MORPHODYNAMIC EVOLUTION, SELF-ORGANIZATION, AND INSTABILITY OF COARSE-CLASTIC BARRIERS ON PARAGLACIAL COASTS

Beaches and barriers on many mid- to high-latitude coasts comprise mix tures of fine and coarse elastic materials forming a distinctive morph odynamic environment. In many cases, the sediments are derived primari ly from limited glacigenic deposits and the coasts are considered para glacial. Over relatively long time scales (decades to centuries), coar se-elastic barriers on such coasts show evidence of self-organisation through large-scale morphological evolution and facies differentiation . This process involves gradual reworking, partitioning, and textural sorting of material toward transport minima. Long intervals of slow ev olution are punctuated by episodes of rapid reorganisation, involving breakdown of stable barrier structures and facies patterns, remixing o f sediment, and accelerated migration of transgressive systems. Drift- aligned systems develop longshore cell structure, sometimes leading to breaching and segmentation, and may evolve toward progressively great er swash alignment under appropriate circumstances. Swash-aligned syst ems may experience catastrophic transformation when appropriate enviro nmental triggers lead to threshold exceedance in the morphodynamics of the shore system. Adjacent barriers may show quite different behaviou r, depending on the antecedent states of individual coastal cells. Whi le appropriate parameterisations and sediment budget formulations allo w us to model the long-term evolution of some barrier structures, the non-linear dynamics that appear to dominate large-scale behaviour may limit predictability. The identification of stability threshold criter ia remains an important research priority.