Pluri-decadal impact of mining activities on coastline mobility of estuaries of New Caledonia (South Pacific)

This work aims at analysing the relative impact of sedimentary supply by rivers versus the effect of sea level rise and other processes over the last 5 decades of some representative coastal stretches of New Caledonia (South Pacific Ocean). The study of twelve coastal estuarine stretches has shown erosion rates ranging from -1.8m/y to accretion rates up to 5.5m/y over the last 50 years. During this time, the climate component of sea level rise has been evaluated around 0.5mm/y in Noumea and can be considered identical on the studied island. Vertical movements affecting New Caledonia are suspected to be different between western and eastern parts of the island. Nevertheless, in areas with homogeneous vertical movement, the coastal stretches studied show various evolution rates. The analysis of accreting estuarine coastal areas shows that this evolution is linked to sedimentary supply from rivers. On New Caledonia Island, the creation of bare soils by nickel mining during the last 50 years has increased erosion processes on the watersheds and the solid discharge of the rivers. The study shows a strong correlation between the percentage of bare soil surfaces (up to 3.7 % of the watershed surface) generated by nickel mining and the coastline evolution rate (from -2m/y to +4m/y) around the estuaries of each watershed. Therefore, mining activity on watersheds is suspected to have been the major forcing factor of recent coastal changes during the last fifty years.