LAKE COMO WEST ARM EXCHANGE ENHANCEMENT SYSTEM (WAEES): FIRST INDICATIONS OF THE EFFECTS INDUCED BY A MIXERS SYSTEM ON THE LAKE WATER QUALITY

The work reports the results of a field experiment (FE) carried out in Lake Como at the beginning of October 2006, to quantify the effectiveness of a pilot mixer to enhance the horizontal exchange between the southern region of the west arm and the main body of the lake. The mixer acts converting mechanical energy into a buoyancy flux, that causes vertical mixing which leads to a horizontal intrusion of the mixed water in to the thermocline. During the FE a pilot mixer (2.5 m diameter) was mounted 2 m below the surface. The performance of the pump was verified by adding 480 l (0.5 %) of rhodamine (R- WT) above the pump over a three-hour period. The R-WT dispersion was then measured by a fluorimeter mounted on a high-resolution profiler. These measurements gave a flow rate for the intrusion of 25-30 m3/s. The water quality (WQ) campaign carried out during the FE underlined a consistent nutrient gradient between the Como bay (25 ?gP/l) and the main lake (15 ?gP/l). The WQ results also show the ability of the mixer in mitigating the nutrient load and the chlorophyll concentrations near the mixer. The encouraging results from the FE suggest that a system of 5 or so impellers could be used to effectively flush the bay to ameliorate the quality of the lake adjacent to the City of Como. These results joined with the historical dataset are currently used to calibrate ELCOM-CAEDYM model that will be used to define the number and distribution of the impellers.