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Keywords:

  • dune slack;
  • coastal dune;
  • England;
  • Wales;
  • ecohydrology

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Coastal dunes are valuable ecological reserves providing a range of ecosystem services from coastal flood defence to recreational amenity. An important feature of dune systems is the slack floors, areas of high ecological value that are closely linked to the prevailing hydrological system in the dunes. Dune slacks are sensitive environments that are impacted by external changes, such as rainfall patterns and foreshore erosion as well as conservation management. Understanding the effects of these influences upon the water balance in a dune system is fundamental to the management of the slacks. Occurrence of seasonal flood depths, area and timing and how far the water table recedes below ground in summer are critical to the flora and fauna that the slacks will support. Change in the hydrological regime directly impacts the ecosystem. Experience is drawn from four sites in Wales and west coast England: Ainsdale on the Sefton Coast, Newborough Warren in North Wales, Whiteford Burrows in South Wales and Braunton Burrows in North Devon. Similarities and differences between the hydroecology of the respective sites highlight common mechanisms and processes and those unique to each site. Copyright © 2013 John Wiley & Sons, Ltd.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Coastal dunes and dune slacks are valuable ecological, as well as recreational reserves, many of which also act as coastal flood defences (Everard et al., 2010). Complex hydrological and ecological interactions exist and play a fundamental role in the development of dune habitat. Coastal dunes are under a variety of threats, including afforestation (Stratford et al., 2007), development (Delbaere, 1998), under-grazing or inappropriate grazing (van Dijk, 1992; Plassmann et al., 2010), dune over-stabilization (Grootjans et al., 1998; Arens et al., 2005), nitrogen pollution (Jones et al., 2004, 2006) and rabbit control (Provoost et al., 2011). Although the dunes tend to stabilize with maturity, they are nevertheless fragile environments readily influenced both by external change and management practices. The impact of drivers of change, such as dune mobilization and migration, and of variations in animal populations and grazing patterns is relatively simple to comprehend compared with the likely anticipated impacts from climate change. Although some of these drivers can be managed by human intervention, others, such as climate change, cannot. To complicate matters further, the various drivers may have similar impacts on the hydrological regime of the dunes, and tearing apart the impact of any one driver can be challenging.

The coast of England and Wales has numerous coastal dune field sites (Figure 1), many of which commenced formation around 5000 years ago (Tooley, 1990). The west coast dune fields characteristically rest over glacial till or lacustrine clay and are hydraulically isolated from bedrock aquifers.

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Figure 1. Location of major dune sites in England and Wales.

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Coastal dunes result from progressive sand ingress over low-lying land. Sand mobility is inhibited by the natural development of vegetation. In some cases, stability has been accelerated by additional planting of species, such as marram grass, to limit undesirable movement of sand onto surrounding land (Packham and Willis, 1997). As active dunes advance inland, wind scour erodes the bare sand left behind. Here, the water table and the wetted capillary layer above it prevent further erosion, leaving relatively flat uniform areas between dune ridges called dune slacks. These range from a few square metres to several hectares and more in area (Figure 2). The elevation of the water table within the dunes is, therefore, critical to the morphology of the dune fields and the elevation at which dune slacks occur. A wet dune slack (humid slack in the European Union Habitats Directive terminology) is one in which the water table rises above the slack floor in winter by as much as 0.2 m, but which recedes beneath the slack floor to a depth of no more than 0.9 m in summer (Ranwell, 1959; Davy et al., 2006). Should the water table decline long term, the slacks will evolve from wet to dry accompanied by a corresponding change in habitat.

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Figure 2. A mature dune slack at Whiteford Burrows, South Wales.

