A flushing weir is a permanently installed, sealing device, consisting of a vertically movable weir plate running on rails. Flushing weirs are used for control purposes and also for the cleaning (permanent surge flushing) of large sewers, either as an individual element or as part of a system of cascading weirs.

The figure shows a flushing weir application, where a 8.2 km long sewer line with nominal sizes of DN 1,200 (1200 mm, 48 in) to DN 2,400 (2400 mm, 96 in) is maintained using three flushing weirs [FI-ASA].