Coastal dune fields in Ireland: rapid regional response to climatic ...

Journal Journalof ofCoastal CoastalResearch Research

SI 64

pg -- pg 293 297

ICS2011 ICS2011 (Proceedings)

Poland

ISSN 0749-0208

Coastal dune fields in Ireland: rapid regional response to climatic change D.W.T. Jackson† and J.A.G. Cooper‡ †Centre for Coastal & Marine Research, School of Environmental Sciences, University of Ulster, Coleraine, BT52 1SA, UK [email protected]

‡ Centre for Coastal & Marine Research, School of Environmental Sciences, University of Ulster, Coleraine, BT52 1SA, UK [email protected]

ABSTRACT Jackson, D.W.T. and Cooper, J.A.G., 2011. Coastal dune fields in Ireland: rapid regional response to climatic change. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), . Szczecin, Poland, ISSN 0749-0208 Coastal dunes represent dynamic geomorphological landforms that respond to changes in climatic regimes. Their spatial and temporal evolution as landforms is dependent on sediment supply, antecedent morphology, accommodation space, and physical forcing parameters such as wind speed and direction. Contemporary (decadal) morphological behaviour is therefore driven by changes in climate which affects precipitation, temperature and wind stress. This paper examines recent historical-scale changes in the stability of coastal dune systems on the west and north coasts of Ireland using aerial photographic evidence alongside climatic data to investigate trends in dune stability. A regional scale analysis of dune sites using image analysis indicates a widespread pattern of progressive sealing of dune fields by vegetation. Results show that dune fields have undergone a rapid reduction in bare surface areas of up to 80%. In recent decades (1985 to 2005) the growing season has seen dramatic increases which have likely driven dramatic regional increases in dune vegetation growth coinciding with particularly rapid resealing events across all dune sites studied. This work demonstrates (i) that coastal dune systems have a particularly high sensitivity to climatic shifts and the relatively fast response is manifest in dramatic re-vegetation patterns; and (ii) the pattern is evident on a regional scale. The results demonstrate the rapid response of coastal dunes to climatic shifts over a very short timescale (circa 5-10 years). ADDITIONAL INDEX WORDS: dunes, climate, coastal, re-vegetation

INTRODUCTION Sand dunes are ubiquitous landforms along many of the world’s coastlines and are indicative of periods when wind, sediment supply, vegetation and local climate all co-existed at suitable levels to result in dune deposition. Once established within an accommodation space, coastal dune fields represent responsive geomorphological landscapes that react closely with changes to forcing parameters. In Western and Central Europe alone, coastal dunes occupy an estimated area of over 5300 km² (Doody, 2008). However, since sediment supply is now largely finite along most of Europe’s coastlines, displaying only limited new marine sediment inputs; European coastal dunes can be classified as ‘relict’ landforms and therefore as signatures of former sedimentabundant periods. Contemporary morphological behaviour (decadal) is dominated by changes in climate which drives local precipitation, temperature and wind stress over dune landforms. Landform instability is a natural phenomenon in any dune field’s evolution, helping move sediment from the initial beach deposition zone to the back dune system, creating landform and species diversity. Local wind stress acts as the transport mechanism in physically pushing sand inland across the coastal hinterland. However, it is vegetation that plays a key role in dune construction, anchoring or retarding sediment movement and building the numerous dune forms found. Once the dune field has

been deposited, it undergoes pulses of stability and instability, spanning anything from seasonal up to decadal/century scale time periods. Initiating factors that drive these phases is ultimately dominated by climate which provides variability in temperature, rainfall and wind regimes. However, the paucity of good quality historical and environmental data has made the examination of dune activity and associated environmental driving parameters difficult (Aagaard et al. 2007). Our poor understanding of multiple controlling factors operating within a range of timescales has provided only speculative insights into what drives fundamental changes in coastal dune field dynamics. Dune response will also vary geographically, according to local weather patterns and to some extent the aspect position of the site, however, strong regional signatures of climate-induced response can be striking. This paper examines recent historical scale changes in the stability of coastal dune systems along west and northwest Ireland using aerial photography and climatic data. We report on the degree of dune re-sealing (vegetation growth) of Irish dune systems and associated climatic patterns that are evident in recent decades. Here, we report recent dramatic decadal-scale changes in vegetation cover in Irish dunes to invoke the hypothesis that a rapid bio-geomorphic response in dunes has been initiated and maintained by recent climate change. Implications of observed changes in the behaviour of dune dynamics are also discussed.