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Grootjans et al. (1998) defined dune slacks as ‘the low lying area within the coastal dune systems where the water table is near the surface but has a high seasonal variation’. They continue ‘Primary slacks originate from beaches which have been partially or fully cut off from the influence of the sea. Desalinisation can occur within a few years, but a brackish character may be preserved by exceptional flooding during very heavy storms. Secondary slacks are the result of intense sand-blowing in dune complexes’. Dune slacks may take various forms, but are distinguished not only as largely flat-lying hollows within the dunes, but also by vegetation assemblage. Primary dune slacks can develop within the protection of a newly formed foredune, evolving from a salt marsh habitat into a fresh water slack environment, whereas development in secondary slacks will be from bare sand to the early successional stages of dune vegetation. The slacks are sensitive to a number of influences, not least the prevailing effective rainfall. The cyclicity of rainfall patterns in western UK is reflected in runs of dryer and wetter years that can impact some of the more sensitive vegetation and lead to a change in the water budget.

There are significant differences between west coast UK dune systems and those in, for example, the Netherlands and Belgium. The British sites tend to be smaller and unlike their European counterparts, do not appear to age inland, i.e. with older sand deposition furthest from the shore line (Grootjans, personal communication, 2011). Other differences are a predominantly alkali sand in west UK sites due to high initial carbonate contents, whereas many of the larger European coastal dunes have lower initial carbonate contents and consequently mature from alkali to acid environments as decalcification caused by leaching progresses (Sevink, 1991). Although conservation management in the UK west coast dunes is aimed at habitats, in the Netherlands and elsewhere, it is also targeted at public water supply and protection of low-lying land from the sea.

The maturation of dune systems from the initial embryonic foreshore dune to mobile dune ridges with associated wet slack floors and subsequently to vegetated dune ridges and slack floors is well documented (e.g. Ranwell, 1959; Davy et al., 2006, 2010). Colonization of the dunes by vegetation promotes a degree of stability, although periodic blow-outs can transform the morphology of parts or all of a dune field, whereas erosion of the foreshore may impact the hydrology of the system (Clarke and Sanitwong Na, 2010). Davy et al. (2006, 2010) describe a series of ecohydrological environments in dune systems ranging from seaward slightly saline slacks, flooded slacks fed by through-flow of groundwater and inland slacks maintained by a near surface water table fed by a capillary fringe. Additionally, there may be slacks situated in dune hollows caused by wind erosion which are either fed by precipitation or which intercept a shallow or perched water table. The total UK dune slack resource is only about 2450 ha of which 430 ha is in England and 620 ha is in Wales but only a very small percentage of these areas comprise wet dune slacks (Dargie, 1993, 1995; Radley, 1994).

Not only are dune slacks small in total area but also wet dune slacks provide a unique, if sensitive, habitat. Important species occurring in dune slacks include Liparis loeselii which is confined to short swards of rich fens and damp calcareous dune slacks, stoneworts and a range of rare mosses and liverworts, including Petalophyllum ralfsii and several species of Bryum (thread mosses). Seasonally flooded pools within the slacks provide a vital breeding site for the Natterjack toad (Epidalea calamita) (Houston, 2008).

The objectives of this paper are (1) to characterize the hydrogeology and ecology of dune slacks and (2) to review the impact of physical and human influences on dune slacks. Examples are cited from four dune sites on the west coast of England and Wales: Ainsdale on the Sefton Coast in Lancashire, Newborough Warren on the island of Anglesey in North Wales, Whiteford Burrows on the Gower Peninsular of South Wales and Braunton Burrows in North Devon. Similarities and differences between these dune systems highlight common mechanisms and processes and those of more local significance. The paper comprises two elements: a review of the understanding of coastal dunes and dune slacks and a description of ongoing work and preliminary findings at the four case study UK west coast dune systems.

COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

The depth to the water table determines the abundance of many slack species and controls the vegetation assemblages (Willis et al., 1959). The seasonality of hydrological variation is also important, with spring water levels, particularly in April to June, controlling the breeding success of European priority species such as the Natterjack toad (E. calamita) (Denton et al., 2003).

