Abstract

The distribution and spatial patterns of surficial nearshore sediments off the north coast of Ireland were mapped in detail using three repeated (over one year total duration) dual-frequency (100–500 kHz) side-scan sonar surveys which were ground-truthed using a bucket-grab sediment sampler. In the study area (40 km², < 30 m water depth), three acoustic facies were identified (interpreted as sand, gravel, and bedrock). Planar sand with small sand waves dominates most of the study area; bedforms developed on sand and gravel substrates are located around the margins of a bedrock-floored bathymetric trench. Unusual sea-going gravel ‘ribbons’, which morphologically resemble rip current chutes, feed into the trench. Temporal changes in substrate and bedform patterns, mapped from the side-scan sonar data, suggest bedform development and facies boundary migration (by > 100 m distance) over the winter season followed by sediment migration back again during the summer. Generally, substrate and bedform mobility is likely to be related to changes in the relative strengths of waves and tides along this exposed, mesotidal coast. More specifically, tidal asymmetry and the formation of a headland leeside eddy can account for scouring of the trench, trench-flank bedform patterns developed in sand, and the formation of the gravel ribbons.