Water intake at Stafel: construction of a grit chamber

The sand traps at Stafel’s water intake were meant to do the same job as a grit chamber. Gravel up to 50 mm in diameter was caught in the sand traps. This made them less efficient, hampered rinsing operations and led to an increase in sediment in the balancing reservoir. These problems could be resolved with the addition of a grit chamber at the water intake. HYDRO Exploitation’s civil engineering team managed all the phases of the project, from the research to the implementation.

Adding a reinforced concrete spillway and increasing the height of the metal spillway

The work focused on designing a new spillway made of reinforced concrete, to be positioned before the sand filtration basin, and renewing the purge system at the intake. The spillway at the bed of the intake was raised using a cofferdam system. These targeted measures offer various advantages: retaining the water reduces the flow rate and allows the gravel to settle so it can then be removed later by emptying the basin bed, which has a new valve.

A procedure implemented under the best conditions

The work involved in the reinforced concrete spillway was completed by SULAG AG, while Inauen-Schätti AG was responsible for designing and installing the metal cofferdams. The work was carried out in October and November 2021, the only period that does not entail any loss of water, as the pumping season ends on October 31st. 

The new spillway was covered with concrete in November. For this to be possible, the work area had to be covered and heated. On the whole, the weather was mild. The risk of snow and floods had been taken into account. In the event of floods, the site would have been shut down immediately. In the case of heavy snow, the snow would have been cleared. Some tasks, such as raising the metal spillway, could also be planned for during the operation thanks to a sand backfill on the water side.