The link between water budget and dune ecosystem is critical to the prevailing environment. Ranwell (1959) grouped slack types into ‘wet’, ‘transitional’ or ‘dry’ (Table 1). This generic classification does not take into account longer-term trends in climate, drainage conditions and other impacts that may affect water table levels: the depth to the water table and moisture content between the surface and the water table control the ability of wind to scour. The depth to which scouring can occur is determined by the upward movement of moisture because of capillary forces and the surface drying of the sand by wind.

Table 1. Range of observed depth-to-water table values for each of the three phases of the annual cycle grouped by plant community type (after Ranwell, 1959).
Slack typeDepth to water table (cm) (+ve = above ground, −ve = below ground)
November to AprilApril to AugustAugust to November
Wet+20 to −60+20 to −90−10 to −90
Transitional0 to −90−10 to −120−50 to −120
Dry−35 to −120−35 to −160−90 to −170

The west coast dunes of England and Wales typically comprise a layer of blown sand, up to 20 m thick, overlying poorly permeable clay and silt (Figure 3). Groundwater circulation within the west coast dunes is contained within the shallow sand aquifer with the upper surface of the underlying clay acting as a no-flow boundary (Davy et al., 2010). Direct rainfall recharge is usually the only source of recharge and amounts to rainfall minus actual evaporation and vegetation interception. Hydraulic connection to bedrock aquifers is possible at Newborough Warren, and at Whiteford Burrows where the sand gains from the Carboniferous Limestone aquifer which is adjacent to the southern (inland) end of the dunes. Runoff is negligible although flow in ditches and via ponds may occur in wet conditions should the water table intercept slacks and other depressions, and some lateral flow may occur over subsurface layers of fine-grained sand. There are no significant gaining or losing streams associated with any of the four case study sites. Aquifer storage is sufficient to allow water to remain in the system for several years depending both on the overall size of the dune field and the prevailing hydraulic gradients towards marginal discharge areas. The specific yield within the saturated sand is normally <0.25 and porosity >0.35 (Bakker, 1990) varying in both instances according to grain size, which reflects depositional energy (wind speed at time of deposition). Groundwater discharge is usually concentrated along the foreshore, with a groundwater and sea water mix seeping through the foreshore as the tide ebbs. Some discharge occurs to land, particularly at Ainsdale and also at Whiteford Burrows, where a rich habitat occurs along the inland margins of the dunes as groundwater is discharged partly as plant transpiration and partly as small but visible seepages.

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Figure 3. Generic cross-section of a typical west coast dune field.

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The water budget for a typical dune system can be stated as

  • display math(1)

where ∆S is the change in the volume of water stored, ∆Sv is the change in moisture storage in the unsaturated zone, P is precipitation, AEt is actual evapotranspiration and GWDischarge is groundwater discharge from the system. Where the unsaturated zone is thick beneath the higher dunes, capillarity will intercept infiltrating moisture which may be held within the root zone inhibiting recharge to the water table (Figure 3). Observations at the four west coast UK dune systems show that the dunes typically contain a groundwater mound with the highest part of the water table furthest from the relatively static water levels that form the perimeter of the system. The water balance cannot easily be reconciled with outflow because discharge cannot be measured directly, as most groundwater flow is usually dispersed along the seaward front of a dune system, although some also discharges to the landward side. The groundwater mound is a dynamic equilibrium between the rainfall recharge, transmissive properties of the sand and silt aquifer and the area and shape of the dune field (the larger the area and rounder the shape containing the dunes the higher the groundwater dome). The transmissive properties of the aquifer are such that prevailing rainfall along the west coast of England and Wales is sufficient to maintain a positive head difference over present day sea levels and this ensures that sea water intrusion cannot occur even though the base of the aquifer is generally below mean sea level. In addition, actual evapotranspiration is usually higher than rainfall in summer, resulting in a water table recession typically about 0.75 m. The water table recovers in the winter when rainfall exceeds evapotranspiration.

OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Monthly groundwater levels have been recorded with electronic dip tapes at the four sites described subsequently over varying time frames. The measurements are made using shallow dip wells in dune slacks, providing long-term (up to 40 years) information on water levels and shorter term seasonal and inter-annual variations. Few measurements are available for the elevation of the water table beneath the high dunes but analogy with work in the Netherlands and reported by Davy et al. (2006) suggests that interpolation between slacks is justifiable as less recharge is likely beneath the dunes where the unsaturated zone is relatively thick. Ranwell (1959) suggests a slight doming of the water table beneath dune ridges.

The dip wells comprise 50 mm diameter plastic pipe, perforated towards the base, with a pointed bottom cap which is inserted into a hole that has been hand augered to beneath the water table, i.e. between 1.5 and 3 m deep. All the dip wells are situated in dune slacks where the water table is shallow. There are also continuous digital water level records for selected wells at each site, recorded with conventional pressure transducers since 2008. Rainfall data are available for all sites from nearby weather stations and automatic rain gauges have been running at three sites since 2009. The hydrographs present a combined system response to variations in rainfall, land management practices and coastal processes. Periods of slack flooding are recorded within the hydrographs as height above ground level, although monthly dip readings may not always be taken during severe flood events. Three of the dune systems are partially forested, with adjacent open warren containing slacks that offer thin alkaline soils. The vegetation coverage of the slacks at the four sites is similar. The slacks are nutrient deficient, but support grass and a variety of other vegetation types. Generic and specific issues at the four sites are summarized in Table 2.

Table 2. Generic and site specific issues impacting the four west coast UK coastal dune examples.
SiteSite specific issuesCommon issues
AinsdaleForeshore erosion and corresponding accretion up to 4.5 m year−1, encroachment by man, and landward groundwater discharge• Slacks tending to flood less frequently
• Management strategy and land use change
Newborough WarrenComplex setting with sand thinning over a central bedrock ridge, some foreshore erosion and partial forestation• Future climate change
• Inhomogeneous sand body
• Groundwater discharge to sea and land not measurable
Whiteford BurrowsStorm erosion of foreshore in 1995 followed by ~1 m reduction in groundwater level elevation• Groundwater monitoring beneath slacks rather than dunes
Braunton BurrowsLong-term decline in rainfall, long-term scrub control• Impact of forestation

Ainsdale is a 25 km long by 3 km deep hindshore system situated on the Sefton coast between the Mersey and Ribble estuaries (Figure 4). The dune system is impacted by longshore scour which is eroding the dune front and transporting sediment northwards to deposition grounds off Southport and southwards towards the Mersey estuary. Ainsdale Sand Dunes National Nature Reserve is located in the central section of the dunes and has been isolated from anthropogenic development since its establishment in 1965. The site is partly forested. In a typical wet winter, approximately 30% of the slack floors flood to a depth of 0.1–0.3 m. The majority of the slacks dry out in summer with the water table falling to around 0.5 m below ground level and only 10% of the slacks remain flooded throughout an average year. These dynamic conditions provide environments for rich assemblages of flora (JNCC, 2007; Jones et al., 2008) and breeding grounds for rare amphibians (Steward, 2001).

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Figure 4. Conceptual groundwater flow systems at each of the four sites: (A) Ainsdale, (B) Newborough Warren, (C) Whiteford Burrows and (D) Braunton Burrows.

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In addition to inter-annual climatic variation, there are other local issues, such as the development of the pumped agricultural drainage system behind the dune system and the expansion of nearby areas of urbanization with an associated road drainage system which intercepts water that would otherwise drain to the water table (Clarke and Sanitwong Na, 2010). A network of dip wells was installed in 1972 and water table levels have since been measured at monthly intervals (Figure 5). The water table measured in the slack dip wells throughout the site typically rises from sea level at +0.2 m AOD (above ordnance datum) beneath slacks nearest to the sea to a maximum +10.5 m AOD beneath slacks situated up to 2 km inland (Figure 4).

