Marine Geography Dissertation 2015

Environmental change and shoreline evolution of Sowley

Marsh, West Solent, and implications for management

Peter Elliott

“Dissertation submitted as part of the requirements for the BSc Degree in Marine

Geography with Honours”

I agree / do not agree* to the use of this dissertation as an example for future

students. (*delete as appropriate)

Signature ______________________________________________________

Date ________________________

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Abstract

The study area of Sowley Marsh is located on the north-western shore of the Solent, an area

renowned for unresolved picturesque beauty. Positioned behind a dynamically variable barrier

beach, marsh conditions are particularly susceptible to change in response to storm conditions or

high water influencing the hydrological conditions of the Marsh landward of the barrier. Such

ephemeral habitats are often of high environmental importance and included in features of

conservation importance in site designation, local, national and international levels. In recent years

concern for the survival of the Lymington to Pitt’s Deep saltmarsh seaward of the barrier has

increased owing to the rapid erosion of the saltmarsh. Consequently the dynamics of the coastal

system are changing, and the effects are illustrated by Sowley barrier. This report describes the

contemporary evolution of Sowley barrier, and its ability to inform coastal management authorities

of the benefits of managed retreat to offset losses of habitat exacerbated by climate change and

rising sea levels. Through the use of ArcGIS saltmarsh erosion rates were calculated. In addition to

this the evolution of Sowley barrier was mapped exemplifying the changes in morphology of the

barrier following a breaching event in 1955/56. Potential scenarios for the evolution of the barrier in

the future were also analysed by creating cross section maps indicating the shape of the barrier in

reference to its vulnerability.

It was found that saltmarsh erosion rates are linear at 3m/y indicating that the saltmarsh will be

eradicated by the turn of the century. Sowley barrier influenced by a storm surge induced breaching

event in 1955/56 has since resealed in 2008, as coastal sediment transport processes have increased

due to the saltmarsh erosion causing a net movement of sediment from west-east. Further evidence

is needed to predict the future morphology of Sowley barrier. Upon assessing current management

of the site it was found that Water Framework Directive (WFD) tools for assessment of lagoon

habitats were not suffice to provide feedback relevant to managing such a variable site. It is

recommended that delineation of the waterbody be amended and it is categorised under

transitional water bodies in the Solent. Coastal lagoons are listed as a priority habitat on Annex 1 of

the EC habitats directive owing to high biodiversity and specifically its specialised species. Due to the

important nature designations for this site (Special Area of Conservation/Special Protected Area) it

was concluded that further management from this site should come from these requirements. The

ephemeral, unique and diverse habitats offered by saline lagoons are an integral part of the coastal

system and thus should be closely monitored and managed as best to inform future shoreline

management plans that best dictate coastal management in the future.

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Contents

1.0 Introduction ................................................................................................................................ 6

1.1 Ecosystem services of a saltmarsh ................................................................................................ 8

1.2 Site Designations ....................................................................................................................... 9

1.3 Aims and objectives ................................................................................................................ 10

2.0 Methodology ................................................................................................................................... 11

2.1 Use of GIS .................................................................................................................................... 13

2.2 Assessment of uncertainty regarding aerial photographs. ..................................................... 17

3. 0 Results ............................................................................................................................................ 18

3.1 – Saltmarsh recession ................................................................................................................. 18

3.2 Saltmarsh loss- ........................................................................................................................ 22

3.3 – Breach/heal cycle ................................................................................................................. 27

4.0 Discussion ........................................................................................................................................ 39

4.1 Saltmarsh erosion ....................................................................................................................... 39

4.1.1 Future saltmarsh projection .................................................................................................... 42

4.2 Breach/heal cycle of the barrier beach ................................................................................... 43

4.2.1 Future breach/heal cycle projection .................................................................................... 45

5.0 Limitations ....................................................................................................................................... 50

6.0 Conclusion ....................................................................................................................................... 52

7.0 Future management ....................................................................................................................... 54

8.0 Acknowledgements ......................................................................................................................... 56

3

References ............................................................................................................................................ 57

Appendices ............................................................................................................................................ 61

4

List of figures

Figure 1. Map of study area in UK context, south coast. ........................................................................ 6

Figure 2. Detailed Location map of study area in the Solent. ................................................................. 7

Figure 3. Setup of GPS base station at Sowley Marsh. ......................................................................... 12

Figure 4. Mobile GPS receiver used to run survey lines. ...................................................................... 13

Figure 5. Polygon shape files used to map seaward edge of saltmarsh indicated by chenier deposits.

.............................................................................................................................................................. 14

Figure 6. Measuring tool on ArcGIS. ..................................................................................................... 15

Figure 7. Reference point to check accuracy of aerial photographs. (Source – CCO) .......................... 17

Figure 8. Aerial photograph of 1946 with position of saltmarsh in 1946 outlined............................... 18

Figure 9. Aerial photograph of 1946 with position of saltmarsh in 1946 and 1980 outlined. .............. 19

Figure 10. Aerial photograph of 1946 with position of saltmarsh in 1946, 1980 and 2001 outlined. . 20

Figure 11. Aerial photograph from 1946 with position of saltmarsh from according years. ............... 21

Figure 12. Aerial photograph of 2013 with outline of saltmarsh position……………………………………….21

Figure 13. A graph showing the average amount of saltmarsh lost in metres- saltmarsh islands east of Lymington stretching to Sowley measured……………………………………………………………………………………..22

Figure 14. Storms and wave heights recorded from Milford buoy 1998-2008 (Source – NFDC storm report for Milford)…………………………………………………………………………………………………………………………. 23

Figure 15. Area of cliff lost due to winter 2014 storm…………………………………………………………………….. 24

Figure 16. Total area lost between 1946-2013……………………………………………………………………………….. 25

Figure 17. Total area lost in km2 ……………………………………………………………………………………………………. 26

Figure 18. 1946 saltmarsh……………………………………………………………………………………………………………... 27

Figure 19. 1954 saltmarsh....…………………………………………………………………………………………………………. 28

Figure 20. Sowley Marsh and barrier in 1980……………………………………………………………………………….... 30

Figure 21. Polyline showing position of 1980 shoreline overlaid onto 1946 aerial photograph………. 31

Figure 22 Polyline indicating the front toe of the barrier beach in 2000/2005 overlain onto 2000 aerial photograph………………………………………………………………………………………………………………………….. 32

Figure 23. Polyline indicating the front toe of the barrier beach in 2008/2010 overlain onto 2010 aerial photograph……………………………………………………………………………………………………………………………33

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Figure 24. Location of beach cross sections used to identify morphological changes……………………… 34

Figure 25. Topographic profile of the barrier beach between 2005 and 2013, profile 2 (see fig.24 for location)…………………………………………………………………………………………………………………………………………. 35

Figure 26. Topographic profile of the barrier beach between 2005 and 2013, profile 3 (see fig 24 for location)…………………………………………………………………………………………………………………………………………. 36

Figure 27. Topographic profile of the barrier beach between 2005 and 2013, profile 4 (see fig 24 for location)…………………………………………………………………………………………………………………………………………. 37

Figure 28. Net loss/gain of sediment on areas of the beach from 2005-2013…………………………………. 38

Figure 29. Future projection of saltmarsh recession at a linear rate (red).…………………………………….. 42

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1.0 Introduction

The study area is located east of Lymington, within the Western Solent, a dynamically variable

estuarine system on the south coast of England (Fig.1) Located within the study area is Sowley

Marsh, a lagoon containing saltmarsh positioned on the landward side of a barrier beach that is

dynamically variable in its morphology. The marsh itself extends north covering an area of 47

hectares (NFDC, 2010) The land located behind the barrier is gently undulating in morphology,

however Sowley Marsh is in a river basin and as such has low relief. Located to the South-east of

Sowley Marsh is an extensive saltmarsh system that stretches 4km east towards Lymington reaching

a maximum seaward extent of 1.2km offshore (fig.2) Sowley Marsh conditions are susceptible to

change due to the especially in response to storm conditions or high water therefore the

hydrological conditions of the hinterland landward of the barrier beach fluctuate between

agricultural land, intertidal and submerged brackish, marine and freshwater. Such ephemeral

habitats are often of high environmental importance and included in features of conservation

importance in site designation, local, national and international levels. These will be detailed in

section (1.2)

(Source - https://www.google.co.uk/maps)

N

Figure 1. Map of study area in UK context, south coast.

