Methodologies for Measuring and Modelling Change in Coastal Saline Lagoons Under Historic and Accelerated Sea-Level Rise, Suffolk Coast, Eastern England

Hydrobiologia (2012) 693:99–115 DOI 10.1007/s10750-012-1089-x

PRIMARY RESEARCH PAPER

Methodologies for measuring and modelling change in coastal saline lagoons under historic and accelerated sea-level rise, Suffolk coast, eastern England

Thomas Spencer • Susan M. Brooks

Received: 17 December 2011 / Revised: 12 March 2012 / Accepted: 16 March 2012 / Published online: 5 April 2012 Springer Science+Business Media B.V. 2012

Abstract Thorough assessment of vulnerable ‘time to extinction’. Loss rates are likely to accelerate coastal habitats, impacted by sea-level rise and considerably after 2015 and a fundamental revision of anthropogenic pressures, requires both the accurate UK saline lagoon creation targets is urgently required. establishment of the evidence base for current status The approach is generic and could be used to assess the and scientiﬁcally-informed forward planning of evolutionary trajectories for other vulnerable coastal expected future status. Coastal saline lagoons are habitats, under a range of near-future environmental transitional, ephemeral habitats of considerable con- change scenarios. servation interest; under European legislation their status requires on-going maintenance of ‘favourable Keywords Lagoon biology Reedbeds Barrier status’. Over decadal timescales, the seaward barriers dynamics Shoreline response models Shoreline that enclose saline lagoons migrate progressively Management Plans Southern North Sea landwards. Geo-referenced and digitised historic maps and aerial photographs are used to create a detailed trajectory of barrier migration and loss of lagoon area Introduction for three saline ‘broads’ on the rapidly retreating coastline of Suffolk, eastern England. The SCAPE Coastal habitats provide considerable challenges for shoreline response model is then employed to extend conservation and management which have become this trajectory, under a range of sea-level rise scenar- more complex and insistent over the last two decades ios, to 2050 and 2095 and to predict saline lagoon (1990–2010). After centuries of low valuation of coastal habitats—accompanied by habitat degrada- tion, fragmentation and loss, from both agricultural Handling editor: Pierluigi Viaroli and industrial land claim (e.g. Bakker et al., 1997; Wolters et al., 2005) and changed estuarine and T. Spencer (&) riverine hydrodynamics (e.g. van der Wal & Pye, Cambridge Coastal Research Unit, Department of 2004)—has come the realisation that coastal ecosys- Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK tems provide a wide range of ecosystem services of e-mail: [email protected] considerable direct and indirect beneﬁt to human populations (Zedler & Kercher, 2005). It is perhaps S. M. Brooks not surprising, therefore, that ‘in the relatively short Department of Geography, Environment and Development Studies, Birkbeck, University of London, space of 20 years we have moved from a position Malet Street, London WC1E 7HX, UK where maintaining the existing line of defence was a 123 100 Hydrobiologia (2012) 693:99–115

