High-resolution geophysical investigations of estuarine and nearshore sediment dynamics, River Bann estuary, Northern Ireland coast.

Abstract

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. 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. Side-scan sonar data, ground-truthed by surficial sediment sampling, yielded 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 lies a relict shore platform (at −30m OD Belfast), likely formed during a late-Pleistocene lowstand. Individual boulders up to 1m diameter are imaged on the bedrock surface. The surface is partially overlain by active, shore parallel gravel wedges surfaced by ripple beds (wavelength 1.5m, height 1.0m) 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 that 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. Chirp surveys revealed two main seismic stratigraphic units, termed inshore and offshore, overlying the acoustic basement, the inshore unit being more internally complex. Four major controls on Holocene nearshore sedimentation on the north coast of Northern Ireland are suggested: (1) antecedent geological constraints, (2) sediment supply, (3) wave climate and (4) tidal current regime. Apart from antecedent geology, these controls have been, and continue to be, modified by RSL fluctuations and storm events.