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(Source – http://www.channelcoast.org/data_management/online_data_catalogue/)

The large area of marsh vegetation and open water located in Sowley Marsh is approximately 500m

long and 350m at it’s widest. There is freshwater infiltration from Sowley pond, located further

north of Sowley Marsh. Furthermore there is a drainage channel that runs to the east of Sowley

marsh to drain Sowley pond. In periods of high rainfall this channel can overflow leading to further

infiltration of freshwater into the Sowley Marsh ecosystem. Accordingly there is an interface where

in there is mixing of salt and freshwater is apparent, contributing to the often ephemeral habitats

found in Sowley Marsh (Bealey et al,.2006) The southern part of the central marsh is dominated by a

hybrid cordgrass species Spartina Anglica, a cross between the indigenous Spartina maritime and an

eastern American species Spartina alterniflora (Ainouche et al., 2004)

Lymington

Pitts Deep saltmarsh Sowley Marsh

N

Figure 2. Detailed Location map of study area in the Solent.

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The south westerly prevailing winds create a net movement of sediment from east-west along the

shoreline. The abundance of salt-marsh acts as a buffer, with little vertical accretion of the marshes,

thus leaving little sediment to be carried along the shore via the process of longshore drift. Also

evident from the study area (fig.2) are small beaches built from accretion of sediment as it travels

from west to east along the shoreline. In contrast to the Eastern Solent the Western Solent is

erosional. It is characterised by a progressive tidal wave, ebb-tidal dominance (Xiankun, 1995) Both

of these characteristics play a fundamental role in the dynamics of this estuarine saltmarsh system.

Further analysis of sediment supply to the area is evaluated in section 3.3.

As with the rest of the Solent maritime regions, Sowley Marsh’ geomorphological evolution since the

late Pleistocene from a coastal fluvial valley to a coastal embayment is continuing. As a result of this

the topography behind Sowley Marsh is still gently rising (Johnson, 2007) The low topography makes

the site susceptible to sporadic flooding, however gently shelving offshore bathymetry prevents

rapid tidal flow close to the shore. The tide surrounding Lymington has been classified as meso-tidal

(2-4m) Furthermore the study area in the past has no experienced large wave heights in normal

conditions due to the aforementioned gently sloping bathymetry. It is only susceptible to large wave

and storm surge events that are of enough magnitude to reach the crest of the barrier beach.

1.1 Ecosystem services of a saltmarsh

Saltmarshes are an effective natural method of flood prevention, offer nature conservation potential

and also provide for the local economy (New Hampshire Environmental Services, 2004) This is due to

the unique habitat they provide that can be utilised by many different species for nutrients, shelter

and food. Ecosystem services offered by the colonisation of saltmarsh are often overlooked however

there are multiple benefits to humans and wildlife. Ecosystem services are essentially anything

beneficial that humans gain from an ecosystem (Bromberg et al., 2009) Saltmarshes and mudflats

afford flood prevention to existing defences and shorelines, absorbing wave energy and decreasing

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the impact of stormy weather, which brings increased rainfall and wave height. Consequently the

ecosystem services provided by saltmarsh are ubiquitous, leading to areas of coastal wetlands

containing more ecosystem services than other coastal environments alike (UNEP, 2005)

1.2 Site Designations

The area landward of Sowley barrier is also important in relation to nature conservation. Nature

conservation designations for this site cover the land and intertidal habitats as they have a range of

different features regarding conservation importance. Sowley Marsh is already designated as a SSSI

(scientific site of specific interest) at a national level, and the saltmarsh is identified as a Ramsar

(conservation of wetlands) site at an international level. This Ramsar site was first designated in

1998 (Cox, 2011) Southampton waters are designated under the SPA bird directive, and the study

area, Sowley Marsh, earned its designation due to the sites providing important refuges for surface

feeding and diving ducks that was seen as an integral part of the marshland system(Natural England,

1984) The Marsh is regularly used by 1% or more of the biogeographic population of regularly

occurring migratory species(Southampton Waters SPA, 1998) Saltmarsh provides critically important

feeding and roosting areas for internationally important numbers of overwintering waders and

waterfowl birds, in particular the Dark Bellied Brent Geese (Branta Bernicla) (Tubbs, 1982) This

species is seen to inhabit the mudflats and saltmarshes found in the intertidal area seaward of the

barrier beach. Moreover, the saltmarsh provides home for important breeding populations of gulls

and waders. These species mostly breed in circum-polar regions and spend winter occupying the

Solent shore. In spring and autumn the Solent also supports populations of migrant Waterfowl that

move to and from wintering grounds on the estuaries of south-western Europe and West Africa.

Gulls in particular nest in colonies on the saltmarshes and associated shell and shingle deposits

(cheniers) to the south-west of the study site. The site is a critical component of the Solent estuarine

system that is decidedly biodiverse.

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It is also important to note that all of the frontage at Sowley is privately owned, and currently there

is no public funding available for sea defences. The implementation of a no active intervention (NAI)

policy has meant this shoreline has been undefended allowing natural processes to take place.

Information for landowners is available from the North Solent Shoreline Management Plan (SMP)

which explains the reasons why this area is undefended (NFDC, 2010)

The corroboration of all these factors means further research into this area of natural undefended

coastline is helpful in informing flood and coastal erosion management authorities of future coastal

management decisions.

1.3 Aims and objectives

This study aims to:

Determine and quantify the temporal and spatial changes of the Sowley barrier beach and

adjacent shoreline. Analysis of the factors effecting shoreline change will help inform coastal

management in the future.

Following this a further aim is to quantify the temporal and spatial changes of the saltmarsh

system, and determine rates of saltmarsh recession. Predicting the rate of erosion of

saltmarsh is an essential practise as to best understand and prepare the changes this can

cause on a coastline.

Succeeding this, this report will identify the environmental impacts of coastal change at the

study site, to illustrate the effects of the eroding saltmarsh and changing morphology of the

barrier beach.

Finally this report aims to identify environmental planning policy and management implications

of future coastal change for the longer term management of the site. The unique environmental

qualities of this site may offer information regarding future management of other similar areas

of coastlines nationally.

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2.0 Methodology

The site benefits from considerable high resolution coastal monitoring data, including georectified

aerial photography, Lidar, topographic surveys, ecological surveys, bathymetric surveys and

hydrodynamic (wave and tide) measurements recorded at the nearby Lymington Racing Platform.

The New Forest District Council (NFDC) has been monitoring this area of coastline longer than the

Channel Coast Observatory (CCO) has been available. Also all data was accompanied by Metadata,

which gives a description of the data used and when it was taken, thus making it more reliable.

Therefore data were freely available for use to show changes in the state of the environment at

Sowley. These pieces of data were made available by the Channel Coastal Observatory the data

centre for national network of regional coastal monitoring programmes of England

(www.channelcoast.org) A map downloader tool on the website allowed the chosen survey area

(Sowley) to be highlighted and all of the aforementioned data could be downloaded by choosing the

year and the data sets from within that year that were desired (Aerial photographs from 1940 –

present of the survey area were also available from the NFDC coastal team) Each photo covered an

area from West of Lymington to East of Sowley thus fully encompassing the survey area and its

surroundings.

In early September a topographic survey was undertaken at the site involving the use of a real time

Kinematic (RTK) GPS to define beach morphology, widths and heights on pre-defined survey lines.

This allows for the same survey to be conducted numerous times over the year, so that data is

relevant, and methods of data collection are consistent. Topographic surveys like these are

conducted regularly to monitor the constant change in elevation of the barrier beach and to make

future predictions on the state/morphology of the area as best to inform future shoreline

management plans.

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(Source – Original)

The use of this RTK GPS device allows the opportunity to record data with a vertically accuracy of +/-

2-3cm and a horizontal positioning of approximately double the accuracy (CCO, 2003) At first a base

station was set up over a known defined position. Following this mobile gps stations were used to

collect the data. Pre-determined survey lines were located on the GPS so they could easily be

identified. Two surveyors operated the mobile RTK GPS, collecting data on defined cross section

profile survey lines, extending from the back of the beach to mean low water. The use of a pole was

essential as to not submerge the GPS equipment. This type of GPS is well suited to coastal surveys

due to the efficiency of the equipment and the large stretch of coastline that can be surveyed by

utilising just one base station.

Once the base station has been setup according to previous way markers on the ground, the two

surveyors could begin to conduct their survey lines running perpendicular to the shore with one

surveyor going east (left) and the other heading right (west)

GPS base station.

Figure 3. Setup of GPS base station at Sowley Marsh.

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In addition to this an area of cliff located 0.5km to the west of Sowley was surveyed using gps points

to show its position in regards to the 2013 aerial photography and previous topographic surveys.