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Figure 5. Long-term water table records at Ainsdale, 1972–2010 showing cyclic trends with clear wetter and drier periods.

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Progressive foreshortening of the dune area has impacted the groundwater levels nearest to the coast, which has been receding at up to 3 m year−1. Significant changes in land use management have also occurred through pine forest removal since the 1990s, but neither reduction in the width of the dune system nor land use change appear to have affected groundwater levels.

Newborough Warren is a combined hindshore and spit dune system extending up to 3 km inland and situated on the southern corner of the island of Anglesey in North Wales. Newborough Warren was the focus for much of the early dune hydrological work in the UK (Ranwell, 1958, 1959, 1960). The dunes reach a maximum thickness of 15 m rising to over 30 m AOD in the vicinity of a rock ridge that traverses the western part of the Warren dividing it into two hydrologically separate components (Figure 4). The dune field has a west and south-west facing coast which is currently subject to erosion, a recurring feature first reported by Ranwell (1958). Long-term average rainfall is 847 mm and surface runoff is essentially absent, except on the flanks of the rock ridge, where the saturated sand aquifer thins to a feather edge, and during intense rainfall events, when flow occurs in drainage ditches in the wooded area and between ponds within partially flooded slacks.

The total area of dunes comprises some 1300 ha of which the northern 700 ha is now managed plantation and the southern part retained as open dune land. The northern area was forested with Corsican pine (Pinus nigra var. laricio) between 1948 and 1965. Hollingham (2006) reported that the water table reacts differently beneath the trees than in the Warren because of interception and evaporation by the trees. Although the present day groundwater regime is affected by the partial forestation, it may also be influenced by altered climatic patterns, digging of drainage ditches, development of vegetation, morphological change and past changes in the controlled stage of a small lake at the boundary of the system.

The water balance at Newborough Warren remains uncertain and attempts to quantify it have so far failed because of an incomplete understanding of the groundwater flow system. A major problem has been the change in constraints which affect recharge (Stratford et al., 2007). For example, the Warren has progressively stabilized since the 1940s because of decline in the rabbit population and maturation of slack soils as well as partial forestation. Much of the mobile system described by Ranwell in the 1950s has now become a stable dune field, with the proportion of bare sand reducing from 70% to around 6% since 1951 (Rhind et al., 2001, 2007). Jones et al. (2010) show that the onset of stabilization preceded myxomatosis by at least 10 years, although the demise of the rabbit population through myxomatosis has since assisted the stabilization of the dunes (Ranwell, 1960). Since the reintroduction of sheep, cattle and ponies in 1986, an element of dune stabilization coupled with shorter grass has evolved. Large-scale managed grazing post-1996 has reduced scrub and tall vegetation (Hodgkin, 1984; Plassmann et al., 2010), thus reducing evapotranspiration and potentially contributing to greater recharge.

Ranwell (1959) described widespread flooding of the slacks in the wet winter of 1950–1951, although only two slacks were flooded during either of the two following winters. He described the groundwater response to a single rainfall event and equated the rise in water level in the slack to be in response to direct rainfall recharge plus discharge from the recharged groundwater ‘dome’ between the slacks (the interfluves), reflecting a rapid discharge mechanism within the sand aquifer. A study of diurnal variation revealed a 2–3 cm increase in water levels after sunset (Ranwell, 1959), providing evidence for the direct influence of evapotranspiration on groundwater levels.

Whiteford Burrows is a 4 km long spit dune system that is 1 km wide. It lies on the northern tip of the Gower Peninsula in South Wales and comprises an area of blown sand over glacial till. The foredunes give way to beach sands and the back of the dunes to the east, to clays and silts of tidal flat deposits (Figure 4). The sand rests on till, or boulder clay deposited beneath a quaternary ice sheet. Steep northerly dipping Carboniferous Limestone strata abut the Burrows at the landward (southern) edge, although the Millstone Grit succession underlies the majority of the dune field. Groundwater in the Carboniferous Limestone ingresses the southern edge of the Burrows sands where iron staining at the edge of a new primary slack is associated with a strongly alkaline environment (Stratford et al., 2011).

