Potential erosion capacity of gravity currents created by changing initial conditions

Abstract

Abstract. We investigate to what extent the initial conditions (in terms of buoyancy and geometry) of saline gravity currents flowing over a horizontal bottom influence their runout and entrainment capacity. In particular, to what extent the effect of the introduction of an inclined channel reach, just upstream from the lock gate, influences the hydrodynamics of gravity currents and consequently its potential erosion capacity is still an open question. The investigation presented herein focuses on the unknown effects of an inclined lock on the geometry of the current, on the streamwise velocity, on bed shear stress, and on the mechanisms of entrainment and mass exchange. Gravity currents were reproduced in the laboratory through the lock-exchange technique, and systematic tests were performed with different initial densities, combined with five initial volumes of release on horizontal and sloped locks. The inclination of the upstream reach of the channel (the lock) was varied from 0 % to 16 %, while the lock length was reduced by up to 1∕4 of the initial reference case. We observed that the shape of the current is modified due to the enhanced entrainment of ambient water, which is the region of the current in which this happens most. A counterintuitive relation between slope and mean streamwise velocity was found, supporting previous findings that hypothesized that gravity currents flowing down small slopes experience an initial acceleration followed by a deceleration. For the steepest slope tested, two opposite mechanisms of mass exchange are identified and discussed, i.e., the current entrainment of water from the upper surface due to the enhanced friction at the interface and the head feeding by a rear-fed current. The bed shear stress and the corresponding potential erosion capacity are discussed, giving insights into the geomorphological implications of natural gravity currents caused in different topographic settings.

Gravity currents over fixed beds of monodisperse spheres

“…Kyrousi et al (2018) extend this line of inquiry to gravity currents propagating over erodible beds, and find that particle erosion is strongest near the head of the current. This is also observed by Zordan et al (2018) and Zordan, Schleiss & Franca (2019) who show the significance of initial conditions and bed material size. They observe a feedback loop where the bed shear stress and the bed material mutually affect one another.…”

Development of gravity currents on slopes under different interfacial instability conditions

“…(2014), in the Romanche fracture channel, clearly indicate that different initial conditions at the beginning of the slope drastically influence the further development of the current. In a recent paper Zordan, Schleiss & Franca (2019) also point to the importance of initial conditions of turbidity current development.…”

“…Field measurement by Van Haren et al (2014), in the Romanche fracture channel, clearly indicate that different initial conditions at the beginning of the slope drastically influence the further development of the current. In a recent paper Zordan, Schleiss & Franca (2019) also point to the importance of initial conditions of turbidity current development. Negretti et al (2017) considered only flow development of large interfacial Richardson number before moving on to concave or linear slopes.…”