(Source – Original)

2.1 Use of GIS

Once the aerial photographs were obtained the next task was to import them into ArcGIS to analyse

the evolution of the shoreline. Due to the size and quantity of the photo files a mosaic dataset was

created, a data model within the geodatabase created by right clicking on a specific folder and

choosing the file geodatabase option in arc catalogue, that’s primary function is to manage a

collection of raster datasets, to collate all photo files from each year of aerial photographs. Once all

of the aerial photographs had been converted into mosaic datasets for their respective years a

variety of GIS tools were used to analyse them.

Figure 4. Mobile GPS receiver used to run survey lines.

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Shapefiles –

These were created by right clicking a specified folder in ArcGIS 10.2.2, and choosing create shape

file. To begin with a polyline was created that followed the shoreline parallel across the survey area,

this was used to show any changes in the position of the shoreline. Following this the extent of the

saltmarsh was digitised via the use of polygon shape files.

The edge of the saltmarsh was indicated by the presence of Chenier deposits to south east,

fragmented shell deposits on the edge of saltmarsh. Where the landward edge of the saltmarsh

visible was mapped an approximate of the saltmarsh edge was used at the identifying factor. These

techniques were used on the available aerial photographs covering a period from 1940 to 2013.

Breach/beach position

A further use of the polyline tool was to map the front toe of the barrier beach, to show its

evolution including its breach cycle. Polyline’s depicted the shape of the barrier beach from each

year, and highlighted the years in which a breach of the barrier was seen, and changes in the

position of the sediment at the front and back toe of the barrier.

Figure 5. Polygon shape files used to map seaward edge of saltmarsh indicated by chenier deposits.

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Figure 6. Measuring tool on ArcGIS.

Area lost

Finally a polygon that covered the whole area of the marsh not just the outline was used to show the

total area of saltmarsh lost throughout the data range. Values were input into excel to show the

total amount of saltmarsh lost in km2 from 1940-2013.

Measuring the erosion -

A measuring tool was used to measure the meters of saltmarsh between each year (fig.6) indicated

by the polygon outline (fig.5) This measuring tool is found on the measuring toolbar and measures

the distance between points chosen by the user. Following this an area measuring tool was used on

the polygon covering the whole area of the saltmarsh to show the total area lost. This can be

selected from the same measure box as the length measuring tool (fig.6)The data gathered from the

measurement tool was input into excel to calculate the average recession rates for each year the

aerial photographs were available, and ultimately the average recession rate since the photographs

began in metres and area.

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Sediment Changes

To begin with a cross section of the beach from 2005 and 2013 was created, using just one polyline.

This was repeated 3 times at different intervals on the beach and consequently allowed for a

comparison between the beach morphology in 2005 and 2013. Following this a cut/fill tool was used

to indicate areas of loss/gain in sediment, again between 2005 and 2013. Elevation data was

obtained through Lidar datasets for both these years made available by the CCO.

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2.2 Assessment of uncertainty regarding aerial photographs.

Due to the erroneous nature of aerial photographs the house visible in the above image was used as

a reference point. Furthermore the accuracy of the older datasets is perhaps hindered by the low

quality images produced, and therefore measurements and rates calculated for coastal change are

more indicative than definitive. If aerial photographs were not aligned then calculations of

saltmarsh recession could be made redundant even by a few metres error. In figure 7 the house was

identified and its position digitised from each of the photographs available. As aforementioned all

images are georectified and accompanied by metadata to make them as accurate as possible, but

discrepancies are still present with historical aerial photographs. It is evident that all the

photographs are geo-rectified and thus the data is aligned. The only anomaly implied by the image is

that of the yellow square, however this is indicative of an extension of the house and not a change in

position (fig.7)

Figure 7. Reference point to check accuracy of aerial photographs. (Source – CCO)

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3. 0 Results

3.1 – Saltmarsh recession

Sowley barrier beach is continuous. Narrow

beach widths limited source and availability

of beach sediments. Extent of saltmarsh

directly seaward of barrier beach providing

some protection to shoreline

Abundance of marsh affording protection to

shoreline. Minimal or no alongshore

transport of sands and gravels.

Figure 8. Aerial photograph of 1946 with position of saltmarsh in 1946 outlined.

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Recession of the saltmarsh is evident indicated by the inland transgression of chenier deposits.

These are fragmented shell deposits on the edge of saltmarsh. This technique was used as the

saltmarsh edge indicator as often it is difficult to identify the boundary between saltmarsh and

mudflats, and with the increasing volume of mudflats due to the saltmarsh recession, chenier

deposits remained a reliable indicator of the saltmarsh edge (fig.9) Chenier deposits located to the

south of barrier beach have shown considerable erosion, with very little left protecting the beach.

Consequently input of sediment to the shoreline has increased.

Figure 9. Aerial photograph of 1946 with position of saltmarsh in 1946 and 1980 outlined.

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There has been noticeable loss of saltmarsh seaward of the barrier beach. (fig.10) Reduction in extent

of the saltmarsh to the west, but still affording some protection to the shoreline from the prevailing

south westerly waves. Furthermore fast eroding saltmarsh to the south-west allows wave energy to

have more of an impact causing sediment shift. The coastline is much more susceptible to storm

events as prior to this the abundance of saltmarsh acted as a ‘buffer’ to stormy conditions.

Barrier Beach

Figure 10. Aerial photograph of 1946 with position of saltmarsh in 1946, 1980 and 2001 outlined.

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Over the 67 year data range between 1946-2013 there has been considerable recession of the saltmarsh seaward of Pitt’s Deep and further to the west of Lymington. The erosion of the saltmarsh system has increased the exposure of

the coastline to tidal and wave conditions and thus changed the orientation and erosional dynamics of the shoreline at Sowley. This long-term process of change, with periods of variable shoreline erosion rates, may have initiated the

breach//heal cycle of the Sowley barrier beach; higher rates of shoreline erosion may have produced increased source sediment to widen and increase the volume of the barrier beach; lower shoreline erosion rates may have reduced

the supply of sediment or even starved the barrier, resulting in a breach following a storm event. Essentially there are a few potential reasons for the changes in morphology of the barrier beach that are to be explored further. Evidence

for this is the beach that has formed (fig.12) where-as there was little or no beach in 1946 (fig.11) Due to the Ebb tide dominance in this saltmarsh system an abundance of sediment is transported out to sea when the saltmarsh erodes

therefore the volume of sediment supplied to the area Is needed to properly deduce the breach/heal cycle of Sowley barrier beach.

Timber Groynes privately built to stop

sediment transport.

Figure 11. Aerial photograph from 1946 with position of saltmarsh from according years. Figure 12. Aerial photograph of 2013 with outline of saltmarsh position.

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3.2 Saltmarsh loss-

The maximum annual loss of saltmarsh on the seaward edge was seen from 1968-1980 at 5.57m

(fig.13) In contrast the lowest level of saltmarsh erosion was from 1946-1954 at just 1.0m. 2010-2013

shows an average of 3.6m being lost per year, the highest seen for 10 years. The average rate of loss

per year was calculated at 3.02m corresponding with the North Solent shoreline management plan

(NFDC, 2010) Standard error was calculated in addition to this figure and is included in appendix 1.

It is proposed that the decreasing level of erosion evident from 2002-2007 is partly correlated with

the frequency and intensity of storms recorded between these periods. Higher rates of saltmarsh

erosion are usually recorded in response to storm conditions wherein the wave height is raised

above its normal rate causing faster erosion of the saltmarsh to the south-west of Sowley (Wal &

Pye, 2004) Disturbance from a storm surge can initialize an erosion process that causes a steep slope

on the seaward edge. Consequently sediment found here is more vulnerable to currents and wave

action. Once an edge begins eroding, the process is often irreversible, until it is protected from new

marsh growing in front of the cliff (Allen, 2009)

0

1

2

3

4

5

6

Ave

rage

m/y

Year

Figure 13. A graph showing the average amount of saltmarsh lost in metres- saltmarsh islands east of Lymington stretching to Sowley measured.

