Long-term shoreline change trends over the last 191 years in Northern Ireland

Abstract

A shoreline change study of the 790 Km coast of Northern Ireland was undertaken to exam the period 1830-2021. Along its coastline (loughs included) and the offshore islands, Northern Ireland’s coast exhibits a variety of natural coast types (cliffs, rocky coasts, sandy beaches, sand dunes, etc.) that were progressively impacted by diverse human uses, including recreational. commercial and industrial uses. Shorelines were manually digitised in a GIS environment using multiple temporal and spatial datasets (historical maps, aerial photos, orthophotos and DGNSS surveys). The temporal resolution of shorelines averaged from 7.8 to 37 years, depending on location and data availability. For consistency among all the distinct types of data, the seaward vegetation line was selected as the primary shoreline proxy, whereas the cliff edge, rock-water line or anthropogenic structures were chosen if more appropriate. The uncertainty of shoreline’s position was calculated using multiple errors inherent from each dataset from which the shoreline was digitised. The ArcMap® tool DSAS 5.0 was used to calculate shoreline rates and distances over more than 24,000 cross-shore transects, each spaced 25 m apart. The maximum recession value was found south of Magilligan Point, in Lough Foyle, where the shoreline progressively retreated up to -283 m (peak Linear Regression Rate (LRR) -1.46 m/year). The largest shoreline advance (+3.7 Km) was highlighted in Belfast due to anthropogenic port expansion. Sandy coastlines exhibited the largest natural changes. If only natural processes are considered, the largest net shoreline advance was found at the Murlough’s sand spit (+240 m) due to the NE directed sediment drift acting in Dundrum Bay and complex tidal channel and ebb tidal delta dynamics  Along the north coast, where the most mobile beaches are dominated by cross-shore sediment exchanges on a seasonal and event (storm) basis, the net shoreline movement since 1830 was negative: Ballycastle (peak LRR -0.28 m/year), Whitepark Bay (peak LRR -0.38 m/year), Runkerry (peak LRR -0.14 m/year), Portballintrae (peak LRR -0.09 m/year), Portrush West Strand (peak LRR -0.36 m/year), southern part of Portrush East Strand (peak LRR -0.48 m/year). The study showed that 58% of the coastline advanced within the last two centuries, whilst 42% retreated. Despite the high percentage of apparent accretion, the accreted shoreline sites, especially among the peak values, were predominantly induced by human interventions, such as, land reclamation in the loughs (mainly during the 19th century), seaward expansions of ports, sea defence structures  or development of industrial areas. On the other hand, the highest recorded retreat values were related to natural hydrodynamic forcing and sediment processes, although human interventions on adjacent coastal sectors cannot be overlooked. High rocky coasts, apart from few localised rockfalls, were less subject to shoreline changes. The work also helped identify seven, first-order sedimentary cells around the Northern Ireland coast and will contribute toward future coastal management plans.