Hydraulic impact assessment of quarry lakes in the flood plains with 2D modelling

Along the Italian rivers — in the flood plains — there are numerous quarries to extract sand and gravel from the fluvial sediments. The quarries can be very deep (more than 50 m). Accordingly, the excavation goes under the groundwater table and the quarries appear as lakes on the fluvial plane. During flood events the ground-water table increases more slowly than the levels in the adjacent river. When the river flow overpasses the riversides and floods on the plain the quarry lakes are being filled. The flux over the lake sides can be very intensive, depending on the hydraulic level of the groundwater table in the lake at the moment of flooding, which can create severe erosion effect on the lake borders. Depending on the position of the quarry lake in the flooding area with respect to the river axes the interaction between the flood and the lake can cause unexpected effects on the general river morphology resulting in, for example, meander cut off. Not only in relation to hydraulics but also in relation to other aspects the quarry lakes can generate environmental impact on the fluvial areas. Recently, the Italian river authorities and other public administrations responsible for the management of rivers (e.g Municipalities, Provinces, Regional Governments, Fluvial Environmental and Na-tional Park Organisations) have started to pay increasing attention to this serious situation. In particular, much focus is placed on detailed analyses of the hydraulic interaction between the river and the quarry lake, which is requested as a fundamental part of the Environmental Impact Assessment studies. 2D hydraulic models are excellent tools for undertaking an analysis of this character. The paper analyzes an example of the hydraulic interaction between the Po River and an adjacent quarry lake (Cava Ceretto) in the Piedmont Region (Northern part of Italy). In the study area the Po River is characterised by an evident meandering asset with many changes of the tracing in recent past. The Cava Ceretto lake is placed inside a left curve of the river and, because of the position, it can potentially cause the cut off of the meander. In the area of analysis the Po River receives from the right a secondary tributary (Meletta River). At peak flow the lateral inflow can damage and erode the opposite Po river side that separates the river from the lake. The purpose of the study has been to conduct a general analysis of the hydraulic behaviour of the fluvial environment during an extreme flood, paying particular attention to the interaction between the flooding and the quarry lake. Also the case of an instantaneous breakdown of the riverside that separates the lake from the river has been analysed with the modelling application. Particular attention has been paid to the construction of the model geometry. The elevation data of the flood plains and bathymetrical data of the river derive from different information sources (regional Digital Elevation Model, aero photographic surveys, bathymetrical surveys). The data of the cross section have been treated with an original software code to perform a realistic interpolation of the river bed geometry. All the data have been homogenised in order to obtain a reliable and complete digital model of river and flood plain. The result analysis has been carried out to define the reliability of the model results. It has been performed in different ways: through a sensitivity analysis on the values of the most important parameters of the model; through a comparison of the results with the application of theoretical formulas; and through a 1D interpretation of the flow in the main channel.