first priority to one where it is only one of a number of consequent upon accelerated sea-level rise and possi- options. Today not only has the enclosure of tidal land ble changes in storminess (Lowe et al., 2009). Such for agriculture ceased … but also habitat restoration environmental futures suggest the need for proactive, and re-creation for conservation and sea defence long-term conservation planning rather than reactive, purposes has become much more acceptable’ [Doody short-term responses to events such as windstorms, (2004, pp. 136–137) in discussing UK coastal policy]. river floods and coastal surges. However, as Leafe et al. (1998, p. 288) point out, this It is clear that better-informed decision-making on shift in thinking has generated dilemmas for nature coastal habitat futures requires improved scientific conservation interests where ‘the issues of sustain- data on status. In addition, there is a need for new ability and biodiversity tend to intertwine and can methodological tools, both to improve near-future easily become confused’. If an engineered structure forecasting of habitat areal change and to better currently protects an area of conservation importance constrain the envelope of uncertainty around such should the defence be removed to create a more forecasts. In this paper, we demonstrate how (i) the use environmentally robust shoreline of likely value in of geo-referenced and digitised historic maps and reducing flood risk or should the conservation interest aerial photographs can be used to create a detailed prevail, particularly if the reason for the designation is trajectory of coastal lagoon change and (ii) environ- of national or international significance? mental modelling can be used to extend such a In the USA, such debates have been sharpened by trajectory into the near-term future, under a range of the contexts of ‘no-net-wetland-loss’ policies and the sea-level rise scenarios. Such an approach provides, instrument of the Clean Water Act (e.g. Zedler, 2004). amongst other benefits, improved input into the In Europe, legislative pressure has been applied by necessity for, and timing of, compensatory habitat European Union directives, most notably by the provision. We illustrate this approach using the Habitats Directive (Council Directive 92/43/EEC). example of the history and likely fate of the saline The purpose of the Directive is to maintain or restore lagoons of the Suffolk coastline, eastern England. at ‘favourable status’ a representative network of Here, the shoreline has been displaced between 300 natural habitats (Annex I) and species of wild fauna and 600 m landwards since the 1880s. This research and flora of community interest. ‘Favourable status’ is thus goes some way towards refuting, for example, defined as a position where the range and area of statements such as ‘although coastal lagoons face habitats are stable or increasing and where sufficient severe threats from global warming (with sea-level areas of habitats exist to maintain viable long-term rise) and associated coastal squeeze in the longer-term, populations of key indicator species (Council of the there are currently insufficient data to quantify these European Communities, 1992). Maintenance of hab- threats’ (JNCC, 2007, p. 14). We stress, however, that itat area is a key measure of maintenance of status and the approach is generic and could be used to assess the thus generates a need for regular monitoring within a evolutionary trajectories for other vulnerable coastal delineated spatial unit. Furthermore, status needs to be habitats, under a range of near-future environmental measured against a benchmark condition recorded at a change scenarios. particular point in time. These demands run counter to the notion of a dynamic coast where both extent and Characteristics of saline lagoons condition oscillate within an envelope of possible natural environmental states. As Pethick (2002, Coastal saline lagoons are areas of comparatively p. 433) notes, this is equivalent to ‘a preservation shallow water, defined by the combination of three policy, immobilizing the undeveloped areas of the characteristics: ‘(i) the presence of an isolating barrier [British] coast by legislation, in the same way that beach, spit or chain of barrier islands; (ii) the retention inappropriate infrastructures have immobilized the of all or most of the water mass within the system developed coast’. Finally, these challenges are likely during periods of low tide in the adjacent sea; and (iii) to intensify in the near future with changing physical the persistence of natural water exchange between the energetics in the coastal system. Greater wave lagoon and the parent sea—by percolation through and tidal activity are to be expected (Nicholls et al., and/or overtopping of the barrier, through inlet out- 2007), enhanced by increased nearshore water depths, flow channels, etc.—permitting the lagoonal water to 123 Hydrobiologia (2012) 693:99–115 101 remain saline or brackish’ (Barnes, 1989, p. 296). On macro-tidal coasts ([4 m tidal range), coastal lagoons are largely restricted to locations where gravel barriers have accumulated under postglacial sea-level rise, migrating landwards and alongshore to enclose river valleys. In such settings, with lagoons at relatively high elevations above sea level, continued freshwater inflows and terrestrial sediment additions lead either to evolution towards freshwater lakes or to continued barrier retreat ‘squeezing out’ the lagoon between the seaward barrier and higher ground to landward. A further subset of lagoon type is where the gravel structures themselves incorporate saline lagoons. The unusual and ephemeral nature of all of these systems—which are typical of the North Atlantic region—thus makes them of considerable conservation interest, not least in the face of near-future accelerated sea-level rise. For gravel barriers at the mesoscale (\102 year), it is decadal scale sea-level rise that is the driver of barrier migration (Orford et al., 1995). Barnes (1989) identifies 41 saline lagoons around the UK coastline. A significant proportion of this resource is found in 11 areas around the coastline of eastern England. In this paper, we focus on a set of three saline lagoons of the Suffolk coast, between Benacre Cliffs and Southwold (Fig. 1). Known locally as ‘broads’, they comprise a series of discrete brackish water lagoons, fringed by reedbeds of Phragmites australis (Cav.)Trin. ex Steud. and located within small river valleys surrounded by farmland and woodland. They are fronted at their seaward margins by sand and gravel barriers (Fig. 2), with crest elevations of 2.0–3.0 m Ordnance Datum Newlyn [ODN (which approximates to mean sea level)], above narrow (*45 m), steep (6–7) and gravelly beaches (Pontee, 2005). Details of the environmental process setting are reported in Table 1. An initial inventory of these lagoon systems was made between 1984 and 1989 by English Nature (now Natural England) (Smith & Laffoley, 1992; Downie, 1996) and described in a series of scientific papers by R.S.K. Barnes (Barnes, 1980, 1987, 1991, 2001; Bamber & Barnes, 1998). The demonstration sites used in this paper relate to two topographic settings: (i) stream-fed lagoons trapped behind sand and gravel Fig. 1 Location of study area, Suffolk coast, UK. Bathymetry barriers, also known as ‘isolated lagoons’ and (ii) from Admiralty Chart 1543 (Winterton Ness to Orford Ness) ‘percolation pools’ fed by seepage through such 17th edition, June 2005. Cross-hatched areas low-lying coastal barriers (Barnes, 1989). Benacre Broad and Covehithe wetlands 123 102 Hydrobiologia (2012) 693:99–115

Fig. 2 The Suffolk broads in 2010. A Benacre Broad; (5–15 cm vertical and 25 cm spatial resolution, profiles B Covehithe Broad; C Easton Broad (NB: Potter’s Bridge extracted from ‘bare earth’ digital terrain model), flown on 23 forms the far left (western) margin of the Broad). Total area of May 2008. Note the barrier breaching evident in the long profile open water and reedbed/marsh digitised from Environment and in a cross profile (transect 2) at Benacre Broad, reducing the Agency geocorrected aerial photographs [supplied by the maximum barrier elevation to below the level of MHWS. A Shoreline Management Group (Anglian Region)]. Long profile similar feature was not evident at either Covehithe Broad or at gravel barrier crest and barrier cross-profiles derived from Easton Broad UK Environment Agency (Geomatics Group) LiDAR data