The burrows' sand acts as a small unconfined aquifer perched over impermeable till. The groundwater system is fed by direct rainfall recharge and has resulted in a dome of groundwater developing in dynamic equilibrium between recharge and groundwater discharge to the foreshore while a component also discharges to the salt marshes behind the dunes (Stratford et al., 2011). There is a small external contribution to the water balance from the adjacent Carboniferous Limestone upland to the south (Grootjans, personal communication, 2011). The site experienced significant foreshore erosion during a single storm event in 1995 which increased the hydraulic gradient away from the dunes to the sea (Figure 6) causing a rapid overall reduction in groundwater levels of about 1 m. This sudden change is evident in the dip well hydrographs for the site, particularly the summer levels, and is most evident in the summer mean levels for all dip wells (Figure 7).

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Figure 6. Change in hydraulic gradient across the foreshore at Whiteford Burrows before and after the 1996 storm event.

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Figure 7. Summer and winter average water levels for dip well 7S at Whiteford Burrows, showing reduction in summer water levels post foreshore erosion event in 1996.

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Subsequent erosion of the foredunes by up to 2 m a year has now stabilized. Some form of amelioration has also recently begun to occur as the water table is slowly recovering in parts of the dune system. There is recent evidence that the foreshore is again accreting blown sand transported from the adjacent bay to the north and this may again be reducing the inclination of the hydraulic gradient across the foreshore.

Braunton Burrows is a spit dune system near Bideford in North Devon. Braunton Burrows covers an area some 7 km by 2 km with the long side adjacent to the sea coast. The sand overlies a clay horizon, possibly of lacustrine origin, which is situated over raised beach gravels. The area comprises a series of north to south oriented dunes and slacks shaped by the prevailing on-shore winds (Figure 4). It has no significant external influences, other than an apparent long-term decline in rainfall (Davy et al., 2010), and so provides a valuable opportunity to monitor a coastal dune area that is not being affected by anthropogenic or coastal processes.

There is a narrow zone of low foredunes up to 5 m high adjacent to the beach which is succeeded inland by sand hills up to 15 m high separated by a discontinuous belt of slacks. The succeeding dunes are high, rising to 38 m AOD. Beyond lies a broad, but poorly defined, belt of lower-lying ground with scattered hillocks and slacks. The slacks are mainly dry but are sensitive to the groundwater elevation and flood in exceptionally wet winters (Davy et al., 2010). The water table is dome shaped, rising to a maximum elevation of 8 m AOD in winter (Figure 4). The potentiometric surface is split by a broad groundwater divide that suggests, under natural drainage conditions, that the Burrows drain largely towards the coast. A small area of the Burrows naturally drains to the east towards Braunton Marsh where the discharge is collected in a ditch with a flow typically <0.5 l s−1 in the summer months (Robins, 2007).

Despite the development of a golf course at the northern end of the Burrows, the remainder of the site has not been affected by anthropogenic influences and is a good indicator of the cumulative effects of climate and management processes. Three dip well transects have been monitored for water table elevation since 1992 when a previous dip well network fell into partial disuse. Rainfall has declined by 6.4% since the 1960s. Robins (2007) determined a 24 month linear regression best fit line through the nearby Bideford rain gauge data and found an overall decline in annual rainfall from 960 mm in 1966 to just 900 mm in 2004. Overall water levels in the sand aquifer have correspondingly receded by about 0.7 m in the winter and slightly more in the summer months (Figure 8) illustrating a link between rainfall, recharge and water table elevation.

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Figure 8. Decline in summer and winter mean water levels at Braunton Burrows reflect a 5% decline in long-term average rainfall over the same 40 year period.

