The Importance of Debris Shape in Experiments on Woody Debris Accumulations at Bridge Piers

Woody debris accumulations at bridge piers induces significant alterations of the flow field, typically leading to exacerbated scour, increased backwater effects and large lateral loads. Research on this topic has been primarily based on laboratory experiments with debris jam models idealised and of arbitrary size and shape. Recent experiments on the formation and growth of debris jams have used either idealised debris elements (e.g. dowels) or natural defoliated non-branched sticks. However, little or no attention has hitherto been given to the potential influence of the characteristics of individual debris elements on the resulting process of formation, including maximum dimensions obtained. This study analyses how the shape and complexity of debris pieces affect the formation and maximum dimensions of woody debris jams at single piers. To this end, 59 experiments have been carried out in a 22 m-long and 1.375 m-wide hydraulic flume at the University of Southampton. These experiments were aimed at comparing the characteristics of growth and size of accumulations formed by debris elements modelled by dowels and branched natural elements of different shapes. Branched debris significantly changed the formation process when compared to non-branched sticks. In particular, accumulations of the former displayed an inherent variation over time, in which elements were frequently removed from the pier, and the jam oscillated about the pier axis during most of the experiment. The maximum size of jams formed by both branched debris and dowel was significantly smaller (up to 40%) than those obtained with non-branched natural sticks.