken from aerial
photography. Image
courtesy of SMBC.
Aerial photograph ©
Cities Revealed. 1972 1992 1997 1999
Report on the Salt Marshes at Marshside, Southport
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Figure 4.3: Expansion of salt marsh edge over time taken from aerial
photography. Illustration courtesy of SMBC.
Report on the Salt Marshes at Marshside, Southport
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a)
b)
Plate 4.2: Photographs of the same area south of the sand winning plant
(Coast Road to the left of both pictures) highlighting the expansion of the
salt marsh at Marshside; a) approximately 1970s; b) March 2007. Image
a) held in SMBC archive.
© V Holden
Sand winning track
Sand winning track
Report on the Salt Marshes at Marshside, Southport
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had failed, assumed at the time to be due to insufficient application of the
herbicide (Holliday, 1976). The report suggests all Spartina south of
Marshside Road (near to the sand-winning plant) be eradicated by
chemical control. The spread of salt marsh vegetation was subsequently
controlled between 1977 and 1988 by the aerial spraying of Dalapon
(Smith, 1982), with additional recent herbicide treatment in subsequent
years (Tomlinson, 1997).
By 1992, the marsh edge displayed a notable expansion in the area to the
north of the sand-winning plant. The salt marsh had also extended along
the sand-winning track by over 1 kilometre, compared to the relatively
small area in 1976. The sheltered area between the southerly side of the
track and Coast Road experienced sizeable expansion of the marsh
(around 80-200 metres lateral growth). To the north of the track, the
marsh edge had prograded northwards approximately 50 metres along the
outer length of the track, and by more than 200 metres in a north westerly
direction. In just three years between 1992 and 1995, an increase in the
density of the vegetation was evident to the north of the sand-winning
track, and to the southern edge of the marsh alongside Coast Road (Plates
4.3 and 4.4).
Between 1997 and 1999, although some expansion of the salt marsh was
evident south of the track (10 – 20 metres), the majority of the expansion
was seen to the north, with around 100 metres additional marsh, and with
marsh flanking the track having grown laterally by approximately 250
metres. Tomlinson, in his vegetation survey of 1999 / 2000 reported the
growth of Salicornia had been particularly prolific during 1999 (Figure 4.4).
Further north, the salt marsh continued to prograde seawards, extending
by 60 – 200 metres around the north west edge of the Marshside marshes,
reaching a maximum additional growth of 300 metres. Areas of least
expansion of the marsh edge appear to coincide with major drainage
channels, with the sections of marsh furthest from these channels
demonstrating the greatest expansion.
The marsh edge had extended only very marginally along the majority of
the previous edge by 2002. South of the sand-winning track, however,
there was a relatively consistent expansion of the width of the marsh of
around 100 metres, with the most noticeable change being the southward
expansion of the marsh parallel with Coast Road by over 700 metres.
Here, the marsh retained its existing shape, curving around the triangular
piece of land between the road and the track, but it was by now three
times its previous width from Coast Road. The tip of the salt marsh shown
at the defined edge of the track in 1992 and 1999 subsequently did not
change in 2002, following the line of the Ordnance Survey marked Mean
High Water. This suggests that, under the current conditions, the salt
marsh has reached its seaward extent at this particular point.
Report on the Salt Marshes at Marshside, Southport
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At time of writing, the sand-winning plant at Marshside / Horse Bank has
ceased operations due to non-renewal of the extraction licence. Proposals
for the site are to include extended facilities for the Royal Society for the
Protection of Birds (RSPB), and reinstating a proportion of the site to the
level of the surrounding salt marsh, to allow colonisation of the area by
salt marsh vegetation.
Plate 4.3: Aerial photograph of Marshside salt marsh in 1992. Image
courtesy of SMBC © Cities Revealed.
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Plate 4.4: Aerial photograph of Marshside salt marsh in 1995. Photograph
courtesy of SMBC © Cities Revealed.
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Figure 4.4: Expansion of Salicornia spp. At Marshside, March 1998 to
February 2000. (Taken from Tomlinson, 1999 and 2000).
Report on the Salt Marshes at Marshside, Southport
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5.0 Current Situation of the Marshside Salt Marshes
The adjacent intertidal flats of the north Sefton coast are an integral part
of the functioning of the salt marsh system, so will be considered
alongside the ‘true’ salt marsh of Marshside. The intertidal flats have very
little vegetation, and are mainly comprised of sand particles (Plate 5.1).