Broad are examples of type (i) and Easton Broad of the European Communities, 1992) and thus designated type (ii). These settings are characteristic of many as Special Areas of Conservation (SACs); (ii) exam- European saline lagoon locations in NW Europe. This ples of habitats which support internationally impor- study thus provides a template for the application of tant bird features designated in Special Protection this methodological approach elsewhere in this region. Areas (SPAs) under the EC Bird Directive; and (iii) With adjustment, a similar methodology might be may be included on the ‘Ramsar List’ of wetlands of applied to the extensive lagoon margins of the international importance (under wetland type j). Mediterranean Sea (e.g. Perez-Ruzafa et al., 2011). Furthermore, as examples of transitional/coastal waters, they fall under the EC Water Framework Conservation importance of saline lagoons Directive whose objectives require prevention of deterioration, with the aim to achieve ‘good ecological The nature conservation importance of saline lagoons status’ by the year 2015. As with the Habitats in Europe is reﬂected by the fact that they are Directive, the Water Framework Directive imposes a (i) a priority Annex I habitat under the EC Habitats requirement to monitor, assess and report the ecolog- Directive (Council Directive 92/43/EEC; Council of ical quality of these habitats (Bamber, 2010). In

123 Hydrobiologia (2012) 693:99–115 103

Fig. 2 continued addition in a UK context, saline lagoons are (i) incor- potential complete loss of this habitat. The broads are porated into National Nature Reserves; (ii) qualifying threatened with the loss of brackish and freshwater features and designated as Sites of Special Scientiﬁc reedbed habitats and their associated SPA designated Interest (SSSIs); and (iii) included as a priority feature bird populations of bittern (Botaurus stellaris) and under the UK Biodiversity Action Plan (UKBAP). marsh harrier (Circus aeruginosus). Loss of nest sites The Suffolk Coast and Estuaries Coastal Habitat and freshwater food resources on increased tidal Management Plan (English Nature/Environment inundation may severely affect breeding bittern pop- Agency, 2002) recognizes that continued retreat of ulations. Recent modelling suggests that the complete the Suffolk coastline will result in reduction and loss of Easton Broad, with neighbouring reedbeds at 123 104 Hydrobiologia (2012) 693:99–115

Fig. 2 continued

Minsmere and Walberswick, will halt growth and send Agency, 2002). In addition, changes to the foreshore the UK population into decline (Gilbert et al., 2010). may result in loss of open coast shingle habitat, Furthermore, these lagoons form part of a habitat potentially leading to the loss or reduction of suitable chain for these species, linking areas to south with habitat for breeding little tern (Sterna albifrons)(a the Norfolk Broads (English Nature/Environment designated SPA interest). 123 Hydrobiologia (2012) 693:99–115 105

