To elevate river water table for providing public water supply, weirs were usually built across rivers in Taiwan. Most of the river beds in western foothills of Taiwan are composed soft sedimentary rocks, such as sandstone, siltstone, and mudstone. Due to low rock strength and the head difference caused by the weirs, rapid scour downstream of weirs is often observed. This type of scour may induce weir instability and accelerate river-bed incision. To mitigate the damages, the scouring mechanism and processes in front of a weir built on soft rock riverbed need to be correctly identified. It is also necessary to determine the possible extent of scour hole and to deliberate the appropriate engineering countermeasure. The project aims at investigating the erosion/scouring problems associated with weirs built on soft rock. The scope of this project includes the following: (1) collection of relevant data for eight weirs built on soft rock; (2) preliminary study of erosion mechanism and erodibility of soft rock ahead of weirs; (3) applicability evaluation of the available approaches for estimating local scouring in front of a weir built on soft rock in Taiwan.