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Groundwater residence time measurements have been determined using SF6 analyses (Busenberg and Plummer, 2000) to support understanding of the groundwater flow regimes at all four sites. Preliminary indications suggest that the overall age of water beneath the dune slacks is between 5 and 15 years (Table 3). This length of residence time supports the contention that the bulk transmissivity of the sand is moderate and that the dune groundwater flow system is both dynamic, yet constrained. At Ainsdale, groundwater beneath the forested area is significantly older than that in the open area, although at Newborough Warren and Whiteford Burrows, there is no significant age difference between woodland and open warren. A transect of three dip wells perpendicular to the shore was sampled and analysed at Braunton Burrows. The longest residence time of 15 years was just 200 m from the foredune, the shortest, of 13 years, from 900 m inland with an intermediate time in the middle. This reflects the overall transport of groundwater towards the foreshore discharge zone.

Table 3. SF6 data and groundwater residence times.
Site and grid referenceCoverSF6 (fmol l−1)Date sampledAge date
Ainsdale
SO2883309766Wooded0.7608/2009~1986
SO2933011862Open3.1708/2010~2009
SO2922711711Open2.7608/2010~2005
SO2987311486Open2.2408/2010~2001
SO3003611360Open2.2408/2010~2001
Newborough Warren
SK4242063758Open1.0108/20091980s
SK4096564065Wooded0.8808/20091980s
SK4068363759Wooded0.7108/20091980s
Whiteford Burrows
SS4466795483Wooded1.3608/20091991–1996
SS4469795505Wooded1.1908/20091991–1996
SS4444995495Open1.7208/20091991–1996
SS4437395210Open1.2908/20091991–1996
Braunton Burrows
SS4584935363Open2.6502/20102004
SS4522235738Open2.4902/20102003
SS4476535888Open2.7302/20102005

The groundwater flow regimes at Ainsdale, Braunton Burrows and Whiteford Burrows comprise simple groundwater domes created by direct rainfall recharge with discharge to the foreshore, whereas the more complex geology at Newborough Warren makes this system more difficult to conceptualize. All sites have been impacted by change, Ainsdale by slow but consistent coastal erosion, Newborough by conservation management, Whiteford by rapid storm foreshore erosion and Braunton by a decline in long-term rainfall. Foreshore erosion increases the hydraulic gradient away from the dune front with a corresponding increase in discharge from the dunes, noted at Whiteford (Stratford et al., 2009), although this is less obvious at Ainsdale. At Braunton Burrows, Newborough Warren and Whiteford Burrows, there is an apparent lowering of the water table – at Braunton over a 40 year period and at Whiteford Burrows during the course of a single storm. Records at Newborough are shorter and intermittent but the longer continuous data at Ainsdale provide a useful backdrop to records elsewhere.

KEY DRIVERS OF ECOHYDROLOGICAL CHANGE

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Geomorphologically active dune fields naturally adapt to changing conditions. However, the overstabilized dune fields which now characterize much of temperate north-west Europe have limited internal capacity to ameliorate external change. They are vulnerable to variations in rainfall, coastal erosion and long-term trends in sea level. Therefore, these areas are likely to experience significant perturbations in the availability of water for ecosystem use resulting from anticipated sea level rise and climate change. A variety of natural influences interact to control the conditions on the site, and these influences take place over different timescales (Table 4).

Table 4. Slack evolution framework.Thumbnail image of

These processes are expected to be perturbed by anticipated climate change in the UK. The UK Climate Projections briefing report (Jenkins et al., 2009) gives details of these effects such as

  • Wetter winters and drier summers
  • Higher temperatures with increased potential evapotranspiration
  • Rise of sea level

The likely impact of these changes on dune hydrology includes a change in total annual groundwater recharge, a change in the timing of groundwater recharge, changes in the hydraulic boundary conditions and changes in the tidal processes driving erosion and deposition.

