Soft-rock scouring processes downstream of weirs

Diversion weirs are typically constructed to elevate river water levels and thereby increase water supplies. Most riverbeds in the western foothills of Taiwan are composed of soft sedimentary rocks covered with an armor layer of varying thickness. Due to the low rock strength and head fall caused by weirs, rapid scouring downstream of the weir often occurs once the armor layer is worn away. To estimate scour depth and mitigate its damage to weir foundations, scouring processes must be identified correctly. Because of different mechanical behaviors and the water-jet conditions, scouring processes downstream of a weir on soft rock may not be that same as those assumed by existing models. Analytical results from a series of studies of scouring downstream of weirs on soft rock indicate that the shape of a scour hole on soft rock differs markedly from that of a scour hole on hard rock. This study identified three basic scouring processes downstream of weirs, namely, (1) plucking, (2) uniform incision, and (3) trenching incision. The suitability of existing approaches for calculating the depth of a scour hole on soft rock were also assessed.