Bottom currents induced by baroclinic forcing in Lake Banyoles (Spain)

Because of the difference in morphometry and in the underground heating of the two lobes of Lake Banyoles there is a differential cooling that causes a density current — the denser water of the shallower lobe plunging into the deepest lobe forming a bottom current — which redistributes water between the lobes and replaces that of the northern lobe about every 5 days. This current has been studied during the mixing periods of the years 1989–1991 from temperature and current measurements and it has been found that it increases or slows down depending on the direction of the wind. Furthermore, it is observed that it mainly flows along the west shore of the lake, deflecting towards the right as an effect of the bottom morphometry. However, due to the long time scale of the density current the influence of the earth's rotation should not be neglected in advance. Finally, a simple numerical model has been used to corroborate the magnitude of this current — whose velocity has been directly measured — which is found to be the most important in the lake during the mixing period and so, it dominates the lake-wide circulation     

Sediment deposition from turbidity currents in simulated aquatic vegetation canopies

A laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders. The cylinders were intended primarily to simulate aquatic vegetation canopies, but could equally be taken to represent other arrays of obstacles, for example forests or offshore wind turbines. The turbidity currents were generated by mixing naturally sourced, poly‐disperse sediment into a reservoir of water at concentrations from 1·0 to 10·0 g l^?1, which was then released into the experimental section of the flume by removing a lock gate. For each initial sediment concentration, runs with obstacle arrays with solid plant fractions of 1·0% and 2·5%, and control cases with no obstacles, were carried out. The progress of the current along the flume was characterized by the array drag term, C_Dax_c (where C_D is the array drag coefficient, at the frontal area of cylinders per unit volume, and x_c is the position of the leading edge of the current along the flume). The downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume. Analysis of these deposits divided them into fine (2·2 to 6·2 μm) and coarse (6·2 to 104 μm) fractions. At the beginning of their development, the gravity currents proceeded in an inertia‐dominated regime until C_Dax_c = 5. For C_Dax_c > 5, the current transitioned into a drag‐dominated regime. For both fine and coarse sediment fractions, the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime, remained approximately constant at the early drag‐dominated regime, and then rose and peaked at the end of the drag‐dominated stage. This implies that, when passing through arrays of obstacles, the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles.     

Localized algal blooms induced by river inflows in a canyon type reservoir

The local response of the phytoplankton community to river inflow processes was investigated with modeling and field analyses in a long and narrow, stratified reservoir in mid-summer. The river water had high concentrations of phosphorus and nitrogen (ammonium and nitrate) and temperature had large variations at diurnal scales. As a consequence of the large variation in river temperature, the level of neutral buoyancy (the depth where the river water spreads laterally in the reservoir) oscillated between the surface (overflows) during the day, and the depth of the metalimnion (interflows) during the night. The reservoir remained strongly stratified, which favoured the presence of cyanobacteria. It is shown that under these conditions, nutrient-rich river water injected during overflows into the surface layers promoted the occurrence of localized algal blooms in the zones where the overflow mixed with the quiescent water of the reservoir. A series of hydrodynamic simulations of the reservoir were conducted both with synthetic and realistic forcing to assess the importance of river temperatures and wind-driven hydrodynamics for algal blooms. The simulations confirmed that the river inflow was the main forcing mechanism generating the localized bloom.     

Effect of submerged aquatic vegetation on turbulence induced by an oscillating grid

In wetlands wind-induced turbulence significantly affects the bottom boundary, and the interaction between turbulence and plant canopies is therefore particularly important. The aim of this study is to advance understanding of the impact of this interaction in submerged aquatic vegetation (SAV)¹ on vertical mixing in a fluid dominated by turbulence. Wind-generated turbulence was simulated in the laboratory using an oscillating grid. We quantify the vertical distribution of turbulent kinetic energy (TKE)² above and within different types of vegetation, measured by an acoustic Doppler velocimeter. Experimental conditions are analysed in two canopy models (rigid and semi-rigid) with seven solid plant fractions (SPFs)³, three stem diameters (d)⁴ and three oscillation grid frequencies (f)⁵ and four natural SAVs (Cladium mariscus, Potamogeton nodosus, Myriophyllum verticillatum and Ruppia maritima).     

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

: 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 Re_p = (u-w_s)d_p/ n {\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 n {\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/(Mo^1/2/ws), D2 = ho/dp, Rep and tan  b {\bf tan \, \beta} (where ho is the height of the particle bed, Mo is the kinematic momentum flux of the jet and tan  b {\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.