The 1979 Var Delta Landslide on the French Riviera: A Retrospective Analysis

Abstract

The Var Delta is a steep-faced Gilbert-type delta that lies on an earthquake-prone, extremely narrow, canyon-dissected margin. The entire subaerial delta and the subaquatic delta-front have been reclaimed for transport infrastructures. In 1979, a major landslide affected the delta front resulting in structural damage and several casualties on the highly developed, French Riviera coast. This landslide was not triggered by earthquakes, which are considered as the main landslide dynamic triggering mechanism in this region. The official report on this event attributed it to turbidity currents on the steep submarine slopes of the delta. These were reportedly generated following exceptionally heavy rainfall and river discharge. However, in the light of recently acquired data on the geology and hydrology of this delta, and of the extensive anthropogenic modifications carried out in the years before this landslide, we view the causes of this event, which involved the failure of both subaerial coastal deposits and submarine sediments, as being due to two factors: (1) delta-front slope steepening, up to the angle of failure, as a result of reclamation fill of the upper delta-front crest and mantling of the delta-front by massive fallout of underconsolidated fine-grained sediments from this fill; and (2) reclamation fill of the subaerial delta plain, leading to heavy sedimentary loading and strong increases in excess pore water and gas pressures, and reduction of sediment strength. In addition to these direct effects on the delta, we also consider that reclamation of the Var Valley upstream favoured build-up of instability through: (1) a drop in suspension load storage upstream and an increase in the amount of fine-grained suspension fallout that was mantling the delta-front slopes, and (2) a lower capacity for infiltration upstream and in the deltaic plain, leading to a heavier short-term discharge and surcharge of a confined delta aquifer that drains seaward at depths below 30 m. The landslide was probably triggered by (1) major gravity failure due to exceedance of the equilibrium delta-front slopes; (2) scouring of the basal delta-front slopes by greater submarine groundwater discharge from the confined aquifer following heavy rains and river discharge at the time; and (3) massive fine-grained sediment fallout on the steep delta front associated with this heavier discharge. The seismicity hazard must also be kept in mind as far as future coastal and nearshore management practices are concerned on this densely populated and highly developed area.