Filtering retention check-dam design in mountain torrents

During floods mountain rivers are affected heavily by transport of sediments in different forms ranging from bedload to hyperconcentrated and debris flows. In the alpine region there exists a long tradition (i.e. Thiery, 1891; Valentini, 1912) concerning the protection of human settlements and activities against such flood events. The most effective strategy is represented by the control of water-sediment flows by means of deposition basins associated with check dams located at their outlet. In the last fifty years closed barriers were replaced by open check dams and thanks to well focused research it has been observed a progressive improvement of their designing and trapping efficiency. After an introduction on the historical evolution of retention check dams, the chapter will deal with current typologies of integrated works (deposition basin-check dam) which are capable of mitigating flood hazards over fan areas. Moreover the assessment of design parameters in relation to the main characteristics of the flood is also considered: form of transport, expected sediment volume, impact force, mode of deposition. The functioning hypotheses will be coupled with the corresponding types of constructive solutions in order to support correct design and building. Some significant examples of open check dams operating in the Italian Alps will be presented and discussed.