Stepped spillways and cascades

Dams and weirs are man-made hydraulic structures built across a river to provide water storage. During major rainfall events, large water inflows into the reservoir induce a rise in the reservoir level with the risk of dam overtopping. The spillway system is an aperture designed to spill safely the flood waters above, below or besides the dam wall. Most small dams are equipped with an overflow structure, the spillway, which includes typically a crest, a chute and an energy dissipator at the downstream end. The energy dissipator is designed to dissipate the excess in kinetic energy at the end of the spillway before it re-joins the natural stream. Energy dissipation on dam spillways is achieved usually by a standard stilling basin downstream of a steep chute in which a hydraulic jump takes place, converting the flow from supercritical to subcritical conditions, a high velocity water jet taking off from a ski jump and impinging into a downstream plunge pool, or a plunge pool in which the chute flow impinges and the kinetic turbulent energy is dissipated in turbulent recirculation. The construction of steps on the spillway chute may assist also with the energy dissipation.