Understanding the physical and human influences that control and affect water table levels in coastal dune systems is crucial to understanding ecological change. Identification of the hydrological processes that are common to all the sites and those processes or factors (e.g. management) that have specific impacts at individual sites enables a generic understanding of the key drivers to be identified. The drivers can also be viewed in terms of their timescale of influence and the permanence of influence (Figure 9). Both of these factors are important when assessing the restoration potential of a site.

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Figure 9. Permanence and ease of reversibility of pressures influencing dune slack development and condition.

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Medium-term dip well hydrographs and climatic records are available at all four coastal dune sites; however, there is a lack of supporting information on aquifer properties and changes in management practices. What is apparent over the last 30–40 years is that there are not only long-term changes but also mid-term (5–10 year) periods of wet and dry sequences. Any short-term study of less than 5 years may detect an apparent trend in groundwater conditions (contrast the periods 1972–1980 and 1981–1990 in Figure 5) that might incorrectly be ascribed to a specific issue such as a land management or a storm event (Clarke and Stratford, 2010).

CONCLUSIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

Work to date, focussing on four west coast UK coastal dune fields, has identified the key generic processes and constraints which influence their hydrogeology. These are direct rainfall recharge, dune field area and shape and to a lesser extent vegetation. Other specific drivers include conservation management, foreshore and foredune erosion, sea level rise and climate change. The main uncertainty, however, is recognizing the many and diverse processes and mechanisms that control recharge to coastal dunes and the consequent effect that they have on ecosystem and habitat. Water table elevation and its seasonal and longer mid-term decadal variation is critical to dune slack ecology and biodiversity. Understanding the prevailing hydrology of the slacks within the context of the overall dune system is, therefore, key to their successful management.

The four west coast sites all have some external drivers whereas one site, Newborough Warren, is constrained by a complex geological setting. Braunton Burrows is influenced by declining effective rainfall but with no obvious anthropogenic pressures and this site is a valuable and simple system with which to evaluate cause and effect (Robins, 2007). Both Ainsdale and Whiteford suffer offshore erosion, Ainsdale consistently over a long term, whereas Whiteford suffered a catastrophic storm event that shortened the beach and increased drainage from the dunes. Whiteford is also influenced by an area of salt marsh on its eastern edge which is dissected by some deeply incised channels. Whether the partial forest cover at Newborough Warren and Whiteford is causing a lowering of groundwater levels beneath the trees is as yet unresolved. Each site, therefore, offers a unique and dynamic ecohydrological equilibrium.

Monitoring of dip well water levels and meteorological data is ongoing. Other field campaigns, which include analysis of groundwater chemistry and measurement of moisture profiles beneath high dunes, will add to the understanding. Analysis of these data includes groundwater flow and stochastic modelling. Stochastic process modelling in particular, in which different processes can be ‘tuned’ until a match is made with historic water table hydrograph data, is already informing process sensitivities. Other analytical methods such as cluster analysis of dip well hydrograph morphology according to ecological indicators will also inform future work.

Changing hydrological conditions remain one of the key drivers to changing ecological conditions. Predictions on ecological sustainability largely relate to the sustainability of the hydrological regime. The importance of this ecohydrological link cannot be overstated and the importance of understanding it is central to all future dune management strategies (Curreli, 2012).

ACKNOWLEDGEMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES

The authors are grateful to their respective employers for supporting this work and to Countryside Council for Wales and Sefton Council. They are grateful also for local field support provided by Natural England, Countryside Council for Wales and Sefton Council as well as by Martin Hollingham, Debbie Allen and Angela Curreli. Generous guidance from Ab Grootjans, University of Groningen and from two anonymous reviewers of the first version of this paper is most gratefully acknowledged.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. COASTAL DUNE HYDROLOGY AND ECOHYDROLOGY
  5. OBSERVED GROUNDWATER LEVELS, ISSUES AND CONTROLS
  6. KEY DRIVERS OF ECOHYDROLOGICAL CHANGE
  7. CONCLUSIONS
  8. ACKNOWLEDGEMENTS
  9. REFERENCES
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