GRAVITY CURRENTS OVER POROUS SUBSTRATES

Results of laboratory experiments are presented in which a fixed volum e of homogeneous fluid is suddenly released into another fluid of slig htly lower density, over a horizontal thin metallic grid placed a give n distance above the solid bottom of a rectangular-cross-section chann el. Dense liquid develops as a gravity current over the grid at the sa me time as it partially flows downwards. The results show that the gra vity current loses mass at an exponential raze through the porous subs trate with a time constant tau; the front velocity and the head of the current also decrease exponentially. The loss of mass dominates the B ow and, in contrast to gravity currents running over solid bottoms, no self-similar inertial regime seems to be developed. A simple model is introduced to explain the scaling law of the loss of mass and the evo lution of the front position. The flow evolution depends on the charac teristic time of the initial (slumping) phase and the time constant ta u, related to the initial conditions and the permeability of the porou s substrate, respectively. Qualitative comparisons with other gravity currents with loss of mass, such as particle-driven gravity currents, are provided.