Influence of local hydrodynamics on ship drift leading to ship-bridge allisions

• Coupled models can predict the allision of a container ship with a bridge pier. • Ship’s southward turn was driven by a cross-channel gradient in water currents. • Ship drift motion is highly sensitive to uncertainties in the ship and environment. • Bridge wreckage affected the local circulation, especially at alternate channels. An increase in commercial shipping has led to an increase in hazards for ship strikes on bridges, to which we refer as allisions . There is a need for a better understanding of how ships are affected by local flows as they approach an allision. We couple region- and local-scale models to simulate the allision of the container ship Dali with the Key Bridge. Simulations are forced with real tides, river inflows, and atmospheric conditions, and then the ship’s motion is predicted as it drifted and then allided with the bridge’s south pier. The trajectory is a close match to observations, and the allision timing is matched within 70 seconds of the real event. The ship’s southward turn was driven by a cross-channel gradient of 0.22 cm/s in the currents. Perturbations show the trajectory sensitivity to ship and environmental conditions, with many scenarios showing ship motion away from the bridge pier, as much as 500-m down-channel or 200-m to the north side. Simulations with wreckage show the depth-averaged currents may have increased by 10 to 20 cm/s in the temporary alternate channels around the bridge. Our findings can inform models for ship motion and management of navigation channels.