Jessica Zordan
Gravity currents are density-driven flows that are able to transport high amounts of sediment and are responsible of great geomorphic changes. Moreover they can have severe repercussions on the environment since they are conveyors of substances, e.g. pollutants, for long
distances depending on their flow dynamics. The latter are determined by the release conditions and by the exchange at the upper and lower boundaries. The aim of this research study is to characterize the turbulent structure of gravity currents, as a consequence of the initial
release conditions, to relate with their transport capacity.
For this purpose, saline gravity currents are experimentally created in a laboratory channel through the lock-exchange technique. Different initial conditions, representing different configurations that can be found in reality, are considered during the experiments. The tested
parameters are: initial buoyancy of the current in the lock, initial volume of release (i.e. lock-length), lock slope and grain sizes of the sediment that form an erodible bed over which the current flows.
High-resolution velocity measurements, performed with an Acoustic Doppler Velocity Profiler (ADVP), allowed to describe both horizontal and vertical structures of the gravity currents. A universal criterion is established to isolate head and body of the current which are characterized
by different dynamics and their extensions vary in relation with the conditions of release. A new parametrization, based on the computed temporal evolution of shear stress, allows to quantify water entrainment from the upper interface and sediment erosion capacity at the bottom.
The effect of the increment of gravitational forces on its erosion capacity is tested by introducing a slope in the lock reach. The range of considered slopes goes from horizontal to S=8%. This latter is identified as a transient case in which two mechanisms compete i.e. on one side, the current entrains more water from the upper interface due to the increment of friction, the current expands, dilutes and therefore slows down; on the other side, the head is fed by the faster rear steady current thus inducing an acceleration.
At the bottom, high shear stress associated with intense ejection and burst events influence erosion and bed load transport. The coupling of hydrodynamic mechanisms and processes of erosion, transport and deposition of sediment are investigated. It is shown that the upward
motion, caused by mean and turbulent velocity components, promote vertical mixing of sediment fromthe channel bed. The feedback between the hydrodynamics of a gravity current and the geomorphic changes of a mobile bed are analysed. The shape of the front changes due
to sediment entrainment and the deposition of sediment downstream creates characteristic patterns whose geometry reflect the coherent turbulent structure of the current.
The scientific contributions elaborated in this research project allow to ameliorate the modelling of these flows, describing their inherent complex mechanisms under various initiation conditions and the interaction with suspended material. This helps to formulate adapted
mitigation measures for the retention of these phenomena which frequently have a negative effect and e.g. induce reservoir sedimentation, subaqueous structure damages and scour processes in the vicinity of submerged pipelines or exacerbate pollutants dispersion.
EPFL2018
