Coastal Defence Report on the Salt Marshes at Marshside, Southport. Version 1.1 March 2008 Prepared by: Sefton Council Vanessa Holden Technical Services Natural, Geographical & Applied Sciences Balliol House Edge Hill University Balliol Road St Helens Road Bootle Ormskirk L20 3NJ Lancashire Tel: +44 (0)151 934 4238 L39 4QP Fax: +44 (0)151 934 4559 Email:[email protected] 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. 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. 1 Report on the Salt Marshes at Marshside, Southport 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). 2 Report on the Salt Marshes at Marshside, Southport Ribble Navigation Channel Marshside Marshes Bog Hole Channel Southport Pier 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. 3 Report on the Salt Marshes at Marshside, Southport 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. 4 © V Holden © V Holden © V Holden 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. 5 Report on the Salt Marshes at Marshside, Southport 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).