Contrasting geomorphic response to structural control: The Loughros estuaries, northwest Ireland

Abstract

Sediment-rich estuaries typically exhibit considerable dynamicity, with channel shifts and shoreline erosion/deposition occurring over timescales of years to decades. Contemporary behaviour is usually attributed to the immediate effect of short-term events (such as storms), longer-term forcing (such as sea-level change) or anthropogenic modifications to system controls (such as wave–tidal–fluvial regime and sediment supply). In this context, geological control is understood to provide an inherited framework within which an estuary will evolve. This paper examines the morphodynamics of two sand-filled, but sediment-limited, mixed-energy estuaries of Loughros Bay in northwest Ireland. Analysis of ebb channel morphometry shows that estuaries can be divided into upper and lower estuary components, which appear to represent a down-valley increasing energy gradient. Examination of historical ebb channel migration reveals three morphodynamic zones delimited by a significant spatial variability in channel mobility that reflects the distribution of specific structural controls along the estuary and their influence on the changing energy gradient. Based upon analysis of low tide channel migration over the last 172 years, three phases of geomorphic response are identified. In Loughros More, the behaviour is characterised by a 19th century phase of steady-state equilibrium; a subsequent period of dynamic response to extrinsic (storm) forcing; and, most recently, adjustment to intrinsically forced changes in structural control. In Loughros Beg, the phases are comparable, but contrast specifically in inlet and ebb-tidal behaviour. This analysis illustrates discontinuities in decadal- to century-scale forcing and morphological response, identified here as complex response. It suggests that the sedimentary evolution of coastal systems that have no appreciable external sediment supply is potentially more strongly determined by local variability in system character than climate or sea-level driven mechanisms.