Hydrodynamic experiments on the impacts of vehicle blockages at bridges

Most urban rivers carry much natural debris, such as vehicles and trees, during extreme flood events, and these large debris, particularly vehicles, can block a local hydraulic structure such as a bridge. Such blockages usually increase the upstream water levels and cause more water to be diverted into adjacent urban areas. A scaled physical river model was constructed in a laboratory flume, consisting of a sketched urban reach with a compound cross-sectional geometry, and experiments were conducted on three model bridges, blocked by vehicles, with the corresponding hydrodynamic impacts of these vehicles blocking bridges being investigated. The main findings obtained from the investigation show that (i) the upstream water depths in the urban river increased significantly due to the vehicle blockage, for the single opening arch bridge and the straight deck bridge, while the upstream depths increased slightly for the three-opening straight deck bridge; (ii) the non-uniformity in the velocity profiles varied along the channel centreline, with the exponents in the power-law velocity distribution reaching their maximum values around the first two blocked model bridges, while the exponent values changed slightly along the channel for the third model bridge due to the middle opening not being blocked; and (iii) the downstream mean velocity was slightly greater than the upstream velocity for the first two cases, with the minimum value occurring just downstream of the bridge. However, the depth-averaged velocities just downstream of the bridge were relatively higher for the third case. Therefore, the obstruction caused by vehicle blockage at bridges has significant impacts on the hydrodynamic characteristics in an urban reach, and these impacts depend on the specified bridge type and the blockage mode.