Local scour estimation in bridge piers is one of the main elements to design new bridges and to monitor existing bridges. It is important to obtain the maximum local scour depth in designing the level of foundation or countermeasures for the existing bridges. In this experimental study, an attempt was made to investigate the effect of cohesion on temporal and spatial local scour around cylindrical bridge pier in sand-clay sediment mixture. Two sizes of sands (with median grain size \(D_{50}\) = 0.59 mm and 0.99 mm) with different percentages of cohesive materials (0–30%) were used. The cohesive material contains 90% kaolinite and 10% bentonite. The results show that the location of maximum scour depth \(y_{sumax}\) varied for different time intervals based on the percentage of clay. Upstream side scouring dominates the scouring process at early stages before propagates to the upstream of pier for low flow velocity and low percentage of clay. Increasing flow velocity and clay percentage gives significant downstream side scouring throughout the experiments. The percentage of clay in the sediment mixture does not necessarily influence the maximum ultimate value of scour depth. The maximum ultimate scour depth was occurred at the upstream, usually at the pier nose, while the minimum ultimate scour depth was observed to occur at the downstream for the sand-clay sediment mixtures with up to 20% of cohesive materials. However, an adverse profile was observed for the sediment mixture with 30% of cohesive material where the maximum scour depth occurred at the downstream.
