Putting a Grill (or Not) in Slit Dams Aiming at Trapping Debris Flows? Lessons Learnt From France

Slit dams are typical structures used in torrent control to trap debris flows and debris floods. While the question of the interval between bars of a grill is widely covered by the literature, the more basic question of whether or not to put a grill in the slit dam is less clear. The Maurienne valley is a debris flow hotspot in France. Since the 1980s, several torrents in the valley were equipped with dams with slits 4-6 m wide, initially with steel grills. Most of the grills were removed after a few events. The first grills were initially set down to the slit bottom thus creating chronic sediment trapping during small events unlikely to cause damage. In addition, grills impacted by debris flows transporting huge rocky boulders, typically 3-6 m in diameter, were damaged if not fully destroyed. Nonetheless, debris flows were partially trapped because such huge boulders often jam in slits and suddenly obstruct them. This note reports why slit dams are no longer equipped with steel grills in the debris flow torrents of the Maurienne valley and in other sites with similar characteristics.