23

Hs represents the highest significant wave height recorded during the storm event surrounding the

study site. Upon examining storm conditions recorded between 1998-2008 (fig.14) it is noticeable

that very few storms reach a wave height that will help to exacerbate erosion rates above an

average of 3m p/y (fig.13) Recordings from the nearby Milford buoy in Christchurch bay maybe

indicative of wave conditions of the western Solent as a whole. From 2008 onwards increased

storminess is responsible for the increasing erosion trend from 2010-2013 (fig.9) The return period

shown is an estimate of the likelihood of an event, in this case it represents the average wave height

that is to be expected at least once per year. Although less in frequency two storms with the highest

Hs occur between 2010-2012. Events like these cause enhanced erosion rates of saltmarsh and the

shoreline, and further increase the coastal squeeze on the saltmarsh. More recent analysis draws

attention to 7 storms exceeding the 1 year return period in early 2014. Although the impact of these

on the saltmarshes hasn’t been quantified, adverse impacts of these storms have been identified

elsewhere (fig.15)

Figure 14. Storms and wave heights recorded from Milford buoy 1998-2008 (Source – NFDC storm report for Milford)

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Saltmarsh erosion rates may become delinearised over the next few years. To justify this the impact

of storm damage recorded post winter 2014 was analysed. The winter of 2013-2014 contained one

of the largest depressions (areas of low pressure) since 1886 (Met Office, 2014) A large storm surge

occurred on February the 14th, sweeping the south coast and causing damage spanning the majority

of the Solent. Gust speeds of up to 65kt were recorded, along with low pressure recordings of

957hpa within the depression that effecting the south coast (Met Office, 2014) This was illustrated

on an area 0.5km west of Sowley barrier, wherein around 10m of a small cliff was lost due to

hydraulic action from the sea. This exacerbated level of erosion is also detrimental to saltmarsh, due

to Sowley marsh being the only area landward retreat is possible as a direct result of its surrounding

topography. Therefore a coastal squeeze is apparent where-by the habitat is being ‘squeezed’

between the fixed landward boundary and the sea, ultimately causing the loss of saltmarsh as it has

no landward retreat other than Sowley Marsh.

Figure 15. Area of cliff lost due to winter 2014 storm

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The total area of saltmarsh lost from Lymington to Pitt’s deep was also calculated as this coupled with

metres lost gives a full understanding of the total amount of saltmarsh lost. It is evident that since

1946 nearly half of the total saltmarsh, from Lymington to Sowley, has been lost (fig.16) Also

noticeable is the saltmarsh to the southeast and seaward of Sowley beach is visible and evident in

1946, but has been completely eroded by 2013. Further saltmarsh studies conducted show an area of

734 hectares of saltmarsh in 1921, decreasing to a total of 297 hectares in 1994, a loss of more than

half in the Lymington/Keyhaven area (Colenutt, 2002)

Figure 16. Total area of saltmarsh lost between 1946-2013.

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The area of saltmarsh lost per year fluctuates with a maximum of 0.03km2 lost per year from 2001-

2005 and the lowest figure of 0.006 between 1968-1980 (fig.17) A range of 0.023km2 shows there is

a fairly consistent rate of saltmarsh recession throughout the range of this dataset. Minor

fluctuations from annual metres lost (fig.13) and annual area lost (fig.17) show a relatively linear rate

of erosion of the saltmarsh. This is indicated by the trend line, although it shows an increasing trend

when compared to the actual figure it only represents a very small increase. The calculation of both

area and annual metres loss was to rectify inherent differences between the two graphs. Area lost

shows a decreasing trend from 2010-2013 in terms of saltmarsh loss, in contrast metres lost (fig.9)

shows an increasing rate of erosion recent years (fig.17) St. error was calculated in addition to this

figure and is included in Appendix 1.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

1946-1954 1954-1968 1968-1980 1980-2000 2000-2001 2001-2005 2005-2008 2008-2010 2010-2013

Ave

rage

km

²lo

st p

er y

ear

Year

Figure 17. Total area of saltmarsh lost in km²

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3.3 – Breach/heal cycle

A comparison of the 1946 and the 1954 aerial photograph provides information and a baseline

condition which may contribute towards an explanation of the first breaching event that occurred in

1955/56. In 1946 the extent of the saltmarsh absorbs the majority of wave energy. Wind and waves

from the prevailing south west direction therefore have little impact on the coastline. Little or no

sediment is seen along the coastline as sediment transport processes are hindered by the saltmarsh,

consequently only a narrow beach is seen at Sowley. The topography of the land behind the barrier

beach is of low-medium height, 1.2-3m, however Sowley Marsh especially is low and topographically

constrained meaning once the beach was breached, the low lying basin can be inundated with saline

water.

Figure 12 – 1946 salt marsh.

Sowley Barrier, narrow

beach.

Abundance of saltmarsh

extending seaward

Figure 18. 1946 saltmarsh extent

28

Although there is only an average of 1m/y lost from the saltmarsh from 1946-1954, below the average

(fig.9) the reason is easily deduced. The loss of saltmarsh reduces the effectiveness of a meso (2-4m

tide) to macro (4m+) tidal saltmarsh in attenuating waves induced by a range of tidal and

meteorological conditions (Hanson & Larson, 2001) Although the breaching event did not occur until

a year after this photograph it can still be partly postulated that the recession of the saltmarsh is

responsible for the breaching event. Consequently the breach is caused by barrier rollover and

overwashing from waves carrying more energy than previous years. Flooding due to overwashing

occurs when the combined effects of raised water level and waves results in water entering an area

at a faster rate than it can drain away (Wadey et al., 2012)

Loss of saltmarsh

allows waves to

refract around marsh

and directly hit

Sowley barrier beach.

Erosion of the

most south

easterly

saltmarsh

decreases the

amount of

wave energy

absorbed.

Figure 19. 1954 saltmarsh.

29

In the Western Solent the average tidal range fluctuates around 2.8m, however due to the complex

tide system found in the Solent an event called ‘double high tide’ occurs. Due to the Solent’s east/west

orientation there is an interaction with different tidal harmonics in the region causing an unusually

long period of high tide. During spring tides, the tide rises after low water, however the tidal stream

slackens around 2 hours before high water. This in turn induces a stand up for a maximum of 2 hours,

named the young flood stand, just before a rapid rise to high water. Therefore in total the flood and

double high water could last up to 9 hours, thus leaving a reduced time of 3 hours for the ebb tide

(NFDC, 2010)

After high tide there is a prolonged period where water level does not vary. Therefore, if stormy

conditions occur during this time the effects could be detrimental to areas of unprotected coastline.

Also heightened wave conditions during this time could lead to increased erosion rates of the

saltmarsh. Storm surges, coastal floods associated with low pressure, are therefore most destructive

at this time. Storm surges in the Solent are the result of low pressure systems that arrive from the

Atlantic and move east over the Solent (Haigh, 2009) There are no sea defences in the immediate area

surrounding Sowley marsh meaning it is susceptible to flooding from these storm surge events.

Furthermore due to the added duration of the double high tide in this area in regards to the length of

a full tidal cycle the Ebb tide cycle is significantly reduced. This in turn causes a greater velocity of flow

on the ebb tide which is responsible for the movement of sediment offshore (Dyer & King, 1974)

Consequently the majority of sediment re-suspended due to saltmarsh erosion is transported out to

sea. For Sowley Beach to continually grow in sediment, it can be partly deduced that its sediment

arrives from alongshore coastal transport processes such as longshore drift. The loss of saltmarsh

increases shoreline erosion and thus there may be more sediment available to be transported.

30

A breaching event is evident (fig.20) with the right hand arm of the spit migrating landwards. An

increased level of saltmarsh recession from 1954-1980 (fig.13) has ultimately led to the decimation

of the marsh directly in front of the barrier beach. A decrease in area of the marsh to the west of the

breach (A) has also allowed sediment transporting processes to have more impact on the area, with

more sediment visible on the shore to the west of Sowley (B) Further evidence of the breaching

event of 55/56 are the halophytic species that have colonised the marsh providing an important

habitat for many bird species(C) The volume of sediment present on the beach is increasing.

A C B

Figure 20. Sowley Marsh and barrier in 1980.

31

To further understand the complexity of the breach/heal cycle the front toe of the barrier beach was

digitised to show its position, and consequently the sediment shift between years of aerial

photography. Initially the front toe from 1946 and 1980 is displayed, which represents the first

breaching event in 1955/56. To the south-west of the marsh sediment has extended seaward, thus

any sediment transported west to east along the shoreline will accumulate on this spit. This is

indicated by the green polyline, showing a seaward extension to the west. A wide breach is evident

on the western side of the barrier beach (fig.20) with sediment from either side of the beach

migrating landward. The eastern part of the agricultural land behind the barrier beach (fig.21) has

been flooded with saline water changing environmental conditions and resulting in a lagoon

landward of the barrier beach (fig.20)

Figure 21. Polyline showing position of 1980 shoreline overlaid onto 1946 aerial photograph.

32

When exploring the morphological change between 1980 (fig.21) and 2000 one alteration is the

large increase of sediment on the western spit of the barrier beach, and noticeably a significant

increase in the area of beach located to the west. This coupled with pre-existing knowledge of

saltmarsh recession freeing up sediment for transport is evidence for a west to east sediment

transport movement channelling sediment along the coastline from Lymington to Sowley barrier.

