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.
Highlights
Gravity currents are common phenomena that may occur spontaneously in nature or triggered by human activities
We show how shear stress at the boundaries is dependent on the conditions under which a gravity current forms, i.e., its initial and boundary conditions
The head of the gravity current is characterized by a high specific flow rate that decreases at the rear of the head, a region in which fluid is recirculated through vortical movements
Summary
Gravity currents are common phenomena that may occur spontaneously in nature or triggered by human activities. These flows are created by differences in hydrostatic pressures at the surface of contact of two fluids that have different densities. Examples of gravity currents generated in the atmosphere are katabatic winds, which are created by temperature inhomogeneities that originate the density gradient. Avalanches of airborne snow and plumes of pyroclasts from volcanic eruptions are atmospheric flows wherein suspended particles play a major role in producing the density gradient. If suspended sediment produces the extra density, gravity currents are called turbidity currents. Among gravity currents caused by human actions, the release of pollutants into rivers, oil spillage in the ocean and desalination plant outflows are of primary importance due to their negative environmental impacts
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