Study of tributary inflows in Lake Iseo with a rotating physical model. (accepted for pubblication in Journal of Limnology on October 16th 2013, and prepublished in Techical Reports of DICATAM, N. 12, 2013, pag. 24)

The influence of Coriolis force on the currents of large lakes is well attested; very few contributions, instead, investigate this aspect in medium-size lakes where its relevance could be questionable. In order to study the area of influence of the two major tributary rivers in Lake Iseo, a rotating vertically distorted physical model of the northern part of this lake has been prepared and used, respecting both Froude and Rossby similarity. The model has a horizontal length scale of 8000 and a vertical scale of 500 and has been used both in homogeneous and in thermally stratified conditions. We explore the pattern of water circulation in front of the entrance mouth for different hydrologic scenaria at the beginning of spring and in summer and in the absence of winds. The primary purposes of the model were twofold: 1) to increase our level of knowledge of the hydrodynamics of Lake Iseo by verifying the occurrence of dynamical effects related to the Earth’s rotation on the plume of the two tributaries which enter the northern part of the lake and 2) to identify the areas of the lake which can be directly influenced by the tributaries waters, in order to provide guidance on water quality monitoring in zones characterized by relevant environmental and touristic value. The results of the physical model confirm the relevant role played by the Coriolis force in the northern part of the lake. Under ordinary flow conditions, the model shows a systematic deflection of the tributary waters towards the western shore of the lake, triggering a clockwise gyre within the Lovere bay and a slow counter-clockwise gyre that returns water towards the river mouth moving along the eastern shore. For discharges with higher return period, when only the contribution by Oglio River is relevant, the effect of the Earth’s rotation weakens in the entrance zone and the plume has a more rectilinear pattern, whilst in the far field the current driven by the inflows keeps moving along the western shore. On the basis of these results one could expect that the north-western part of the lake between Castro and Lovere, although not allineated with respect to the tributaries axis, is more sensitive to accumulation effects related to river-borne pollution. The results obtained with the physical model are critically compared with data obtained from different sources: the trajectory of a Lagrangian drogue; a map of reflectivity data from the lake floor; a map of water turbidity at the intrusion depth. They are also confirmed by the results of a 3D numerical model of the lake.