The low marsh or pioneer zone has a finer texture of sediment towards
silty sand, and is characteristically comprised of Salicornia and Spartina
species of vegetation (Plate 5.2). The mid marsh zone of Marshside
includes the floral species of Halimione (Atriplex) portulacoides (sea
purslane), Puccinellia maritime (sea meadow grass) and Aster tripolium
(NVC, 2002). There is usually complete ground coverage by vegetation in
this zone (Plate 5.3). The vegetation becomes much more terrestrial in
nature in the high marsh zone at Marshside, with species such as Juncus
maritimus, but with continuing considerable coverage by Puccinellia
maritime and Aster tripolium. The vegetation tends to be notably taller in
the high marsh, up to tens of centimetres (Plate 5.4). The sediment is
predominantly fine silt (Pye and French, 1993b).
Plate 5.1: Typical environment of the intertidal flats adjoining Marshside
salt marshes.
© V Holden
© V Holden
Report on the Salt Marshes at Marshside, Southport
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Plate 5.2: Spartina anglica and Salicornia europaea as early colonisers of the
pioneer zone at Marshside. (Body of tape measure = 60 mm).
© V Holden
Report on the Salt Marshes at Marshside, Southport
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Plate 5.3: Mid marsh vegetation at Marshside, including a) Aster tripolium; b)
Puccinellia maritime; c) Halimione portulacoides; d) Limonium vulgare. (Body of
tape = 60 mm).
a)
d)
c)
b) © V Holden
© V Holden
27
a) b) Plate 5.4: a) Aster tripolium on the high marsh at Marshside, measuring around 60 cm; b) The highlighted marker post stands
approximately 60 cm tall.
© V Holden © V Holden
Report on the Salt Marshes at Marshside, Southport
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The rate at which a salt marsh can build up (its sedimentation or accretion rate)
can vary depending upon the localised environment, the stage of development
that the marsh is already at, the local hydrology, tidal energy, the quantity and
characteristics of the inorganic sediment supply, and levels of organic input from
vegetation (Carter, 1988; Reed, 1988; Adam, 1990; Reed, 1990; Orson et al.,
1998; Woodroffe, 2002; Goodman et al., 2007). The stage of marsh
development is important as it can influence the elevation of any given location, it
reflects the species and extent of vegetation that are likely to be present, which in
turn controls the amount of sediment that can be trapped by each tidal inundation
(Gleason et al., 1979), and wind blown sources if such a supply is available. The
input of fresh sediment in turn brings nutrients, which encourages plant growth.
This increased vegetative growth further increases the amount of sediment that
can be trapped during tidal inundations (DeLaune et al., 1989).
Areas such as low and mid marsh zones that experience regular tidal inundations
tend to build up by a mixture of inorganic sediment deposition and organic
deposition from the vegetation. However, areas such as the high marsh, that
receive relatively little tidal inundation tend to build up more through organic
input from the leaves, stems, seeds and roots of the vegetation on and below the
surface than through the deposition of fresh sediment (Hatton et al., 1983;
Bricker-Urso et al., 1989; Neubauer et al., 2002).
Recent studies by the author on the Marshside salt marshes have shown that the
average rates of sediment build up (accretion) across the entire marsh are
between 10 and 28 mm per year, with their being negligible erosion. However,
over time, this build up of sediment becomes compacted by its own weight, and
so the annual accretion is effectively compressed to result in a much reduced
level of sediment accretion. The highest levels of sedimentation are found within
the low marsh environment. This is in accordance with other published studies
that show that low marsh areas build up more rapidly than the other salt marsh
zones, as they are at a lower elevation, so receive more tidal inundations, whilst
having some vegetation (compared to the largely un-vegetated intertidal flats)
that are able to help accumulate sediment (Steers, 1948; Pethick, 1981;
Temmerman et al., 2003). Accordingly, the intertidal flats demonstrate the next
highest levels of sediment build up, as although they are at a lower elevation than
the low marsh, they experience higher energy conditions from waves, so cannot
support sufficient vegetation. The intertidal flats also contain higher numbers of
invertebrates that feed on micro-organisms that produce biofilms that help to
adhere sediment particles together, hence the less micro-organisms, the less able
the sediment is to accrete (Coles, 1979; Adam, 1990; Andersen, 2001). The mid
marsh shows the next highest level of accretion, with high marsh areas
demonstrating the lowest levels. This can be accounted for by the relative
elevation of the environments, which is an important factor in the level of
sediment deposition, due to it determining the number and length of tidal
inundations, which subsequently influences the level of sediment deposition from
the water column (Richards, 1934; Carter, 1988; Stoddart et al., 1989; Allen,
Report on the Salt Marshes at Marshside, Southport
29
1990; Allen and Duffy, 1998; Neubauer et al., 2002; Woodroffe, 2002; van
Proosdij et al., 2006).
