Experimental study on dynamical blocking conditions of net exchange flows

Experimental results are presented from a series of large-scale laboratory investigations of the internal flow of restricted net exchange flows across a submerged trapezoidal sill, intended to give higher Reynolds numbers that are more representative of real situations (e.g. semi-enclosed estuaries and fjords). The experimental program, funded under the EU FP7 Hydralab IV Initiative, was conducted in the LEGI experimental facility in Grenoble. Measurements focused on obtaining velocity and density profiles in the vicinity of the obstruction to observe and quantify interfacial mixing processes under a range of parametric forcing conditions (i.e. variable saline and fresh water flow rates; density differences; water levels). Detailed synoptic velocity fields were analysed across the obstruction using particle image velocimetry (PIV), with high-resolution density profiles obtained by micro-conductivity probes, to aid qualitative and quantitative interpretation of these internal flow processes. These measurements were processed to define parametric conditions under which the saline intrusions became arrested through interface ventilation at the intrusion nose by the upper, counter-flowing, freshwater layer. An internal flow hydraulic model has been developed to help interpret the parametric conditions under which this saline flow is blocked in such bi-directional, estuarine exchange flows. The results from this study are aimed primarily at improving representation of interfacial mixing processes within semi-enclosed estuaries and fjords, as well as providing datasets for future validation of advanced numerical models (CFD).