This accumulation of sediment on the western side of beach reaches its maximum seaward extent in

2000, and consequently a decrease in the area of sediment is seen in the progression of the beach

from 2000-2005 as sediment migrates eastward. In 2005 the area of sediment has decreased and

migrating east towards the breach, thus flattening the shape of the shoreline and causing the breach

to migrate eastwards. Furthermore the size of the breach has been reduced, illustrated by the

purple line (fig.22)

Figure 22 Polyline indicating the front toe of the barrier beach in 2000/2005 overlain onto 2000 aerial photograph.

33

A further eastward migration of the breach is visible in 2008 compared to 2005 (fig.22) This then

leads to the closure of the breach on the eastern side of the barrier beach, the opposite side to the

initial breach location in 1955/56. This illustrates the net movement of sediment from west-east on

this shoreline. The closure of the breach has further implications on the physical state of Sowley

Marsh, analysed in section 4.3.

Figure 23. Polyline indicating the front toe of the barrier beach in 2008/2010 overlain onto 2010 aerial photograph.

34

This figure was created to show the position of the cross sections analysed using LIDAR data from

2005 and 2013.

Figure 24. Location of beach cross sections used to identify morphological changes.

35

By using LIDAR data a cross section of different parts of the beach was created to show morphological changes between 2005-2013. Upon

analysing the changes it is evident that this part of the beach has retreated landward, and the crest has also migrated landward, indicated by

the red line peak at 70m distance. Therefore the beach width has become narrower at profile 2, and more sediment has accreted towards the

back of the barrier.

0.0000

0.5000

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0 4 10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 74 78 82 86 90 94 98 102 106 110 114 118 122 126

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ght

(m)

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Figure 25. Topographic profile of the barrier beach between 2005 and 2013, profile 2 (see fig.24 for location)

36

Profile 3 intersects the area of the beach where the breach is located in 2005. Evident from the 2013 analysis is the closing of this breach,

located between 22-44m on the distance scale (fig.26) Following a similar trend to point 2 (fig.25) the beach has rolled back, with the crest now

being found 20m landward of its 2005 location. This has further implications for the future morphology of the barrier beach evaluated in section

4.2.1. The height of the crest has increased by 0.22m from is 2005 extent.

0

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Figure 26. Topographic profile of the barrier beach between 2005 and 2013, profile 3 (see fig 24 for location)

37

Profile 4 is located on the eastern side of the barrier beach. Here the crest height has reduced, and more sediment can be seen on the

landward side of the barrier from 30-44m distance. This part of the beach still illustrates a small retreat of the crest from 2005-2013 but it is less

pronounced than point 2 and point 3 due to the alongshore sediment processes that provide a constant source of sediment on the western

side. Narrowing of the beach as a whole has been identified from analysing points 2 3 and 4 (fig.25,26,27)

0

0.5

1

1.5

2

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0 2 4 6 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94

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ght

(m)

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2005lidar 2013lidar

Figure 27. Topographic profile of the barrier beach between 2005 and 2013, profile 4 (see fig 24 for location)

38

This aerial photograph of 2013 illustrates the areas of loss and gain of sediment from 2005-

2013. This figure is further evidence for the narrowing of the width of the beach, with a net

loss of sediment from the seaward side of the beach shown by the blue area. Small areas of

net gain appear on top of the barrier, showing the increase in crest height and the landward

retreat of the crest. Evident is the closing of the breach indicated by the large red area on the

east of the beach (fig.28)

Figure 28. Net loss/gain of sediment on areas of the beach from 2005-2013.

39

4.0 Discussion

4.1 Saltmarsh erosion

The saltmarshes and inter tidal mudflats spanning the area from Lymington to Sowley are of vital

importance for numerous reasons. They afford coastal protection by buffering hydraulic wave

action and therefore limiting the effects of heightened storm conditions. Furthermore the

saltmarsh and mudflats provide habitat for a number of important species in particular migratory

birds such as the Dark Brent Bellied Goose (Branta bernicla) The plethora of benefits provided by

saltmarshes are therefore thoroughly considered upon making coastal management decisions. It is

essential that sustainable saltmarsh management can therefore balance the evident conflicts

between socio-economic requirements of commercial and recreational, and furthermore coastal

defence obligations with nature conservation interests, of which Sowley Marsh is abundant

(Colenutt, 2002)

Due to saltmarsh recession instigating changes in the sediment budget the morphology of the

coastline is constantly evolving. The recession of the saltmarsh as aforementioned to the south-east

of the study area is causing the shoreline to become increasingly exposed illustrating the

fundamental control of coastal configuration concerning the transport and deposition of sand and

consequently the shingle found on Sowley barrier (Kiyoshi, 1981) This in turn is influencing the

source and volume of sediment input into this coastal system that is available for transport. In the

past analysis of the movement of this shoreline has been undertaken to further knowledge of

impacts of alongshore sediment transport (Cundy & Croudance, 1996)

Historical mapping is an established method of shoreline mapping that has been used a useful tool

to outline shoreline changes over a series of years (Bradbury, 1995) Evident from findings in section

3.0 the saltmarsh stretching from Lymington to Sowley has been eroding at a linear rate

(3m/y,fig.12) from 1940 to present. The loss of saltmarsh recorded in this thesis supports previous

observations of large reduction in the extent of saltmarsh found from Lymington to Sowley

40

(Colenutt, 2000) Due to the large gaps between years in the dataset it is challenging to identify

singular years where saltmarsh recession was above the average rate of 3m/y, conversely the

highest rate recorded was seen from 1968-1980 where the average rate was 5m/y indicating a

consistent rate of higher than 3m/y. Spatial and temporal variations are apparent between different

studies, understandably due to the error existing in aerial photographs (Xiankun,2007) The recession

of the saltmarsh accordingly has ramifications for the morphology of the area, the nature

conservation and the environmental state of the land behind the coastline, specifically Sowley

Marsh. One of the most influential factors in the recession of the saltmarsh is the effect of wind

wave action. This causes an acceleration of erosion concerning the seaward side of the saltmarsh,

but there is also potential for increased accretion and marsh development in the upper levels of the

marsh. In spite of this, in an undisturbed system, such as the saltmarsh from Lymington to Sowley,

there may only be a landward retreat of the saltmarsh and with Sowley marsh being the only low

lying land that supports the colonisation of saltmarsh, the net effect is therefore loss of saltmarsh

(Boorman et al., 2001) In recent years there is a visible increase in storm conditions (fig 14) which in

turn brings intensified wave conditions. This exacerbates erosion rates on the seaward side of the

saltmarsh consequently leading to faster marsh loss. Alongside wave action there are other

mechanisms causing the loss of saltmarsh.

As aforementioned the introduction of the hybrid Spartina Anglica in the late 1890’s (Hubbard,

1965) meant a large area of the south coast was colonised by this species. Its ability to trap sediment

and reproduce rapidly is the reason for large areas of saltmarsh appearing during this time. It is more

tolerant to a tidal submergence than other native Spartina on British saltmarshes meaning it could

colonise areas of mudflats that previous species such as Spartina Maritima could not (Goodman et

al., 1969) After the rapid colonisation of this species there has been dieback whereby the plant dies,

leaving organic matter and open sediment. Where the loss of these plants occurs there are no roots

to hold the sediment, causing the area to erode and ultimately causing retreat of saltmarsh. These

41

rates are intensified by the strong ebb tide dominance in the western Solent, causing sediment to be

taken offshore.

This ebb tidal regime is also influential in the recession of saltmarshes. Saltmarsh plants trap

sediment allowing the accretion of the mudflats and thus allowing the plant to colonise the area.

Wave and current processes on the seaward edge of the saltmarsh can lead to erosion of the mud

supporting the root mass, eventually causing the saltmarsh plants on top to collapse leaving behind

blocks of saltmarsh. Loss of sediment from the surface of the saltmarsh leads to abrasion platforms

(where wave action erodes the base) and in due course leads to the loss of vegetation on the

seaward side of saltmarsh (Allen, 2000) Upon examining the tidal regime of the Solent

(Townend,2002) it can be deduced that the fine grained sediments will be eroded offshore. The

resultant action is therefore an exacerbation of the lack of sediment supply that is input into the

saltmarshes, leading to often rapid rates of erosion.

One of the identifying factors in digitising the saltmarsh erosion was the presence of cheniers.

