Understanding the formation of woody debris jams at bridge piers

Woody debris accumulations at bridge piers can significantly increase the risk of flooding and bridge failure because of increased afflux upstream of bridges, additional structural loads and exacerbated scour. Despite the importance of this problem, limited research has been conducted on the topic. In this study we experimentally analyse the process of accumulations of woody debris (modelled with twigs and natural sticks) at single piers exposed to flow and a continuous supply of debris. Results show that these debris jams follow a three-phase growth: unstable (where growth occurs rapidly but debris are easily disengaged); stable (growth assumes a less pronounced trend and debris are less likely to be escaped); critical (the accumulation begins to oscillate about the pier and ultimately drifts away, i.e. fails). The dimensions of the accumulation at failure were observed to plot as well-defined functions of the flow and debris characteristics, and provide a worst-case scenario that can be useful for engineering design. In particular, while the cross-sectional and longitudinal dimensions of the accumulations were observed to decrease with increasing flow, the vertical component displays an opposite trend.