Table 1 Environmental process setting for the study area (see road crossing at Potter’s Bridge (Fig. 2C) as this Fig. 1) provides an unambiguous limit for the coastal section Predicted tidal levels (m ODN) of this river valley. 1.40 Highest Astronomical Tide (HAT) As well as defining total area for each broad, the 0.90 Mean High Water Springs (MHWS) area of open water was also extracted from the maps 0.11 Mean Sea Level (MSL) and aerial photographs. The boundaries of the broads -1.00 Mean Low Water Springs (MLWS) and the water edge were carefully digitised by a single 1.90 Mean Spring Tidal Range (MSTR) operator for each year of map survey and aerial Measured extreme water levels (m ODN) photography, and stored as polygon features in a 3.44 31 January 1953 shapefile. The area of each feature was found using the 2.71 29 September 1969 measure feature tool in the software package ArcMap 9.2 (www.esri.com). 2.69 1 February 1983 2.68 3 January 1976 Measurement of historic lagoon areas, 1883–2010 2.68 21 February 1993 2.63 9 November 2007 Initial analysis involved a combination of 1:10,560 Wave heights (m) historic OS maps, surveyed in 1882–1883 (plotted as 7.3–7.8 Modeled 1 in 100 year offshore 1883), 1905, 1921–1928 (plotted as 1925) and 1947, (48 km from coast) wave height and vertical aerial photographs taken as part of the UK 4.3–4.5 Modeled maximum annual offshore Environment Agency (EA) Sea Defence Management (48 km from coast) wave height System (SDMS) monitoring programme. The historic 0.4–0.5 Measured mean annual inshore maps are available digitally (at www.edina.ac.uk/digi Wave height, 1975–1979 map/), and the aerial photographs were supplied in a Sources: Predicted tidal levels and modelled wave heights from georeferenced format by the Environment Agency Pye & Blott (2006); extreme water levels from Pye & Blott Shoreline Management Group (Anglian Region). (2009); measured inshore wave heights from Fortnum & Hardcastle (1979) Eight photographic tiles (1 km 9 1 km) covered the coastline between Benacre and Southwold. Photog- raphy from the years 1994 [the start of the first Materials and methods reporting period for the EC Habitat Directive (Euro- pean Commission Environment, 2011)], 2000 (the end Definition of lagoon boundaries of the first reporting period), 2006 (the end of the second reporting period) and 2010 were used. Maps The boundary between the area occupied by the low- and aerial photographs were first individually regis- lying broads and the marginal slopes under agriculture tered against the 2010 Environment Agency aerial and woodland is well defined and can be readily photograph, using ArcMap 9.2 within the British extracted from both map sources, back to the 1880s, National Grid (OSGB36) co-ordinate system. Errors and aerial photography. Under such a boundary associated with georeferencing and digitising shore- definition, the change in the perimeter of a broad is lines from maps and aerial photographs have been restricted to the changing position of the enclosing reported in Brooks and Spencer (2010), using indepen- barrier. This position is best defined by the landward dent control points. On historic timescales ([100 years) margin of the barrier as this is represented on maps and errors were reported as 6.46 m, which is 1.7% of the aerial photographs by a clear boundary between the total retreat on this timescale. The errors decline as maps sands and gravels of the barrier and the seaward become more recent, being 4.50 m (1.2% of total margin of both the lagoon water mass and marginal retreat) in the period 1947–2008. For aerial photographs reedbed vegetation. taken annually since 1992, errors have been found to be At Easton Broad, the low-lying marshes of the less than 1 m. These linear error terms were used to enclosing valley are particularly extensive, extending determine errorsassociatedwiththe calculation ofbroad over 3.5 km inland from the coast into the Easton valley. area, for each year of analysis. The methodology Here, the landward lagoon margin was delimited by the involved creating a buffer around each digitised broad, 123 106 Hydrobiologia (2012) 693:99–115 with the size of the buffer being given the same value as 2.47 ± 0.23 to 2.57 ± 0.33 mm a-1 (1956–2006; the linear error for each respective date. The 1883 dig- Woodworth et al., 2009). In addition, from a visual itised broads used a buffer of 6.5 m and the 1947 anal- inspection of this tide gauge record, Pye & Blott (2006) ysis included a buffer value of 4.5 m. All recent aerial argued for a significant upward trend in sea-level since photographs were assigned buffer values of 1.0 m. This the mid 1970s, identifying a rise of 13 cm between generated maximum errors in the 1883 broad areas of 1975 and 2005. This corresponds to a rate of relative 4.3, 5.4 and 5.2% for Benacre Broad, Covehithe Broad sea-level rise of 4.3 mm a-1. Based upon this estimate, and Easton Broad respectively. By 2010 the 1 m max- a figure of 4.4 mm a-1 was used initially in this imum linear error in the aerial imagery generated errors analysis for the periods 1990–2050 and 1990–2095. in the broad areas of 0.8, 1.7 and 1.0% for Benacre This rate broadly corresponds to the median (50th Broad, Covehithe Broad and Easton Broad, respectively. percentile) of the medium emissions scenario of the UK Climate Impacts Programme, UKCP09. Measurement of cliffline retreat, 1947–2010 A further set of sea-level rise predictions for Lowestoft was obtained from application of the Using shorelines for 1947 (clifftop edge from the OS UKCP09 user interface (http://ukclimateprojections. historic map) and 2010 (clifftop edge from EA aerial defra.gov.uk/content/view/1102/500). These were photography), the Digital Shoreline Analysis System selected to examine potential shoreline change under (DSAS) extension of ArcMap 9.2 (Thieler et al., 2005) higher sea-level rise scenarios. The 95th percentile of was used to establish spatially detailed estimates of the medium emissions scenario predicts a rise in rel- shoreline retreat. At every 10 m interval, shore- ative sea level (against a 1990 baseline) of 34 cm by normal transects were cast from a baseline to intersect 2050 and 70 cm by 2095. For modelling purposes, the two clifftop edges in 1947 and 2010, producing a these increases were translated into linear rates of total of 490 estimates of the annual retreat rate (in relative sea-level rise of 5.7 and 6.7 mm a-1 to 2050 DSAS terminology the ‘End Point Rate’) over this and 2095, respectively. These rates are broadly com- time period. parable to earlier sea-level rise estimates provided by the UK Government’s Department of Environment, Determination of future lagoon areas at 2050 Food and Rural Affairs (DEFRA) of 6 mm a-1 (and and 2095 see Halcrow Maritime, 1991) and to the UKCIP 2002 prediction of 69 cm of regional sea-level rise to the Future lagoon areas were determined from predictions 2080s (i.e. sea-level rise of 7.7 mm a-1 for the period of future shoreline positions at 2050 and 2095. This after 1990; Hulme et al., 2002). The UKC09 high- required (i) the selection of appropriate future regional plus-plus (High ??) scenario was also evaluated. sea- level rise scenarios; (ii) the choice of an appro- This predicts a sea-level rise of 93 cm by 2095 (Lowe priate shoreline response model to relate the rate of et al., 2009). The resulting rate of sea-level rise, anticipated sea-level rise to the rate of shoreline 8.9 mm a-1, is comparable to the DEFRA (2006) sea- recession; and (iii) application of the ‘best fit’ shoreline level rise figures of 8.5 mm a-1 (2025–2055), rising to response model. 12.0 mm a-1 (2055–2085).

Regional sea-level rise scenarios Shoreline response models

At Lowestoft, twentieth century mean sea-level rise For the second stage of analysis, we evaluated a range has been estimated at 1.81 ± 0.48 mm a-1 (Shennan of different shoreline response models; full details are & Horton, 2002), similar to estimates of global sea- reported elsewhere (Brooks & Spencer, 2012). Clear level rise over this time period (Church & White, and unambiguous definition of the exact position of 2006). However, the more recent record from the UK broad, convex gravel barrier crests in sequences of National Tide Gauge Network station at Lowestoft aerial photographs is very difficult to achieve [see shows a higher rate of relative sea-level rise. This has Moore (2000) for further discussion on shoreline been calculated variously at between 2.4 mm a-1 definition and mapping]. However, it is clear from (1964–2001; French & Burningham, 2003) and both historic maps and aerial photographs (see Fig. 6, 123 Hydrobiologia (2012) 693:99–115 107