Box 5.1: Published rates of sediment accretion in the Ribble Estuary
Author Location Environment Method Accretion
Rate
Gresswell, 1953
1930 Southport embankment (near
P27) ‘Beach’ Marker post
25.4 mm a-
1
Berry, 1967
Possibly Lytham Low marsh Unknown 18 mm a-1
Mamas et al., 1995
Lytham Main Channel,
Nazemount, & Preston Docks.
Intertidal sediments
137Cs/134Cs 15 – 120 mm a-1
Brown, 1997
Warton and Banks Salt Marsh Artificial turf 1.3 – 6.9 mm a-1
Holden, 2008
Marshside Intertidal Flats and Salt Marsh
Various Artificial Marker
Horizons
10 – 28 mm a-1
6.0 The Future
6.1 Potential Future Changes
A rise in local sea level and its effects is potentially one of the major factors that
will impact naturally upon the Marshside salt marshes in the future. It has been
predicted that there will be a global sea-level rise of approximately 1 metre over
the next 100 years, (Dyer, 1994; McLean and Tsyban, 2001), equating to an
averaged 10 mm per year. The case along many northern European coasts at the
present day however, is a much more propitious 1-2 mm per year (Woodworth,
1999).
Salt marshes can respond to sea-level rise in one of three ways. Provided there is
sufficient sediment supply and other conducive physical factors, they can accrete
vertically and develop horizontally and effectively continue to grow and develop at
a rate beyond that of local sea-level rise. Alternatively, there can be a form of
equilibrium, whereby the salt marsh keeps pace with the sea-level rise. The final
alternative is that salt marsh cannot develop sufficiently either vertically or
horizontally to remain within the zone between High Water Spring Tide (HWST)
and High Water Neap Tide (HWNT) and if unable to migrate landwards (e.g. due
to the presence of a sea wall or other hard sea defence), then the salt marsh will
be lost to the sea (Orson et al., 1985; Patrick and DeLaune, 1990; Reed, 1995;
Haslett, 2000; Masselink and Hughes, 2003).
Report on the Salt Marshes at Marshside, Southport
30
It is anticipated that the long-term effects of sea-level rise will cause a gradual
landward encroachment of both the high and low water marks, leading to
increased erosion, increased siltation, raised groundwater, drainage impedance,
saline intrusion and migration of ecological zones (Haslett, 2000; Pethick, 2001;
Valiela, 2006).
Although still debated, studies have shown that as an estuary naturally infills with
sediment, so the volume of water that enters the estuary at each high tide is
reduced, which results in an associated reduction in water velocities, thereby
enabling further accretion (van der Wal et al., 2002). Seaward development of
salt marsh is generally accepted to be indicative of the positive ability of a
particular salt marsh system to be able to accrete sediment at a rate greater than
that of relative sea level rise (Doody, 2001; Hughes and Paramor, 2004). This
situation has been confirmed at Marshside, with analysis of historical data,
predominantly aerial photographs and map evidence, showing a development of
the salt marsh in both a seaward and a southerly (towards the outer estuary)
direction, which has been continuous, but has slowed in recent decades.
With an anticipated rise in sea level, so each of the zones within salt marshes are
expected to move landwards, so the whole salt marsh effectively migrates inland
so that it retains its same position within the intertidal zone between low and high
tide marks. As the sea level rises, so at any given elevation there will be an
increase in the number of tidal inundations, with the deeper water in the estuary
increasing the amount of energy available to waves (Dijkema et al., 1990; Siefert,
1990). This leads to erosion of the seaward sediments of the salt marsh, and
their deposition higher up the intertidal zone. This process is responsible for the
gradual landward movement of the salt marsh.
Previous studies have shown that as sea level rises, so salt marshes can generally
build up by sedimentation at a rate that is sufficient to keep pace with the rise in
sea level (e.g. Orson et al., 1985; Bricker-Urso et al., 1989; Childers et al., 1993;
Orson et al., 1998; French and Burningham, 2003).
An important factor at Marshside is the presence of the Coast Road sea wall. If
the environmental conditions lead to the salt marsh migrating landwards, then the
presence of the wall will prevent this. In extreme conditions, this would mean
that the marsh reverted back to low marsh or even intertidal flats, and could lead
to the loss of the salt marsh, and subsequently its effectiveness as a sea defence.
