Fluidization of sediments in a conical basin by subterranean springs: relevance to Lake Banyoles

Abstract

An experimental study was carried out to investigate the resuspension of particles by a momentum jet discharging from below into a conical basin. The work was motivated by its relevance to Lake Banyoles, where sediments are suspended in different conical basins by bottom jets fed by a groundwater karstic system. Two different flow regimes were identified: a "Jet Flow" regime (JF) and a "Lutocline Flow" regime (LF). In the LF regime, the particles were resuspended, forming a well-established interface along the entire cross-sectional ara of the cone. This regime occurs when the particle Reynolds number \( {\bf Re_p = (u-w_s)d_p/ \nu} \) where u is the mean velocity of the flow at the jet entry, ws is the settling velocity of particles, dp is the diameter of that particles and \( {\bf \nu} \) is the kinematic viscosity of water) is below a critical number Rec that depends on the slope of the basin. The maximum height to which particles rise in the LF regime was also determined as a function of four non-dimensional variables: D1 = ho/(Mo1/2/ws), D2 = ho/dp, Rep and \( {\bf tan \, \beta} \) (where ho is the height of the particle bed, Mo is the kinematic momentum flux of the jet and \( {\bf tan \, \beta} \) is the slope of the basin). Application of the results to the basins of Lake Banyoles showed a good agreement provided that the aggregate properties of the suspensions are used. It is also shown that, in the LF regime, inflows to the basins can be estimated by a simple technique based on a balance between the sedimentation flux and the upward advection of sediments by the mean flow.