Experimental results of gravity currents travelling over a fissured bed

Gravity currents are geophysical flows having large impact on human life and on the environment. The need to prevent and mitigate their negative consequences justifies the multitude of studies carried out to understand their physics. However little has been done concerning gravity currents travelling over porous bed, which is a frequent scenario in real life. Existing studies of gravity currents over porous substrates deal with lock-exchange gravity currents. In this paper we will present preliminary results of 23 experiments reproducing continuouslyfed brine currents initially developing over a flat bed and then running over a porous region of varying porosities. In correspondence of this region, 2D density maps are acquired by means of an image-analysis technique. Simultaneously an Acoustic Doppler Velocimetry Profiler allows to record a 3D instantaneous velocity profile. Results describing the effect of the bottom porosity on density and velocity profiles are presented. Surprisingly the current does not slow down while travelling over the porous substrate: the relaxation of the no-slip boundary condition together with fresh water incorporation from the bottom cavities allows high velocities in the near bed region. This modifies significantly velocity and density gradients at the lower current interface. Conversely, varying the bed porosity has no remarkable effect on the upper current interface.