An initial sign of deterioration of a marsh system is when the vegetation
composition reverts to more salt tolerant plant species (van Wijnen and Bakker,
2001). However, the photographic evidence collected by the author at Marshside
over four years, and the SMBC archive of aerial photography indicates that this is
not occurring at Marshside, with the salt marsh continuing to develop (Plate 6.1).
31
Plate 6.1: Development of the
Marshside salt marsh over
time: the left hand
photograph shows a point on
the marsh during June 2004;
the right hand photograph
shows the same site in May
2006. The Coast Road is to
the right of the image, with
the sea to the left, showing
that the marsh at this point
has continued to develop and
become increasingly
vegetated in a seaward
direction.
© V Holden © V Holden
Report on the Salt Marshes at Marshside, Southport
32
With an increase in sea level, so sediment deposition within the estuarine channels
will be expected to increase due to the increased water depth resulting in a
decreased drag on the channel bed (e.g. as shown by Pethick (1993) in the
Blackwater Estuary). Deposition within the Ribble navigation channel will additionally
be in all probability increased due to the termination of the dredging operations
within the channel. For the Mersey estuary, an increased deposition rate within the
channel would require increased dredging, which would result in increased dumping
of dredge spoil offshore in Liverpool Bay, which from documentary evidence would
potentially increase the available supply of sediment to the foreshore at Southport.
Pethick (1993) identified that widening of estuarine channels is the natural response
to sea level rise within an estuary. Although prevented in many situations due to the
presence of training walls, within the Ribble this may be a feasible adjustment in the
long term due to the abandonment of maintenance of the training walls along the
navigation channel. If this response can occur, it may alleviate potential flooding
within the coastal zone and prevent salt marsh loss.
6.2 Management
Of all the coastal environments, salt marshes are one of the most susceptible to a
rising sea-level (Reed, 1990). As many areas of salt marsh are backed at present by
land and buildings of economic value, they have a substantial value as a sea
defence, with the vegetation being an important factor in wave attenuation (King and
Lester, 1995).
In contrast to the human pressures that have been placed upon the Marshside salt
marshes, they are nationally and internationally recognised as being of major
importance for their habitat provision, being designated a Site of Special Scientific
Interest (SSSI), a National Nature Reserve, a Special Protection Area (SPA), and a
RAMSAR site. The area has a number of landowners, with many other stakeholders,
such as the Royal Society for the Protection of Birds (RSPB) and golf clubs, involved
in the management of the area. The various bodies work together under the banner
of the Sefton Coast Partnership, created in 2001 from the Sefton Coast Management
Scheme.
There is now a fundamental change in thinking by policy makers, whereas
previously, coastal flooding would have seen the building of defences and attempts
being made to keep coastal water away from the land, now there is a shift towards
flooding being more ‘controlled’, with compromises being made to re-create and
encourage wetland environments to allow for the increased coastal inundations that
are to be expected with rising sea levels and climate change. The presence of the
soft defences of the salt marsh along the north Sefton coast are therefore of great
consequence to the future planning and policy development, not only in terms of
their value as a sea defence, but also as a habitat for wildlife. The need to
understand the processes concerning the salt marsh specific to this area is therefore
central to developing the most viable, informed policies concerning the coastline and
its future development.
Report on the Salt Marshes at Marshside, Southport
33
As the estuary’s natural tendency is towards infilling, so the ability of the intertidal
zone to accept a given volume of water is reduced, particularly in the case of
reclaimed land that occupies an area that was once intertidal. Therefore, it is not
unreasonable to suggest that the ‘excess’ water that no longer enters the estuary
must be directed elsewhere along the coastline. When sea level rise and increased
water volumes are added to this natural trend, so the importance of having
environments that can readily accept the water, such as salt marsh, become
increasingly important.
Similarly, as management strategies are required along adjacent stretches of the
coastline, such as the management of coastal erosion at Formby Point, so the actions
taken along the coastline need to be considered carefully, as it has been shown
historically, that actions in one area of the coast have resultant effects in other
areas.
Salt marshes are known to be a sink for many pollutants, such as heavy metals and
radionuclides, particularly adsorbed to the fine sediment that characterises the
environment (Plater et al., 1991; Berry and Plater, 1998; Ridgway and Shimmield,
2002). The pollutants can originate from industrial activity releasing waste products
into fluvial or coastal waters, and from airborne deposition. Should erosion of the
marsh surface occur, stored pollutants can be re-mobilised in biologically available
forms that can be re-suspended and redistributed via tidal waters (Adam, 2002).
