Weir structures are used when there is a sudden change in river bed levels or on steep gradients in water courses to reduce water velocity. The weir is effectively a dam which allows water to flow over the crest controlled by introducing a notch that can be rectangular or other shapes.
The water can flow over the notch to a lower bed level. The weir is designed as a retaining structure with an apron protection to prevent erosion of the bed. In instances where there are high flows over the weir, it may require a stilling pool to be included in the design generally with an additional counter weir. These structures require the hydraulic conditions within the water course to be considered to determine the protection of the river bed on the down stream side. The flow conditions are turbulent and the mattress protection depth will be greater than for normal gabion mattress protection.