Author(s): L. Merlino; V. Cavedon; G. Vignoli; C. Lanni; S. Simoni

Linked Author(s):

Keywords: Sediment management; Transversal weirs; Hydropower; Environmental sustainability; CFD simulation

Abstract: The optimization of the maintenance operation of transversal weirs, obtained by means of an innovative approach, develops by focusing on the improvement of the quality of the fluvial downstream ecosystems and the operator company objectives. Alperia Greenpower LTD is located in the Autonomous Province of Bolzano in Northern Italy. It is an operator company which manages 41 hydropower plants which produce on average approximately 4.5 TWh of energy per year. In the Province of Bolzano, since 2002, and in Italy, since 2006, laws and directives focusing on environmental and water quality protection have introduced limit values for sediment flushing downstream dams and weirs. The indicated values are: (i) the maximum sediment concentration in the downstream flow (1% in volume) and (ii) 1 hour as minimum time period to return to the environmental flow after the flushing operation. The regulation aims at ensuring the safety of the aquatic fauna, which is stressed by turbidity, drift and stranding. Turbidity is one of the causes of the bed clogging, that influences strongly the deposition of eggs: the material suspended by the cleaning operations tends to deposit and hence to aggregate. Variations of the magnitude of flow rate due to the operations of sediment flushing cause drift and stranding, and their impacts depend on the velocity of increasing and decreasing of the water depth and on the initial discharge. The values indicated in the directives are not suited and adapted to the environmental and physical characteristics of each considered river and flow, but they are the same for all the water courses in the Province of Bolzano. On the contrary, many researchers recognise that stranding and drift are also strongly dependent on morphology. According to the literature, the physical habitat is a key factor in evaluating the ecological status of rivers. Given the increasing sensitivity towards the fluvial ecosystems in the past years, the last concession agreements prescribed that Alperia Greenpower elaborated within 2016 the sediment management plans for reservoirs, weirs and desanders of each of its own hydropower plant. The difficult aim of the project was to solve the prescribed requests by starting from two different points of view: that of the operator company, which is interested to apply correctly the directives, but by using less water as possible. From the second point of view, the legislator who has given parameters in the directive which are very generic. The project team chose to elaborate innovative and partially scientific solutions that consider the following factors and objectives: i) to reduce, as far as possible, the production losses (namely the volume of water necessary for the cleaning); ii) to optimize the efficiency of cleaning operations; iii) to verify the environmental sustainability in accordance to the directives; iv) to elaborate a protocol of opening and closing of the transversal weirs which can be easily applied by the workers. The boundary conditions are particularly complex, given that: transversal weirs and desanders were built in the 50s and 60s, when the environmental sensitivity was different from the current one; the existing directive is based on data that are not site specific, but empirical; the literature is often lacking, or there are only site specific or case studies applications; the sediment cleaning depends not only on the seasonal trends of natural conditions and flows, but also on the management of upstream and downstream hydropower plants. Given that the cleaning operations depend on the velocity of opening and closing of the transversal weirs and on the cleaning time, the aim of the project was mainly to correlate the parameters which describe the environmental and biological deficit of the river stretch with the operations of opening and closing of weirs. For this reason, the project team chose to face the problem by analysing the real and existing characteristics of the downstream stretch and then by measuring and simulating (by using a CFD simulation for flow and sediment transport) the environmental impacts (sediment transport and turbidity, morphology, drift and stranding risks), to try to identify for each stretch the most critical factors. Once the important factors were identified, new protocols of opening and closing of the weirs and the desanders were tested by means the same methodology: hydraulic simulations and field measurements. The final results of our investigation procedure are the sediment management plans adapted to the environmental conditions and to the technical characteristics of each of the power plants.

DOI: https://doi.org/10.3850/978-981-11-2731-1_264-cd

Year: 2018