Brooks & Spencer, 2012) that the gravel barrier crests 2095; (b) 5.7 mm a-1 to 2050 and then 6.7 mm a-1 to on the Suffolk coast are broadly aligned with the 2095 and (c) 8.9 mm a-1 to 2095; and known rates of cliffed sections that lie to either side of the broads. The shoreline recession in the period 1947–2010 (R1), the precise processes of inland migration of the shoreline SCAPE model could be used to ascertain the likely remain to be quantified and evaluated, but given that in future shoreline position in the years 2050 and 2095 every historic map and aerial photograph available under the different sea-level rise scenarios, (a) to (c). since 1883 this alignment is evident, the migration of the cliffline reflects the migration of the barrier. Any Alongshore variations in shoreline retreat rates temporal lag that may exist between cliff migration and changing broad areas, 2050 and 2095 and subsequent barrier migration is smoothed over the timescale of analysis reported in this paper. The SCAPE model was then used to generate inland The clifftop edge is a clear and unambiguous retreat of the shoreline, for each of the sea-level rise marker that can be readily extracted from maps and scenarios, at the 490 points alongshore. These points aerial photographs. It is argued, therefore, that (i) the were then converted into eastings and northings. This coastal recession rates derived from the change in enabled the shoreline position to be precisely located, clifftop edge position can be used as a surrogate for including the position of the gravel barriers enclosing changing lagoon barrier position and (ii) response the broads. Polygon features were then created by models that best describe cliff retreat are likely to be digitising the entire boundary for each broad under appropriate in describing lagoon barrier migration. each of the sea-level scenarios. Total broad area to Shoreline retreat was established from map evi- 2050 and 2095 was measured using the measure dence for the period 1883–1947 when the average rate feature tool in ArcMap 9.2. of regional sea-level rise was 1.8 mm a-1. Shoreline retreat was then measured between 1947 and 2010, a period characterised by a mean sea-level rise of 2.5 mm a-1. Five shoreline response models were Results then used to predict retreat in the periods 1947–2010 and 1992–2010, using 1883–1947 as the baseline Historic analysis of changing lagoon areas, period. Comparisons were made between the modelled 1883–2010 and the actual retreat of the clifftop edge, from historic maps, ground surveys undertaken annually at 1 km Planform change in each of the three Broads in the spacing alongshore by the Environment Agency and period 1883–2010 is shown in Fig. 3. Each Broad from aerial photography. Results showed that the Soft experienced a decline in total area, with the precise Cliff And Platform Erosion (SCAPE) model, validated nature of the change being different for each Broad. on the UK coast at The Naze, Essex (Walkden & Hall, Until recently, changes in the area and location of 2005) and in North Norfolk (Walkden & Hall, 2011), Benacre Broad (Fig. 3A) have been influenced by the provided the best goodness-of-fit between expected northward migration of Benacre Ness. In 1883 Bena- and recorded patterns of shoreline change. The model cre Broad extended to the shoreline through a prom- (Walkden & Dickson, 2008) has the form: inent southward trajectory; Benacre Ness was located immediately seaward of the broad thereby preventing rffiffiffiffiffi an outlet from developing at this point. However, by S R ¼ R 2 ð1Þ 2010 Benacre Ness was located entirely to the north of 2 1 S 1 Benacre Broad, enabling an easterly aligned outlet to where R1 and R2 are the historic and future coastal develop. Between 1883 and 2010 Benacre Broad -1 recession rate (m a ) respectively; S1 and S2 are the migrated 600 m inland in response to the very high, historic and future rates of relative sea-level rise sustained rates of shoreline retreat at this locality. (mm a-1), respectively. Covehithe Broad (Fig. 3B) also showed a high rate Then, using a baseline sea-level rise of 2.5 mm a-1 of inland migration, of 500 m between 1883 and 2010.

(1947–2010) (S1 in the SCAPE model); post-2010 sea- Here, the migration was focused in a north-westerly -1 level rise rates (S2) of (a) 4.4 mm a to 2050 and direction. Inland movement was lower at Easton 123 108 Hydrobiologia (2012) 693:99–115

bFig. 3 Change in planform of each of the three broads, 1883–2010. A Benacre Broad; B Covehithe Broad; C Easton Broad. Note the northward trajectory in the migration of Benacre Broad, particularly evident until 1947, after which inland movement took place in a westerly direction, a more shore-normal alignment. At Covehithe Broad and Easton Broad landward migration was more consistently in a northwesterly direction

Historic change in the total area of each broad resulting from inland migration is shown in Fig. 4.In 1883 the three broads together had a total are of 174 hectares, with Easton Broad and Benacre Broad having a far greater extent than Covehithe Broad. By 2010 the total area had declined to 115 ha, an overall loss of 33%. The greatest percentage loss of area was experienced by Covehithe Broad (40%), with Benacre Broad having a 27% loss and Easton Broad a 39% loss of area. However, Covehithe Broad is the smallest of the three systems and has, therefore, suffered the least absolute areal loss per year (Fig. 4). The greatest absolute areal loss (28.5 ha) was seen at Easton Broad which also showed the greatest loss rate per year (0.23 ha a-1) over the 127 years of record. Benacre Broad and Covehithe Broad experienced areal loss rates of 0.15 and 0.09 ha a-1, respectively. The greater alongshore extent of Easton Broad more than com- pensates for the slightly lower rate of shoreline retreat. Figure 4 illustrates a marked acceleration (threefold) in the rate of areal loss at Easton Broad, as well as a slight increase in the rate of areal loss at Covehithe Broad since 1947. Conversely, the rate of loss at Benacre Broad has been relatively invariant over the entire monitoring period.