Alongside erosion seen from the saltmarshes, gravel and shells are transported in an east-west

direction during storm events in the Solent. Such material is deposited on the saltmarsh edge as

cheniers. As a result of this they represent accretional features, and thus indicate a variation in

environmental conditions, leading to an environment dominated by wave erosion(Schwartz, 2005)

The rate of accretion is seen to be very slow in the Solent due to the previously mentioned strong

ebb tidal currents. Therefore when there is an increase in water levels the lack of sediment causes

the marsh to deteriorate (Solent Forum, 2011)

Due to the loss in height and structure of the saltmarsh water logging of the inner areas of the

saltmarsh can occur. The effect of this process is the creation of an anaerobic environment in the

mudflats surrounding the saltmarsh. This is another contributing factor to the dieback of Spartina

(Doody, 2008)

42

4.1.1 Future saltmarsh projection

It is important to note that future projections for the erosion of the saltmarsh seaward of the study

site are based on linear rates of saltmarsh erosion calculated from the average erosion rate of the

marsh from 1946-2013 (3m/y) Upon examining the results it is clear that by 2062 the saltmarsh will

be eradicated, leaving behind shallow mudflats. This has serious ecological/morphological and social

impacts on the area as a whole, due to the role of saltmarshes as a buffer to wave action.

This loss of saltmarsh has potential to bring problems for management authorities, however this

data is not unknown to authorities, and thus proper preparation and management options should be

considered as to best manage the impacts of the loss of saltmarsh on the area. Figures of increased

return period for significant wave heights increased dramatically in the winter of 2014 (fig.14) and

with increasing impacts of climate change it can be hypothesised that conditions are likely to

continue worsening, thus exacerbating and delinearising rates of erosion seen (Bradbury,1995)

(fig.29) This will likely have an impact on the morphology of the barrier beach, evaluated in section

4.2.

0

0.5

1

1.5

2

2.5

1940 1960 1980 2000 2020 2040 2060 2080

Km

2 o

f sa

ltm

arsh

Year

Figure 29. Future projection of saltmarsh recession at a linear rate (red)

43

4.2 Breach/heal cycle of the barrier beach

Historical mapping of Sowley, dating back to 1920, reveals a breach/heal cycle in response to sub cell

development, changes in the sediment input to area of coastline, and barrier over washing, usually

during a storm event. The profile attributes of a barrier beach develops at a rapid temporal scale in

response to wave activity, acting on a passive water level datum (Powell, 1990) Barrier overwashing

is well known event concerning shingle barrier beaches as heightened wave conditions during storm

events often surpass the barriers pre-storm height. This temporal scale of evolution is of significance

to barrier stability regarding shoreline management. This coupled pre-existing knowledge of south

westerly prevailing wind and tide direction (Bradbury, 2000) insinuates that wave and tide induced

sediment transport are dominant processes in controlling the morphology of the barrier and tidal

inlet. However the relative forcing of these controls is perhaps weakened by evidence of increased

storminess and saltmarsh recession. Therefore storm conditions and saltmarsh recession are having

a more significant impact on the over morphology of the area than before, adding to the effects of

wave and tide induced transport (Baily & Pearson, 2007)

Although no aerial photographs are available from 1955-56 it is recorded that the first breaching

event of the barrier beach occurred during this period. This first breach, indicated by landowners,

was in response to a storm that hit this area of coastline, causing overwashing and overtopping. The

primary forcing variables acting on a shingle barrier are wave climate, superimposed on tidal

elevation that is known to occur for longer periods of time due to the double tide effect. This can

therefore be affected by storm surges and the local wave setup (Carter & Orford, 1993) The elevated

wave heights induced by storms caused the barrier to breach, leading to the inundation of saltwater

into the low lying topography behind the barrier beach. The original breach occurred on the western

side of the barrier. This initial breach led to the shoreline becoming more dissipative, with an

increasing width apparent in 1980, alongside wash over fans extending landward into the Marsh

area (fig.20)

44

During the epoch of the breach, there is evident backtoe retreat of the barrier beach due to rollback

and overwashing of the barrier caused by the 1955-56 storms responsible for the breach. This

coastal system has become increasingly dynamic due to the initial breach (By 1980 and eastward

migration of the breach is visible) as sediment transported along the shoreline begins to accumulate

on the western side of the breach. There is an accumulation of sediment 0.4km to the east of the

breach that was not seen in 1968 after the breach that is likely present due to the erosion of

saltmarsh to the south-east allowing coastal sediment transport processes such as longshore drift to

have more of an impact on the area. From 1980-2000 this accumulation of sediment has migrated

eastwards onto the eastern spit of the barrier. The beach has extended landward as more sediment

has accumulated. By 2005 the eastern side of the beach has migrated landward at an angle. The

western spit is extending rapidly towards the eastern spit, and the beach is close to resealing. By

2013 the barrier has resealed and more sediment is present on the eastern side of the breach, as the

barrier has moved seaward and assumed a more horizontal position facing the sea.

The importance of studying the history of the barrier is essential in understanding the specific

morphodynamic evolution and hydrodynamic behaviour of this coastal system. Although more

sediment is now visible on the beach than in 1946, increased storm conditions and relative sea level

rise will both impacts on the morphology of this beach in the future (Bradbury, 2000) This combined

with the continued erosion of the saltmarsh leaving the coastline more exposed with have further

impacts on the ecological characteristics of the area and thus have implications for the conservation

and designation of Sowley Marsh and its SSSI status.

45

4.2.1 Future breach/heal cycle projection

Shingle barriers move vertically and horizontally in response to overwashing or overtopping (Orford

et al., 1991) Upon examining results from section 3.3 there is evidence that shows steepening of the

vertical profile, and loss of sediment at the front of the barrier, thus narrowing the beach and

changing the position of the crest. The heightening of the vertical profile and narrowing of the beach

could therefore lead to the failure of the barrier and subsequently a movement landwards overtime.

Although this process is common in barrier beach evolution, it is important to note that due to the

rising topography of the land behind the barrier as significant further retreat of the beach landward

is unlikely. Primary forcing variables that rework barrier beaches are wave climate, seen to be

relatively low in the Solent, and the tidal elevation, where periods of double high tide occur in the

Solent. (Forbes et al., 1991) Due to the strong ebb tidal dominance caused by tidal asymmetry in the

western Solent there is a lack in the net input of sediment to the barrier systems. Wave energy in

the northwest Solent has been increasing since the 1950’s (SCOPAC, 2011) exacerbating the erosion

of the saltmarsh landward of the barrier meaning there is less protection for the coastline during

storm events. This therefore could lead to increased shoreline erosion. Net erosion consequently

causes a positive feedback loop wherein morphological irregularities along the shoreline caused by

saltmarsh erosion were amplified by the increasing force of wave action, in turn causing further

morphodynamic change (Orford et al., 1991) Furthermore the erosion of the saltmarsh has allowed

sediment transport processes such as longshore drift to have more impact on the area. Findings

insinuate that the barrier morphology responds mainly to sediment transport from littoral drift

(Bradbury, 1998) This is indicated by the eastward migration of the breach as it sealed, as longshore

drift caused a net movement of sediment from west to east along this stretch of coastline. This

process continues as the barrier tries to reach a dynamic equilibrium (Nicholls, 1985)

The first breaching event that occurred in 1955/56 was in response to a storm surge, that elevated

waters levels such that the breach occurred due to rollback and overwashing of the barrier.

46

Overwashing and overtopping events are often more common in areas where the vertical profile of a

beach is narrow and steep, as the impact of storm surges can often be profound, particularly at

meso tidal or macro tidal sites (Bradbury, 2000) The sealing of the breach in 2008 indicates there has

been an increase in sediment on the shoreline, thus allowing the breach to be sealed. However as

aforementioned and evident from figure 14, there has been a large increase in storms exceeding the

1 year return period in 2014. Wave conditions in these storms are significantly higher than normally

found. Therefore there is potential for another breaching event to occur as it did in 1955/56 in

response to a storm surge event. Conceptual models created by leading scientists infer that

overwashing can occur when wave run up elevation is much lower than the pre storm crest height

(Bradbury, 2000) Subsequently Sowley barrier could be more susceptible to overwashing events

infiltrating the lagoon water. At most the barrier beach only reaches an elevation of 2.5m ODN, and

wave heights recorded from early storms often exceed this height, however true predictions cannot

be made due to wave heights being relevant to chart datum (fig.14) The crest elevation of the beach

can be altered by a number of processes. Firstly the crest can be initially raised by overtopping

events, this occurs when limited overtopping of the barrier occurs. This process is present at Sowley,

as the supratidal beach comes higher and narrower.

