Abstract Pressure flow due to vertical contraction occurs under a bridge when the river water level rises above the bottom chord of the bridge. The flow accelerates inside the contraction and diverges downstream of it, resulting in increased bed shear stress in the vicinity of the contraction. The present study investigates the effect of vertical contraction on the flow field and bed shear stress inside and downstream of the contraction by conducting laboratory experiments in a smooth, rigid-bed channel. The velocity field is measured using particle image velocimetry (PIV). The effect of independent variables, namely, streamwise length of contraction, amount of contraction, and approach velocity on the velocity and turbulence fields, is studied. Bed shear stress is estimated from the measured velocity field. It is observed that the viscous stresses are amplified inside the contraction. Downstream of the contraction, increased turbulent stresses are observed close to the bed. The local bed shear stress exhibits a dual peak, one inside the contraction and the other downstream of the contraction, suggesting the possibility of two different locations for scour inception. The stochastic nature of the bed shear stress is studied and its implications on scouring are discussed.
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