Historic change in lagoon open water/reedbed distribution

The total loss of area of broadland between 1883 and 2010 is reported in Table 2. However, as the broads have declined in area they have also undergone changes in the relative proportion of open water and reedbed/marsh (Table 2; Fig. 4). The ‘isolated lagoons’ of Benacre Broad and Covehithe Broad showed considerable gains in the relative as well as absolute area of open water compared with reedbed Broad (Fig. 3C), at 300 m between 1883 and 2010, over the study period. The area of reedbeds has although the most recent period (2006–2010) showed declined by over 50% in both systems. Furthermore, as greater inland migration than either Benacre Broad or a percentage of the total broadland, the reedbeds Covehithe Broad. changed from occupying over 90% of the total 123 Hydrobiologia (2012) 693:99–115 109

Fig. 4 Changes in total broadland area for the years 1883, 1905, 1925, 1947, 1994, 2000 and 2010, sub- divided into area occupied by open water and reedbed/ marsh. A Benacre Broad; B Covehithe Broad; C Easton Broad. Note the large increases in the extent of open water for Benacre Broad and Covehithe Broad associated with the reduction in total area, compared with the steady decline in the area of open water at Easton Broad

broadland space in 1883 to\70% in 2010. By contrast, total areal loss to 2095 is 81 ha. At 8.9 mm a-1 to at the ‘percolation pool’ of Easton Broad, both the 2095, this ﬁgure rises to 89 ha. Under these latter absolute and the relative proportion of open water scenarios, the area of low ground that can accommo- declined over time. The reedbed/marsh area at this site date broadland is reduced to 33 and 25 ha, respec- declined in absolute terms but as a proportion of the tively. Hence, under the most extreme sea-level rise total broadland area it increased from 76 to 97% scenario tested in this study just 14% of the 1883 between 1883 and 2010. These differences are broadland area remains by 2095. discussed further in the ‘‘Discussion’’ below. The largest area of remaining broad in 2095 is at Benacre Broad, due to the combination of a relatively Predicted lagoon areas at 2050 and 2095 large initial area and a lower overall rate of areal decline when compared with Covehithe Broad and Table 3 and Fig. 5 show the projected future changes Easton Broad (Fig. 4). Whilst Easton Broad had a in the area of the three broads to 2050 and 2095, as very similar areal extent to Benacre Broad in the time calculated using the SCAPE shoreline response model period 1994–2006, it shows a more rapid predicted under a range of sea-level rise scenarios. Under the loss of area after 2010. Whereas the loss of area continuation of the recent rate of sea-level rise (i.e. at Benacre Broad under the lowest sea-level rise 4.4 mm a-1), the additional total areal loss between scenario (4.4 mm a-1, 2010–2095) is expected to be 2010 and 2095 is predicted to be 69 ha, at an accel- 0.23 ha a-1 between 2010 and 2095, at Easton Broad erating rate, from 0.5 (2010–2050) to 0.8 (2050–2095) the loss rate is almost double this ﬁgure, at 0.42 ha a-1. ha a-1. The remaining area that might be occupied by The lowest rate of predicted areal loss occurs for the broads by 2095 is 45 ha, 26% of the area of low-lying lowest sea-level rise scenario (4.4 m a-1) for Coveh- ground (174 ha) in 1883. At a sea-level rise rate of ithe Broad (0.17 ha a-1). However, even under this 5.7 mm a-1 for the period 2010–2050, accelerating to most conservative estimate of sea-level rise, the 6.7 mm a-1 between 2050 and 2095, the predicted remaining area of low-lying ground is only 3 ha by 123 110 Hydrobiologia (2012) 693:99–115

2095. The higher sea-level rise scenarios (5.7–6.7 mm a-1, 8.9 mm a-1) exacerbate these trends (Fig. 5). At the highest rate of sea-level rise, 8.9 mm a-1, only Benacre Broad

Reedbed/ marsh area (%) potentially occupies a viable area of low-lying ground to landward of its present position by 2095 (predicted potential area that remains = 21.5 ha). Easton Broad is reduced to a

Water area (%) barely viable area (3 ha), and Covehithe Broad effectively ceases to exist (\0.5 ha).

Reedbed/ marsh area (ha) Discussion

‘Time to extinction’ for saline lagoons Water area (ha) Figures 4 and 5 shows historical (1883–2010) and predicted (2010–2050/2095) changes in saline lagoon Total area (ha) area respectively for Benacre Broad, Covehithe Broad and Easton Broad. Trendlines through these plots, based on an extrapolation post-2000 (the point at

Reedbed area (%) which broadland loss rates appear to accelerate, particularly at Easton Broad), enable ‘time to extinction’ to be estimated for each broad.

Water area (%) Different shoreline response models will produce different ‘times to extinction’ for the Suffolk lagoons. For this coastline, we have found that the SCAPE

Reedbed area (ha) model provides the best goodness-of-ﬁt to coastline change in the period 1947–2010 (Brooks & Spencer, 2012). The prediction of time to extinction is also

Water area (ha) underpinned by the rate of sea-level rise chosen for the baseline period, in this case 2.5 mm a-1. Hence for predictions elsewhere the baseline sea-level rise rate

Total area (ha) needs to be chosen with care. Finally, the digitising errors and associated errors in the calculated areas of the broads reported above potentially contribute to

Reedbed area (%) errors in the estimation of time to extinction. However, this paper presents analysis that suggests the maximum error terms for the post-2000 extrapolation