Journal of Coastal Research, Special Issue 64, 2011 293

Growing Days /year

Annual Precipitation (mm)

Coastal dunes and rapid climatic response

Rainfall

Growing Days

Annual precipitation 10 year average

Growing Days 10 year average

45 40 35

Gale Days

30 25 20 15 10 5 0

Year

Figure 1. Location map showing the dune sites investigated on the western seaboard of Ireland. Each site number is bracketed with the corresponding reduction in bare sand area that has taken place between 1995 and 2005. Sites are: 1. Portstewart; 2. Tramore (NW Donegal); 3. East of Gola Island; 4. Strandhill; 5. Bartragh Island; 6. SE of Rinroe Point; 7. Newtown; 8. Doolough; 9. Carricknaroun; 10. Omey Island; 11. Magherabeg; 12. Inch. The wind rose also shows the predominant west/southwest wind regime during 1956-2010.

Figure 2. Meteorological data for the Belmullet station from 1957 to 2009 showing mean annual (10 year running mean) precipitation and growing season (top graph) and gales days. contains most of the major dune fields and we therefore draw upon this region in our study.

METHODS STUDY AREA Ireland as a whole has over 140 km2 of dune fields scattered around its coastline (Doody, 2008), many of which are located well beyond the present beach elevation. For the most part, the bulk of Irish dunes were initially emplaced 5-6000 years BP (Carter and Wilson, 1993) at, or just after, the mid-Holocene sealevel peak during an era of abundant sediment supply. With a sealevel fall, sediment was moved onshore through aeolian action, accumulating as beach ridges and dunes. From around 4000 years BP the principal sediment budget along coastlines has persisted in a strongly negative phase, provoking a reworking of many dune systems by marine and atmospheric processes, particularly on their seaward edges. In contrast to many other European coastal dunes, a large number of Irish dunes are isolated and largely free from direct human impact, helping to constrain any variables that are driving their morphological change to just a handful of natural parameters. Due to a number of physiographic reasons (sediment supply, onshore winds, coastal aspect and its location within a high precipitation area) it is the west coast of Ireland (Fig. 1) that

Recent historical-scale (decadal) change at 11 coastal dune sites in western and north western Ireland were examined using aerial photographic evidence and climatic data to investigate the trends in dune stability. Using geo-referenced imagery, unvegetated sand surface areas were identified for each of the years available and used as a proxy for the amount of instability. Climatic records (wind, temperature and precipitation) were examined for corresponding trends (fig. 2) to explain dune field behaviour. A general comparison between sites was undertaken using 1995 and 2005 aerial photo coverage and a detailed case study using image analysis (aerial imagery 1964, 1975, 1983 and 2004) was used to highlight the progressive re-sealing of dune fields. Climate data from the Belmullet meteorological station (Co. Mayo, 54°13'40" N, 10°0'25"W) was used as a reference location to examine the climate that has forced these dune changes during the study period. As a proxy for temperature behaviour related to dune vegetation growth, a 10-year running mean of growing season (number of consecutive days when temperature > 5 ⁰C) was used. Analysis of gale days (O’Connor et al., 2011) during the same period provides a time-series of wind stress in the region (fig. 2).


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Figure 3. Bare areas highlighted at three selected coastal dune sites during 1995 (left images) and 2005 at (i) Tramore [site 2] (ii) Newtown [site 7] and (iii) Inch [site 12]. All sites were geo-referenced with total areas calculated and compared to show percentage drops in bare sand areas.


