Journal of Sedimentary Research, 2011, v. 81, 38–51 Research Article DOI: 10.2110/jsr.2011.3 DECADAL BEHAVIOR OF TIDAL INLET–ASSOCIATED BEACH SYSTEMS, NORTHWEST IRELAND, IN RELATION TO CLIMATE FORCING MARIANNE C. O’CONNOR, J. ANDREW G. COOPER, AND DEREK W.T. JACKSON Centre for Coastal and Marine Research, School of Environmental Sciences, University of Ulster, Coleraine, County Londonderry, Northern Ireland, BT52 1SA e-mail: m.oconnor@ulster.ac.uk ABSTRACT: Shoreline and ebb channel evolution between 1834 and 2008 was recorded at three embayed beaches in northwest Ireland with historically stable relative sea level. The beaches are geologically constrained within bedrock valleys and have minimal contemporary sediment input. Their behavior is strongly influenced by tidal-inlet dynamics of adjacent estuaries; major observed changes in shoreline position are linked to fluctuations in inlet ebb-channel orientation. Comparison of the available historical record of geomorphological change with instrumental and proxy climate records suggests that inlet changes may be driven by fluctuations in average storminess: periods of high storminess perturb the inlet system, causing major changes in ebb- channel configuration, after which sediment redistribution returns the system to its former state over a prolonged period (years to decades). INTRODUCTION Sea level, wave climate, storminess, sediment supply, and geological setting are important drivers of long-term shoreline behavior on sandy coasts (Woodroffe 2003), but the identification of their respective roles at decadal timescales is hampered by a paucity of data on both drivers and geomorphological responses (Viles and Goudie 2003). On beaches adjacent to tidal inlets, additional influences on shoreline behavior arise from changes in channel dimensions and position, reconfiguration of tidal deltas, changing tidal prism, as well as interaction with other channels (Bruun and Gerritsen 1959; FitzGerald 1982; Hicks and Hume 1997; FitzGerald et al. 2000; Morton et al. 2007; Cooper et al. 2007). Decadal- scale ebb-channel variability at tidal inlets has been attributed to shoreline erosion and extreme storms (Morris et al. 2001), coastal defense structures (Elias and van der Spek 2006), co-seismic subsidence, and increased energy conditions associated with El Nin ˜ o events (Morton et al. 2007). Tidal-inlet migration (Reddering 1983), formation, and closure (Davis and Barnar 2003) influences adjacent shorelines as sediment is redistributed, and nearshore wave patterns change. Isolating the factors that cause inlets to change can be problematic, particularly on linear barrier island coasts. In this paper we investigate the historical record of beach and tidal inlet variability on a strongly compartmentalized coast in which tidal inlets and associated barriers and beaches occur in bedrock- confined embayments that significantly constrain the mobility of tidal channels. Relative sea-level change is practically zero and contemporary sediment supply is negligible. Nonetheless, significant decadal to century- scale shoreline changes do occur within the bounds of the embayments (Burningham and Cooper 2004; Burningham 2008; Cooper et al. 2007). The aim of this paper is to investigate the evolution of three inlet- associated beach systems in Donegal (Glenree, Loughros Beg, and Five Finger Strand; Fig. 1), northwest Ireland over a 170 year period. The morphological changes are assessed in conjunction with historical measurements of climate change or appropriate proxies to determine whether the driving forces of change are internal (autocyclic) or external (allocyclic) (Morton et al. 2007). Analysis of the relationship between shoreline processes and shoreline response is hampered by variations in temporal and spatial resolution of data (Cooper 2009). External parameters (e.g., wind, waves, tides, and sea level) impact on the long- term movement (signal) and short-term changes (noise) in shoreline position. These parameters are also affected by larger atmospheric variability reflected in the North Atlantic Oscillation (NAO). Using the available datasets which are, perforce, incomplete, we investigate potential linkages between climate forcing and long-term system changes at these tidal inlet–associated beaches. REGIONAL SETTING Donegal is located in northwest Ireland between latitudes 54.2u and 55.5u N. Relict, glacially derived sediment was driven into reentrants on the indented bedrock coastline during the Holocene transgression (Carter and Wilson 1993), forming numerous small headland-bay beaches and estuary-mouth systems (Carter 1988). In many embayments, sand-rich estuaries are connected to the sea via tidal inlets bounded on one or both margins by a mobile sandy beach. Ebb deltas are present at the tidal inlets, and extensive, vegetated coastal dune systems are present on the adjacent beaches (Carter and Wilson 1993; Cooper et al. 2004). The contemporary supply of modern continental-shelf or fluvial sediment to the coastal system is negligible (Cooper et al. 2007). Relative sea-level (RSL) change on the north coast of Ireland is strongly influenced by isostasy (McCabe et al. 2007). The late glacial (22– 13 cal. ka BP) of higher than present RSL was punctuated with two main periods of RSL fall. The last of these falls was to a late glacial lowstand of c. 230 m , 13.5 cal. ka BP (Cooper et al. 2004; Kelley et al. 2006). Subsequent RSL rise continued into the Holocene to reach a maximum of c. +2–3 m MSL , 7.5 cal. ka BP followed by a fall to present levels (Carter 1982). Tide-gauge records from Malin Head and Belfast show that RSL has been relatively stable in the latter half of the Twentieth Century (Orford et al. 2006) (Fig. 2). Copyright E 2011, SEPM (Society for Sedimentary Geology) 1527-1404/11/038-051/$03.00