Water area (%) period are just 1.6%. Hence, we suggest that predictions of time to extinction should ideally incorporate the most recently available imagery and not be based upon the total period of record available historically. Reedbed area (ha) We predict lagoon extinction by 2157–2236 at Benacre Broad, 2092–2112 at Covehithe Broad and

Water area (ha) 2098–2117 at Easton Broad. The longest survival times relate to the extrapolation of a sea-level rise of 4.4 mm a-1 beyond 2095, whereas the shorter times Total area of individual broads (ha) and total and percentage areas of open water and reedbed/marsh, 1883–2010

Total area (ha) to complete loss are associated with the further extension of the sea-level rise rate of 8.9 mm a-1 1883 71.01905 69.41925 6.3 65.81947 6.5 60.61994 64.7 6.4 55.42000 62.9 6.1 53.02010 59.4 6.1 8.9 52.1 15.6 54.5 9.3 19.8 49.3 91.1 9.8 37.4 90.7 10.0 32.3 90.2 11.0 29.1 90.0 29.4 26.5 0.7 89.0 38.1 25.4 70.6 0.7 23.7 62.0 1.3 28.4 18.6 1.7 17.9 25.8 6.6 17.3 24.1 5.8 2.3 22.1 5.5 2.7 12.0 97.7 12.0 5.1 97.3 7.1 11.9 35.5 94.9 73.6 32.5 92.9 72.0 64.6 31.6 17.7 67.5 70.1 16.2 64.7 68.4 55.9 50.0 14.9 48.4 55.7 9.0 45.1 1.3 55.2 2.1 24.0 55.7 1.1 48.8 22.6 46.3 76.0 21.3 44.0 77.4 13.9 78.7 2.5 86.1 4.2 97.5 2.4 95.8 97.6 Table 2 Year Benacre Broad Covehithe Broadafter 2095. These estimates Easton Broad assume that the broads 123 Hydrobiologia (2012) 693:99–115 111

Table 3 Projected future change in broadland area (ha) to 2050 and 2095, calculated using the SCAPE shoreline response model Year Benacre Broad Covehithe Broad Easton Broad 4.4 mm a-1 5.7/6.7 8.9 mm a-1 4.4 mm a-1 5.7/6.7 mm a-1 8.9 mm a-1 4.4 mm a-1 5.7/6.7 8.9 mm a-1 mm a-1 mm a-1

2010 52.1 17.3 45.1 2050 44.5 43.2 9.1 8.6 27.2 24.8 2095 32.4 26.2 21.5 3.2 1.6 0.5 9.7 5.6 3.2 Range of sea-level rise scenarios: (a) 4.4 mm a-1 to 2050 and 2095; (b) 5.7 mm a-1 (2010–2050) and 6.7 mm a-1 (2050–2095); (c) 8.9 mm a-1 to 2095 will continue to migrate landwards within the low- can significantly retard growth in P. australis (Hart lying coastal valleys within which they lie. However, et al., 1991). By contrast, the ‘percolation pool’ of it has been argued that the relatively rapid steepening Easton Broad experiences low, or no, incidence of of the valley sides in the area into which Benacre breaching and associated tidal exchange. In this saline Broad might migrate implies ‘very limited scope for lagoon type, reedbed is not environmentally stressed the landward transgression of habitats’ (Suffolk and lost by incursions of highly saline waters. Rather, Coastal District Council/Waveney District Council/ the landward movement of the barrier has steadily Environment Agency, 2009). If steady migration does ‘squeezed out’ the area of open water. Whilst it is not not take place, it is clear that saline lagoon extinction possible to extend the analysis of the future scenarios will take place sooner than the earliest dates suggested to include the possible future areas of open water v. above. reedbed/marsh, it is likely that the historical trends of Furthermore, seaward enclosing barriers may increasing area of open water at Benacre Broad and become increasingly susceptible to breaching, or more Covehithe Broad, and a decreasing area at Easton extensive morphological collapse, as sea-level rises Broad, will continue, or accelerate, towards 2095. and storminess potential increases (Lowe et al., 2009). Finally, our analysis takes no account of the possible Even if barriers remain in place, changes in wave additional stresses, including increased summer tem- climate may result in increased frequency of barrier peratures, changes in the volume and timing of overwashing. Barrier crest elevations are typically ca. freshwater inputs and the input of nutrients and 2.5 m ODN (Fig. 2) and since January 1964 there have pollutants from terrestrial runoff, that may also lead been 6 southern North Sea storm surge events in which to irreversible deterioration in lagoon water quality water levels exceeded this height (Table 1). Thus (Jones et al., 2011). changes in environmental forcing may lead to associated changes in lagoon water quality, with funda- Habitat creation targets mental ecological change in saline lagoon habitats and species composition, independent of changes in A key purpose of the EC Habitats Directive is to lagoon area. It is therefore of interest that historic maintain or restore at ‘favourable status’ a represen- changes in broad area have been accompanied by tative network of natural habitats (Annex I) and changes in the ratio of open water to reedbed/marsh. species of wild fauna and flora of community interest. Different behaviours can be identified for different ‘Favourable status’ is defined as a position where the environmental settings. At Benacre Broad and Coveh- range and area of habitats are stable or increasing and ithe Broad, the significant increase in the area of open where sufficient areas of habitats exist to maintain water and loss of reedbed through time may be a result viable long-term populations of key indicator species of a greater incidence of barrier breaching and tidal (Council of the European Communities, 1992). The exchange in these ‘isolated lagoons’, occurring at a UK Government’s return for the status of all UK saline frequency so as to prevent full recovery of the lagoons 2001–2006 concluded that ‘the current extent reedbeds following intrusions of saline water through is stable and not less than the favourable reference the barrier. It is known that increased salinity levels area. Therefore, in accordance with EC guidance, the

