ABSTRACT
A sill is a horizontal, structural part of a structure near the bed level on a foundation or pilings or lying on the ground in earth-fast construction. Sometimes a sill is used to reduce the mixing of different types of water in an estuary. In an estuary, a sill has to be designed for flow in two directions: flood flow and ebb flow. In rivers, for example, a sill may be used as part of a scheme to maintain a minimum water level. The bed in the direct neighbourhood of hydraulic structures is generally protected against currents and eddies. The length of the bed protection depends on the permissible scour and the geotechnical conditions of the soil involved. The curvature increases in the downstream direction; especially, near the point of reattachment. Vortices with a vertical axis will occur when the flow pattern is influenced by vertical ends or wing walls.
