Report on the Salt Marshes at Marshside, Southport.corepoint.ucc.ie/FinalDeliverables/Publications... · Report on the Salt Marshes at Marshside, Southport 4 3.1 Salt Marsh Morphology

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

Report on the Salt Marshes at Marshside, Southport

1

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.


2

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).


3

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


4

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


6

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


7

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


8

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


9

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.


10

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


11

• 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.


12

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


13

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.


14

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).


16

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


18

Figure 4.3: Expansion of salt marsh edge over time taken from aerial

photography. Illustration courtesy of SMBC.


19

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


20

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.


21

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.


22

Plate 4.4: Aerial photograph of Marshside salt marsh in 1995. Photograph

courtesy of SMBC © Cities Revealed.


23

Figure 4.4: Expansion of Salicornia spp. At Marshside, March 1998 to

February 2000. (Taken from Tomlinson, 1999 and 2000).


24

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


25

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


26

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


28

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,


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).


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


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.


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.


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


35

interference. Monitoring of the evolution of this site may prove beneficial when

considering similarities to managed realignment sites.

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