Contribution of trapped air, deck superelevation, and nearby structures to bridge deck failure during a tsunami

Failure of the Utatsu concrete girder highway bridge in Minamisanriku, Miyagi Prefecture during the 2011 Great East Japan Tsunami was puzzling because the bridge decks were not pushed off their piers but rather were flipped off the landward side of the bridge piers after being deeply submerged by the surging tsunami. To determine what caused this to happen, two simulations were conducted. The first was a large-scale Delft shallow-water simulation (beginning with published tsunami source free surface deviation) to determine the behavior of the tsunami (time series of flow depth and speed) at the bridge site. The second was a small-scale two-dimensional (2D) (profile view) software volume-of-fluid (VOF) simulation of flow over the bridge deck, with boundary conditions taken from the Delft model. The VOF model then allowed calculation of lift force, drag force, and overturning moment on the bridge deck. Results show that factors contributing to failure included the presence of a seawall near the bridge, inclination (superelevation) of the deck upward toward the ocean, sediment entrained in the water, and air trapped between girders.