Fluvial and coastal landform changes in the Aceh River delta (northern Sumatra) during the century leading to the 2004 Indian Ocean tsunami

Abstract

River deltas are strongly affected by demographic growth and by the intensificationof land use. The migration of deltaic coastlines is often rapid, threatening urban set-tlements, coastal farming, and coastal biotopes. Some deltas benefit from centuriesof monitoring, such that the evolution of their coastline is well documented. For mostdeltas, however, such long records do not exist. The study of their geomorphologicalevolution can benefit from overlapping maps drafted over time, combined with aerialphotographs and satellite images, to track the evolution of fluvial and coastal land-forms. Both fluvial and coastal landforms are sensitive to variations in water and sed-iment supply, such that covariations in the evolution of these landforms, or the lackthereof, provide clues on the contribution of water and sediment supply to delta evo-lution. We document the evolution of river channels and coastlines in the delta ofthe Aceh River in northwest Sumatra, by overlying maps, ortho-rectified aerial photo-graphs, and satellite images covering the past 130 years. We assess the accuracy ofthe overlays, and then use multivariate statistics to analyze the co-evolution of fluvialand coastal landforms. We propose that a progressive decrease in sediment supplyspurred river channel lengthening and narrowing, landward migration of the shore-line, and narrowing of beach ridges. The 2004 Indian Ocean tsunami generated aninstantaneous retreat of the coastline that amounts to 53% of the coastal retreatfrom 1884 to 2019CE. Post-tsunami evolution is marked by an irreversible accelera-tion of previous trends. Beach ridges located up-drift of rivers and tidal channel mou-ths are more sensitive to long-term landward retreat and tsunamigenic erosion.