Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways

Stepped channels and spillways are used since more than 2,500 years. Recently, new construction materials (e.g. RCC, gabions) have increased the interest for stepped chutes. The steps increase significantly the rate of energy dissipation taking place along the chute and reduce the size of the required downstream energy dissipation basin. Stepped cascades are used also in water treatment plants to enhance the air-water transfer of atmospheric gases (e.g. oxygen, nitrogen) and of volatile organic components (VOC). A first part describes the historical progress of stepped channels and spillways from the Antiquity up to today. Then the monograph reviews the hydraulic characteristics of stepped channel flows. Two different flow regimes can take place : nappe flow regime for small discharges and flat channel slopes, and skimming flow regime. The hydraulics of each flow regime is described. The effects of flow aeration and air bubble entrainment are discussed. Further the process of air-water gas transfer taking place above stepped chute is described. Later practical examples of hydraulic design are presented : e.g. stepped fountains, stepped weirs, gabion stepped spillways, earth dam spillways with precast concrete blocks, roller compacted concrete (RCC) weirs, debris dams. At the end, the author presents a critical review of the risks of accidents and failures with stepped channels. It is shown that the hydrodynamic forces on the step faces are much larger than on smooth chute bottoms.