NUMERICAL MODELING OF A MID-SIZED GRAVITY FLOW - THE 1979 NICE TURBIDITY-CURRENT (DYNAMICS, PROCESSES, SEDIMENT BUDGET AND SEA-FLOOR IMPACT)

The 1979 Nice turbidity current is modelled using a visco-plastic anal ysis of flow velocity because the initial flow concentrations are expe cted to have been very high. The complete history of the failed sedime nt from debris flow to turbidity current plume is therefore addressed. The turbidity current portion is considered as a steady state flow di vided into a dense bottom flow and an upper plume. Model results show that a dense flow can be generated from the debris flow by the disaggr egation of the initial slide. The dense flow would be strongly erosive and able to create and maintain a low-density plume at its surface. T he depth of erosion of the channel floor by the dense flow is predicte d to reach 6-11 m in overconsolidated sediments, with the main erosion taking place in Var Canyon and the Upper Fan Valley. The eroded volum e (10(8) m(3)) provides additional material to the sediment mass of th e initial failure. The dense flow appears able to inject fine sand and silt into the overlying plume during 90 km, and would disintegrate be fore being able to deposit sediment. The extensive sand layer along th e travel path of the turbidity current may have been deposited from th e tail of the trailing plume: a result of the velocity difference betw een the plume and the dense flow. Observations on sedimentary structur es, erosion features and distribution of the sand deposit are quite in agreement without modelling approach. For example, gravel waves can b e generated when loose deposits are reworked by the supercritical dens e flow. The methodology and equations presented here provide a good es timate of the geological consequences of a high-velocity gravity flow undergoing rheological transition.