Coastal dunes and rapid climatic response

RESULTS When climatic data are examined alongside observations of dune vegetation cover the coincidence of particular trends is remarkable. Results show that between 1995 and 2005 dune fields across Ireland have undergone rapid reductions in bare surface areas. At all of the sites examined, remarkable differences were observed with significant re-vegetation having taken place across all major erosional surfaces. Changes at each of the 11 sites (shown in fig. 1) are coincident with the increases in growing season over the 1995 to 2005 period. Percentage decreases in bare sand areas ranged from 51% at Inch, Co. Kerry (site 12) to 84% at sites 5, at Bartragh Island. Three western coastline sites are shown in detail in Fig. 3. At each site the widespread resealing behaviour is evident across the entire dune field, each with dramatic percentage drops in bare sand areas. This is particularly evident at Inch, widely regarded as Ireland’s most active contemporary dune field (Orford et al., 1991). A similar situation has unfolded at Portstewart, Co. Londonderry, where reductions in instability of 86-96% were noted between 1964 and 2004 (Fig. 4), demonstrating a similar pattern of increasing dune stability, albeit over a longer term. Temporal changes in reductions of bare sand areas of Portstewart’s dune field (site 1) between 1964 and 1975 showed a reduction of 63.8% corresponding to a 5% increase in the growing days over that time. Between 1975 and 1983 the number of annual growing days levelled off resulting in a corresponding steady level of relative difference in bare sand areas. After 1983, a progressive 16% increase in the number of annual growth days, was accompanied by 55 to 78% drops in the amount of bare sand in these areas over this time. Wind velocity records at Belmullet in the form of annual gale days recorded over the study period shows an overall stable level up to 1990 of 10 to 20 events. During 1990 a dramatic increase to 40 gale days yr-1 but since then a steady decrease in gale days is observed (McElwain and Sweeney, 2003)

DISCUSSION These observations reveal the unique sensitivity of coastal dune systems to rapid climatic shifts and their relatively fast response in the form of dramatic re-vegetation patterns. In terms of morphological impacts, re-sealing events will restrict contemporary dune dynamics, fixing landforms until climatic trends dictate a subsequent transgressive behaviour, redistributing sediment within the system again (Carter, 1991a; Carter and Wilson, 1993). Results in this study demonstrate the remarkably rapid timescale that this response can take place within, spanning very short timescales (circa 5-10 years). This suggests a closely linked process-response relationship of dune morphodynamics. Similar changes have been observed across other dune systems in West/Northwest Europe with increases in biomass and stability a common trend in the latter half of the 20th century (Provoost et al., 2009; Rhind et al. 2001).

the dune field. However, under conditions of enhanced vegetation growth, which looks likely to occur alongside sea level rise, the predicted increase in coastal dune mobility in NW Europe may not arise. Indeed, it is much more difficult to destabilise a dune field from wind stress alone than to seal it, due to the stabilising strength and resilience of the vegetation once established (Tsoar, 2005). Average temperatures are predicted to rise across northern Europe (Hulme and Jenkins, 1998), with a greater increase particularly over winter months. This, added to expected rises in annual precipitation levels, will compound the dune stability response. Furthermore, it is suggested that CO2 enhancement is likely to amplify rates of photosynthesis, resulting in general increased plant growth (Blackmore and Reddish, 1996). The results in this study suggest that dune-fringed coasts under a global warming scenario would have a heightened resilience where re-vegetated conditions would prevail under longer growing seasons and increased precipitation. Under circumstances where there are unstable areas in the dune, particularly at the coastal edge, sediment is usually able to travel from the beach through transport corridors inland by wind action. This helps redistribute or ‘leak’ sediment deposited at the back beach from wave and wind action into the terrestrial domain. Future scenarios of climate change are likely to halt this sediment pathway, therefore reducing the amount of traditional accommodation space in which the sediment can move into. Higher foredunes and increases in local beach volumes may be a direct future response, leading to relocation of sediment alongshore and/or a forced accumulation in the nearshore zone. This work has important implications for the management of coastal dune systems in the immediate future, where approaches may have to adapt more rapidly to fast response mechanisms of the dunes from climatic shifts.

