HIGH-RESOLUTION GEOPHYSICAL INVESTIGATIONS OF ESTUARINE AND NEARSHORE SEDIMENT DYNAMICS, RIVER BANN, NORTHERN IRELAND COAST

The River Bann, Northern Ireland, discharges onto the high-wave energy, swell-dominated North Atlantic coast. The funnel-shaped estuary, buttressed by Tertiary basalt outcrops, is partially barred by dunes developed on a mid-Holocene gravel barrier. Since the 1870s river outflow, and its associated ebb tide delta, has been deflected by jetties. The meso-scale evolution of the estuary and nearshore zone therefore demonstrates the interaction of fluvial, marine and coastal processes. Echo sounder, side-scan sonar and Chirp sub-bottom profiler were used to map bathymetry, surficial sediments and sediment stratigraphy respectively. Interpretation of side scan sonar data identifies 3 distinct acoustic facies, interpreted as sand (90% of study area) bedrock (9%) and gravel (1%). Oblique nearshore sandwaves, (wavelength 20 - 60m, height 0.8 - 2.4m) are replaced 1.5km offshore by planar sands (20m water depth). Adjacent to the bedrock headland of Portstewart Point a relict shore platform (at -30m OD Belfast), likely formed during a late-Pleistocene lowstand, is partially overlain by active, shore parallel gravel wedges (wavelength 1.2m, height 0.15 m ) which are partially overlapped by planar sand. Repeat seasonal surveys show general onshore surface bedform migration during winter and offshore re-distribution of sediment in summer. These changes suggest west to east tidal currents re-circulating sediment between the offshore and nearshore zones are modified seasonally by changes in wave regime and river discharge. Storm wave activity may be an important control on overall bedform dynamics.