Therefore, the continued existence of the salt marsh is important in preventing such
pollutants from re-entering the marine system. Adam (2002) highlights the
importance of estuarine fringing salt marshes in intercepting terrestrially derived
nitrogen, thereby preventing eutrophication in other habitats within an estuary. As a
larger proportion of the land immediately adjacent to the Marshside salt marshes is
agricultural land, there is potential for a significant quantity of agricultural run-off to
enter the salt marsh system, which will then be retained within the system rather
than entering coastal waters.
Under conditions of rising sea level, a landward migration of the Marshside salt
marshes would not be feasible under the present land use approach, due to the
presence of the sea defences that form the Coast Road, with the various historic
reclamations that have agricultural uses, or indeed have importance as brackish
marsh for habitat provision and increased biodiversity. Although the presence of the
Coast Road as a solid, immovable feature may in theory lead to the phenomenon of
coastal squeeze, so at present the growth of the salt marsh in a southerly and
seaward direction, at rates of accretion above local relative sea level rise, it can be
concluded that at present, future management should continue to encourage the salt
marsh as an important and valuable habitat, whilst being mindful of changes that
may occur in the wider environment of Liverpool Bay, which may impact upon the
condition of the marsh. Continued monitoring of the salt marsh area by aerial and
topographic surveys, along with the condition and operating of the marsh, by
accretion and vegetation monitoring, is an important step in recognising any changes
that may occur in the area, and allow for management to be enhanced accordingly.
Report on the Salt Marshes at Marshside, Southport
34
7.0 Summary
All the evidence collated and analysed suggests that the salt marsh at Marshside has
been expanding spatially and stratigraphically for at least the last 200 years. Until
recently, much of this expansion has been actively encouraged by human
intervention to enable reclamation of land. It is estimated that approximately 2,230
hectares of land has been reclaimed in the estuary since 1800 (Pye and French,
1993(b); Healy and Hickey, 2002). The majority of salt marsh at Marshside is
relatively immature, fronting the Coast Road embankment that constructed between
30 and 70 years ago.
The rates of vertical sediment accretion at Marshside, recorded by high resolution
monitoring over the last four years, have been in the region of 10 – 28 mm per year,
with the lower rate generally applying to the high marsh environment, with the
higher rate being recorded in the low marsh. As the rates of sediment accretion are
usually more rapid in younger salt marshes (Steers, 1948; Pethick, 1981), so the
rates at Marshside can be expected to slow down over time.
Between 1860 and 1980, the relative mean sea level at Liverpool increased by an
average of 1 mm per year (Blott, et al., 2006). If the current rate of local relative
sea-level rise (1-2mm) remains constant, then it would appear that the salt marsh at
Marshside will be able to continue to develop. This is, however, reliant upon the
continued supply of sufficient sediment. Therefore, any change in the current
sediment transport processes within the wider estuary may have a detrimental effect
on the ability of the salt marsh to continue to accrete sufficiently to maintain a rate
of development in line with relative sea level rise.
Although not anticipated by current trends, if increase in the rate of sea level rise
occurs, then the ability of the salt marsh to maintain its position within the tidal
frame may be compromised. This may ultimately lead to a reduction in the area of
salt marsh, with a regression in the environment and vegetation away from high
marsh species back towards low marsh species (Lefeuvre, 1990). This would result
in a reduction in the ability of the marsh to further accrete through the addition of
both sediment and organic matter (Day and Templet, 1989; Reed, 1995). A
subsequent reduction in the ability of the marsh to act as a sea defence, would be
compounded by the presence of hard defences, particularly at Marshside by the
Coast Road sea wall, which would prevent dissipation of wave energy, and hence
lead to further erosion of the reduced salt marsh. A similar situation could be
anticipated should a sufficient supply of sediment be precluded from the salt marsh
system. A reduction in the salt marsh area would have consequences for the salt
marsh environment, with marked changes in the range of intertidal habitat available.
If the situation arose, then adoption of an appropriate coastal strategy such as
managed retreat may be required.
With the closure of the sand-winning plant at Marshside, so the site of the plant may
be allowed to develop both ecologically and morphologically with minimal human
Report on the Salt Marshes at Marshside, Southport
35
interference. Monitoring of the evolution of this site may prove beneficial when
considering similarities to managed realignment sites.
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