123 112 Hydrobiologia (2012) 693:99–115

a Benacre Broad hectare and a similar rate of change is likely in the 80 period 2007–2012. This ﬁnding is thus in line with UK 70 Government reporting under the Habitats Directive 60 and the reporting of the UK National Ecosystem

50 Assessment (Jones et al., 2011) which concluded that 40 coastal habitat losses due to sea-level rise have been 40 Vertical exaggeration x2 relatively small so far. 30 30 However, under scenarios of accelerating rates of 20 sea-level rise and shoreline retreat, these 6-yearly

10 reporting periods may distort the longer-term picture 20 considerably. With time, additional areas of saline 0 lagoons will be needed to compensate for losses as a Covehithe Broad b 80 result of continued shoreline retreat. In 1993, for the UK as a whole it was estimated that 120 ha of saline 70 10 lagoon would need to be created by 2013 ‘if the 60 Vertical exaggeration x2 predicted effects of greenhouse warming become a 50 0 reality’ (Pye & French, 1993, p. 4). As for other 40 habitats (e.g. saltmarsh: Friess & Webb, 2011), this

30 figure has proved remarkably persistent. It has been carried forward in subsequent assessments, with the 20 need for saline lagoon creation at a rate of 6 ha a-1 10 between 1995 and 2015 (UKBAP, 2008). The Suffolk 0 Habitat Action Plan advocated the creation of 5–10 ha c Easton Broad of saline lagoon along the Suffolk coast by 2010 to 80 restore the 1992 habitat area (Suffolk Coastal District 70 Council, 2008). This study has demonstrated that in 30 60 2010 the total broadland area for Benacre Broad, Covehithe Broad and Easton Broad was 114.6 ha. This 50 20 was 9.4 ha below the area measured in 1994, an 40 Vertical -1 exaggeration average loss rate of 0.6 ha a and closer to the 10 ha x2 30 estimate for broadland creation by 2010, rather than 10 Total area (hectares)Total 20 area (hectares) Total area (hectares) Total the lower bound of 5 ha. In future, even in a best case scenario where the continued landward migration of 10 0 existing broads is unimpeded, and allowing for a 0 -1 1850 1900 1950 2000 2050 2100 2150 conservative rate of sea-level rise of 4.4 mm a ,by Year 2015 the total loss in area of these broadlands will rise to 4.5 ha (almost 1 ha a-1). The required compensa- Fig. 5 Historic and projected future change in the area (ha) of tory broadland that needs to be created based upon a Benacre Broad, b Covehithe Broad and c EastonBroadto2050and 2095, as calculated from map analysis (Fig. 3;Table2)andthe these three broadlands alone is almost 17% of the application of the SCAPE shoreline response model. Closed symbols figure suggested for broadland creation by 2015 for the relate to historic changes to 2010 and predicted changes post-2010 whole of the UK. -1 under a conservative sea-level rise scenario of 4.4 mm a to 2050 Under higher sea-level rise scenarios, the percent- and 2095. predicted changes in total broadland area for higher rates of sea-level rise of 5.7 mm a-1 (to 2050) and 6.7 mm a-1 age of replacement broadland required will inevitably (2050–2095); predicted areal change with extreme rate of sea- be greater in both absolute and percentage terms. level rise of 8.9 mm a-1 to 2095. See Table 3 for further information Under the H?? sea-level rise scenario there will be a predicted total loss for these broadlands of 7.6 ha, a conclusion is Favourable’ (JNCC, 2007, p. 5). We rate in excess of 1.5 ha a-1. This would account have shown that the area of three Suffolk lagoons for 25% of the entire UK broadland creation if a figure declined in the period 2000–2006 by less than one of 6 ha a-1 is used. Given that the area of these 123 Hydrobiologia (2012) 693:99–115 113 broadlands is approximately 2.3% of the UK total policy is likely to be most successful when informed broadland area, we conclude that a fundamental by a strong knowledge base and when underpinned by revision of saline lagoon creation targets is urgently methodological approaches which allow a range of required. near-future environmental scenarios to be established This detailed worked example shows how the and then evaluated. analytical strengths associated with the methodologies contained within Geographical Information Systems Acknowledgments David Welsh, Shoreline Management and Remote Sensing approaches combined with Group, UK Environment Agency, provided exceptionally effi- cient supply of aerial photography. LiDAR imagery was supplied insight gained through applying physically-based by the Geomatics Group, UK Environment Agency. We also environmental modelling can provide significantly thank the staff of the University of Cambridge Map Room, better inputs into conservation management decisions University Library, Cambridge for assistance and advice. Access than those provided hitherto. Most studies have to field sites was generously allowed by Edward Vere Nicoll of the Benacre Estate. Dr Steve Boreham, Chris Rolfe & Sean Taylor utilised environmental change estimates of uncertain provided valuable assistance with field surveys. This research was provenance which, in the absence of these contempo- first initiated through a Caird-Crown Estate Fellowship, awarded rary analytical approaches, persist in forming the basis by The UK Crown Estate in association with The UK National for policy decisions. The inherent strengths in the Maritime Museum, to Dr S. 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