The impact of sea level rise has long been documented in the Solent, however fluctuations in results

found infers that there is no consistent pattern of sea level rise. Regardless of this sea level rise is a

contemporary issue among coastal scientists, and the Solent is likely to be influenced by higher sea

levels in the future. Although not a concerning issue for the Solent, the impacts of sea level rise on

coastal areas are well recorded (Nicholls & Cazenave, 2010) In particular sea level rise in the Solent

will cause increased shoreline erosion. The effect of this could potentially lead to net increase in the

sediment input into this coastal system. The longshore drift process from west to east at Sowley will

therefore lead to an influx of sediment onto Sowley barrier and in turn could increase the barriers

stability.

47

However evidence to predict the future evolution of this barrier beach is not conclusive enough and

therefore conclusions should be seen as interpretative rather than definitive. The barrier could

potentially breach in response to adverse wave conditions, or could become more stable due to

increases in sediment. The future of this barrier beach will have important implications for coastal

management authorities, due to the potential for different environmental conditions and species

surrounding Sowley Marsh.

Ecological impacts 4.3

Brackish habitats are an ideal habitat for creating conservation at marine and estuarine sites, in that

they are often appropriate to many coastal landforms, and are of recognised environmental

importance (Kjerfve, 1994) When considering the designation of Sowley marsh it is important to

consider the highly variable and dynamic system, with freshwater infiltration from Sowley Pond and

saltwater infiltration either due to the breach in the barrier or through a small creek to the south-

east of the site. These highly variable inputs to the system lead to an unpredictable state of Sowley

Marsh. Prior to the sealing of the barrier beach Sowley Marsh provided a landward retreat for

saltmarsh. Spartina colonised the marsh after the initial breaching event in 1955/56. Due to the

resealing of the barrier between 2008-2010, the marsh is now permanently inundated. Owing to the

numerous designations for this site (SSSI,SPA) one management view could be orientated towards

trying to control the environmental state of the marsh, as to preserve its current nature

conservation status. Nevertheless, due to the dynamic nature of the site it is difficult to classify the

Marsh as the environmental conditions could change in response to a number of factors (Bradbury,

2000)

Currently the site is classified as a lagoon by Natural England (NA) and the WFD. This classification is

seen to be correct due to the current state of the sealed barrier. However the water framework

directive has set objectives for the lagoon to reach ‘good’ water quality by 2027 (Environment

Agency, 2015) This status means both good ecological status as well as good chemical status. Under

48

such classifications human interference would be needed to reach this goal. Considering the

coastlines NAI policy, this could potentially mean coastal management would need to change as to

stop Sowley marsh from changing environmental state. This seems unfeasible and unnecessary, as

the dynamic processes seen at the site are crucial to its nature (ABP Mer, 2011) The dynamic

processes at Sowley may been seen as exclusive, or alternatively the process at Sowley could be a

microcosm for other sites where statutory authorities try to designate the site and set future

objectives that would require human interference.

One possible route would be the de-classification of the site from the WFD. It is apparent that other

statutory authorities have previously seen the difficulties with designating and monitoring such a

dynamic site. Sowley marsh was previously designated under the Shellfish Waters Directive that was

aimed at ensuring a suitable environment for the growth of shell fisheries and promote water of

good quality. The Sowley site was de-designated in 2009 as reaching targets set would be expensive

in the short-term and difficult to maintain (ABP Mer, 2011) Habitats seen at Sowley Marsh are

unique and often incomparable to other standing water bodies, due to the environmental variables

associated with the site.

One contemporary issue with the management of Sowley is the saltmarsh found landward of the

barrier’s constant need for tidal inundation. The sealing of the breach has consequently lowered the

saltwater infiltration into the lagoon, thus changing the conditions. This means that the saltmarsh

plants have to adapt to survive in new conditions, and are often replaced by other more tolerable

species (Bamber et al., 1992) Sowley in particular is a unique site due to the landward retreat of

saltmarsh as the saltmarsh seaward of the barrier is eroding. Therefore the loss of the saltmarsh

behind the barrier has implications for the distinctive lagoon flora and fauna.

Under the WFD there is currently no standardised method of assessing coastal and lagoons habitats,

such as Sowley (NFNP, 2012) Essentially this means that current assessments conducted by the WFD

may not result in feedback that is viable to apply to the management of this site. The Solent Forum,

49

a forum that represent interest groups and stakeholders and provides information to its members to

aid consultation and raise awareness of coastal and marine issues, have further investigated this site.

This further investigation was undertaken to gather evidence regarding the WFD classification for

this site, as this status would be the basis of future management plans for the area. In coastal areas

such as Sowley marsh, there is a need to consider the obligations of European legislation directly

relating to protecting this sites integrity, and its favourable conservation status of species and

habitats. As before stated, it was found WFD tools for assessment were not suitable for this lagoon.

Therefore other statutory bodies that are involved with this site were further investigated as their

assessment tools may be better suited. Due to the important nature designations for this site

(SAC/SPA) it was concluded that further management from this site should come from these

requirements, as best to fulfil the need for the many important species of birds and invertebrates

found in Sowley Marsh and foreshore (Solent Forum, 2013)

There is a clear need to liaise coastal management with land management for this area, as best to

management it to its full potential. However it has been established that current management

options are often too vague. The numerous statutory bodies involved in the management of this site

often leads to overlapping and conflict, a common problem with coastal management (Conway,

2007) It is clear for this site that stakeholder participation needs to be organised such that statutory

bodies know when and how to be involved. When considering the nature of Sowley Marsh it may be

necessary for statutory bodies such as the WFD to take a more relaxed approach and let bodies such

as the Solent Forum that are more informed on this area to take charge, as to best fulfil the need to

balance nature conservation and land management regarding Sowley Marsh.

Following a report the Solent Forum produced containing information on the multiple designations

of the site, the Environment Agency gave feedback on the best route of management for the site

(Solent Forum, 2013) It was stated that the delineation of the waterbody may be altered so that it

becomes part of the transitional Solent water body. The feedback also stated that the delineation of

50

the waterbody from the WFD would be preferred. The NAI policy instated on this shoreline means

that the ability of Sowley marsh to respond to natural conditions is not influenced, and therefore

supporting natural development. The North Solent Shoreline management plan assesses the current

policies in place on this shoreline and whether they contribute to reaching environmental objectives

set by the WFD (NFDC, 2010) Information provided reveals that although leaving the site to fluctuate

in response to natural conditions may hinder the sites ability to reach environmental objectives, this

is not an issue as it provides further information regarding the landward migration of saltmarsh. In

addition to this the complex management needs of this site can be used to pilot further studies

involving complex hydrodynamic areas in the Solent.

Although the loss of saltmarsh is a concern for land based management, in terms of nature

conservation it is not seen as a large concern. The conversion of saltmarsh into intertidal mudflats

could provide more breeding space for many internationally important overwintering species of

birds that nest on the mudflats. Therefore it is the further loss of mudflats in the future that will

have the most effect on the SPA designation for this area.

5.0 Limitations

Prior to this study a number of reports indicate the erosion seen on the saltmarsh stretching from

Lymington to the study site at Sowley marsh. However one key issue when using historical aerial

photographs to illustrate erosion rates is the erroneous nature of historical photos. All photos were

geo-rectified and accompanied my metadata however error is still present. Errors involved in the

georeferencing and digitising process therefore mean that quantitative figures should be viewed

regarding the tendencies they represent rather than actual numbers. Although this is the case for

this thesis, results identified are similar to results found from numerous surveys of saltmarsh

recession in the Solent (NFDC, 2010) In addition calculations for the area and metres lost of

saltmarsh contain large standard error. However this is due to limited data points. Again results

found are in accordance with the North Solent Shoreline Management plan.

51

Due to time restrictions the total volume of sediment present in 2005 and 2013 could not be

calculated. This would perhaps give a better indication of the morphology of the coastline in the

future, and thus management options presented could be more tailored to the specific dynamics

found at Sowley Marsh. Furthermore there is very limited research into the current environmental

conditions at Sowley Marsh. However this could be seen as an opportunity instead of a limitation, to

further knowledge of saline lagoon habitats nationally.