CONCLUSIONS From meteorological and aerial photographic evidence we have detected significant changes in the amount of vegetation cover across 11 Irish coastal sand dune systems. These changes have been extremely rapid, occurring over a decadal time scale (1995 – 2005) and are closely linked to the increased number of annual growing days which has seen significant rises over the measured period. Other longer term pulses of this re-vegetation are evident in the dunes of Portstewart, representing important temporal changes in dune dynamics, driven by vegetation cover. Dramatic (up to 80% at some sites) re-vegetation of dune fields appears to be forced largely by temperature fluctuations (driving growing seasons). Future climate change and predicted temperature rises are likely to further enhance this phenomenon and therefore moving many of Europe’s dune systems into an immobile state. Sediment dynamics will therefore be forced to reside more within the beach and nearshore areas with inland sediment pathways being halted or severely restricted.

Sea level rise, associated with rapid climate change scenarios, is normally tied with instability at the front edge of a sand dune coast (Carter, 1991b; Saye and Pye, 2007). Under this scenario there is a predicted increase in vertical growth and eventual mobility of foredunes, leading to a transgressive response across the rest of


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Figure 4. Changes in bare sand areas (orange) across the Portstewart dune system (site 1) between the years 1964 to 2004. The aerial photograph (2004) shows the location of each of the four reference boxes (Areas of Interest) analysed at each year of study.

Carter, R.W.G. and Wilson, P. (1991a) Chronology and geomorphology of the Irish dunes. In: A guide to the sand dunes of Ireland. European Union for Dune Conservation and Coastal Management, pp. 18-41. ISBN 0951777904. Carter, R.W.G. (1991b) Near-future sea level impacts on coastal dune landscapes. Landscape Ecology, 6, 29-39. Carter, R.W.G. and Wilson, P. (1993) Aeolian processes and deposits in northwest Ireland. Geological Society, London, 72, 173-190. DOI: Special Publications, 10.1144/GSL.SP.1993.072.01.15 Doody, J.P. (ed.) (2008) Sand Dune Inventory of Europe, 2nd Edition. National Coastal Consultants and EUCC - The Coastal Union, in association with the IGU Coastal Commission. Hulme, M. and Jenkins, G.J., (1998) Climate change scenarios for the UK. In: UKCIP Technical Report No. 1, 80pp Climatic Research Unit, Norwich. McElwain, L. and Sweeney, J. (2003) Climate change in Ireland recent trends in temperature and precipitation. Irish Geography, 36, 97-111. O'Connor, M., Cooper, J.A.G. and Jackson, D.W.T. (2011) Decadal behavior of tidal inlet-associated beach systems, Northwest Ireland, in relation to climate forcing. Journal of Sedimentary Research, 81. ISSN 1527-1404 Orford, J.D., Cooper, J.A.G. and McKenna, J. (1999) Mesoscale temporal changes to foredunes at Inch Spit, south-west Ireland. Zeitschrift fur geomorphologie, 43 (4), 439-461. Provoost, S., Jones, M.L.M. & Edmondson, S.E. (2009). Changes in landscape and vegetation of coastal dunes in northwest Europe: a review. Journal of Coastal Conservation. DOI 10.1007/s11852-009-0068-5. Rhind, P.M., Blackstock, T.H., Hardy, H.S., Jones, R.E. and Sandison, W. (2001) The evolution of Newborough warren dune system with particular reference to the past four decades. In: Houston, J., Edmondson, S.E. Rooney, P. (eds) Coastal dune management, shared experience of European conservation practice. Liverpool University Press, Liverpool, 345-379. Saye, S.E. and Pye, K. (2007) Implications of sea level rise for coastal dune habitat conservation in Wales. Journal of Coastal Conservation, 11, 31-52. Tsoar, H. (2005) Sand dunes mobility and stability in relation to climate. Physica A, 358, 50-56.

ACKNOWLEDGEMENTS We thank Thomas Smyth and Ruth Lyttle for their help in preparation of figures. This paper is a contribution to the IMCORE project funded through the Interreg IVB North-West Europe programme.

REFERENCES Aagaard, T., Orford, J. and Murray, A.S. (2007) Environmental controls on coastal dune formation; Skallingen Spit, Denmark. Geomorphology, 83, 29-47. Blackmore, R. and Reddish, A. 1996. Global Environmental Issues. Hodder and Stoughton (eds), 354pp.
