Sills are used to reduce the flume slope to decrease erosion processes at the flume bottom. Usually, they are designed as a step downstream. Fish ladders are often made of small sills. Bridge pier foundations may have the same effects as a sill.
Sills cause a reduction of the flow cross-section.
The behaviour of open channel flow at a reduction of the flow cross-section can be observed using the sill HM 161.44.