52

6.0 Conclusion

Under the NAI shoreline policy implemented on this area of Solent coastline it is calculated that the

total eradication of the saltmarsh by 2062 or earlier if rates becomes delinearised. The loss of

saltmarsh will change the dynamics of the coastline, as assessed in this report. The conservation

value of this intertidal zone is well appreciated as the entire inter tidal zone of the Solent is

categorised under the Solent Maritime Candidate Special Area of Conservation (cSAC) Continuing

transition of saltmarsh to mudflats may prove beneficial to the numerous overwintering birds found

on this shoreline, as mudflats often provide valuable feeding sites for species such as the Dark Brent

Bellied Goose (Branta Bernicula) Saltmarshes and mudflats are considered of equivalent standing in

relation to biodiversity and nature conservation (Maddock, 2008) However the ubiquitous benefits

both of these ecosystems provide may be lost as it is hypothesised that they could be transitory

features located in a developing estuary. Therefore the total abolition of the mudflats would have

detrimental impacts on the internationally importance bird populations, and additionally the general

well-being of the Solent’s plant and marine populations.

The variable salinity found in Sowley Marsh is what gives rise to the distinctiveness of this site given

that the flora and fauna is restricted (due to the dynamic conditions) and often portray specialised

adaptations to cope with the fluctuating environmental conditions (Lamptey & Armah, 2008) Flora

and Fauna located in Sowley Marsh are often categorised into 3 groups, species that are tolerant of

high salinity, species tolerant of low salinity and lagoonal specialists. Often lagoonal specialists

exhibit different traits to identical species found outside of the lagoon area, and for this reason these

specialist species make the conservation value of Sowley marsh considerably higher as it is vital that

high biodiversity is maintained (Allen et al., 1995)

The aforementioned impacts of climate change on the Solent, and the UK’s coastlines as a whole

provides evidence for ongoing research into managed retreat, and creation of habitats behind the

coastline as to accommodate species that have lost their natural habitats. Coastal lagoons are listed

53

as a priority habitat on Annex 1 of the EC habitats directive (JNCC, 1996) and the Solent is considered

to be one of the most important areas for lagoons in the UK. Therefore due to Sowley Marsh’s high

conservation value, it is essential that research be done into the evolution of Sowley barrier, as best

to inform future management regarding the marsh. Although no conclusive evidence for the future

state of Sowley Barrier is provided, future research regarding the calculation of volumes of sediment

and focusing on wave overtopping could provide beneficial information on managing the site.

Bradbury (2000) created a proposed parametric framework for calculating the evolution of barrier

beaches. The validation of the barrier inertia threshold was achieved at Hurst spit, located west of

the study site. This framework was considered valid for a range of conditions found at Hurst spit, and

it may be applicable to the shoreline at Sowley. It is recommended that this framework is applied

and tested at other sites under carefully controlled measurement conditions, and research at Sowley

could prove beneficial for management authorities, or to further improve the model created so that

future research into barrier beach evolution can be conducted at higher confidence levels.

There are numerous threats currently facing many lagoons in England and Wales (Joyce et al., 2005)

One contemporary threat is pollution. Due to the restricted habitat influenced by the sealing of the

breach in 2007/8 nutrient enrichment could lead to eutrophication which could have considerable

negative effects on the numerous species found in Sowley Marsh. Furthermore increased freshwater

infiltration from Sowley Pond to the north could cause salinity levels to fall. In contrast to this a

further breaching event could lead to a saltwater influx that again would change the environmental

conditions at the site. In addition to this further research into climate change that induces higher sea

levels and increased storminess has revealed that increased incidence of storm events could lead to

an increased level of desiccation to the intertidal zone restricting the distribution of intertidal

species (Maltby, 1991) It is the variable dynamics of lagoon environments that make them of such

critical importance, and thus future management decisions must be thoroughly planned as best to

suit the needs of the environment presented at Sowley Marsh, and to offset future coastal losses

induced by climate change and rising sea levels.

54

7.0 Future management

An important aspect considered when applying the NAI policy is that the total erosion of the inter

tidal foreshore conveys a natural response to climate change and sea level rise (NFDC, 2010) Saline

lagoons are a unique and decidedly transient habitat. For this reason the inclusion of numerous

statutory organisations in the management of Sowley Marsh may not be the best solution for

management. The current designation of the site as a lagoon by the WFD is seen to be correct,

however reaching goals for high water quality by 2027 (Environment Agency, 2015) may not be

viable or necessary. Under the WFD there are currently no standardised method of assessing lagoon

habitats. Consequently the first management option to be considered for Sowley Marsh is the

delineation of this waterbody from the WFD.

As aforementioned the Solent Forum, a group of stakeholders that assess the state of the Solent

have conducted pilot surveys regarding the management of lagoon sites. It has been made clear that

conflicting views from statutory bodies concerning this site often makes management a difficult task.

Therefore it is evident that a meeting of stakeholders is required to collate management views. The

Solent Forum as an organisation provide a lot of local knowledge not just of Sowley Marsh but of the

Solent as a whole. The success of their ‘catchment based approach’ has proved that they are capable

of raising awareness of coastal and marine issues.

The dynamic processes at Sowley may been seen as exclusive, or alternatively the process at Sowley

could be a microcosm for other sites where statutory authorities try to designate the site and set

future objectives that would require human interference. As aforementioned one of the key points

regarding the management of this site is retaining its high biodiversity specifically its specialised

species. As a result of this it is recommended that management options for this site should be

provided by the SAC/SPA designations. It appears that there is no need for a ‘classification’ for this

site regarding its environmental characteristics, as fluctuations in any of the factors that effect this

55

site (Climate change, freshwater/saltwater infiltration) could result in a need to change the

classification.

Although the loss of saltmarsh is a key point for land based management, in terms of nature

conservation there are other implications. The conversion of saltmarsh into intertidal mudflats could

provide more breeding/feeding space for many internationally important overwintering species of

birds that inhabit the mudflats throughout the winter. Thus it is the further loss of mudflats in the

future that will have the most effect on the SPA designation for this area. For this reason research

into the opportunities provided by managed retreat is essential.

Saline lagoons can be created artificially to offset losses to habitat seaward of the coastline.

Therefore it is important to identify areas along the south-coast where this is possible. A large

contribution to the conservation of lagoonal species could be derived from temporary sites with a

short life span (Hills et al., 2009) There is also an opportunity to improve public participation in the

management of Sowley Marsh. The use of interpretation boards (board with information regarding

the site/area) and the involvement of local people in managing them could prove influential in the

management of this site. Following this there is also a need to liaise with the few local inhabitants

concerning the future of this stretch of coastline. Considerable flooding from the storm surge on the

14th February 2014 is already an indicator of the potential damage that could be caused on this

coastline(fig.15) More information for the public can be found in the North Solent Shoreline

Management Plan (NFDC, 2010)

It is evident that saltmarsh found from Lymington to Sowley Marsh and Sowley Marsh itself offer a

wide range of ecosystem services to the surrounding faunal floral and human community. Therefore

it is critical that further research into this area is conducted, as to sustain the extent of the lagoon,

and to best inform management authorities concerning other coastal lagoons nationally. It is advised

that Sowley marsh is left alone to let natural processes determine the site. The ephemeral, unique

and diverse habitats offered by saline lagoons are an integral part of the coastal system and thus

56

should be closely monitored and managed as best to inform future shoreline management plans

that best dictate coastal management in the future.

8.0 Acknowledgements

The author would first like to thank Andrew Colenutt from the Channel Coast Observatory (National

Oceanographic Centre) for his feedback and continued research that contributed towards the 2010

North Solent Management Plan and ultimately gave the author scope for this dissertation.

Furthermore to Lauren Saggers and Clare Wilkinson for making the secondary data used for this

dissertation readily available to the author.

57

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Appendices –

st dev st error

1946 0.384447656 0.192223828

1954 0.432078311 0.216039155

1968 0.403515799 0.201757899

1980 0.387760665 0.193880333

2000 0.242950612 0.121475306

2001 0.241798814 0.120899407

2005 0.234804884 0.117402442

2008 0.239774199 0.1198871

2010 0.22686266 0.11343133

2013 0.227229693 0.131191124

st.dev st.error

1946-1954 11.16035714 3.945782106

1954-1968 21.29945226 10.64972613

1968-1980 52.91007646 19.99812916

1980-2000 33.48797462 11.16265821

2000-2001 3.35824028 1.119413427

2001-2005 6.333333333 2.111111111

2005-2008 2.571208103 0.857069368

2008-2010 3.370624736 1.123541579

2010-2013 3.968626967 1.322875656

Appendix 1 – Standard deviation and error of

metres lost recordings for the saltmarsh.

Although relatively high levels of st.error are

shown, results are in accordance with numerous

saltmarsh studies undertaken on the Solent,

showing an average of 3m loss per year.

Appendix 1.1 – Standard deviation and error of

area loss recordings for saltmarsh. These results

show much less st.error than metres lost and

are also still in accordance with previous

studies.