Influence of Debris Jam Formed by Trees on Bridge Pier Scour

Abstract

Debris jams contribute to bridge pier failures. Previous investigations showed that a debris jam can constrict the flow cross section, enhance flow intensity, and result in significant scour. Physical modeling was conducted to investigate the influence of debris jams on scour depth. Instead of using a static block to represent a debris jam, dynamic debris jams composed of real tree seedlings were investigated to represent jams forming of woody debris with roots and branches. The dynamic jam was achieved by continuously releasing individual seedlings from upstream. The resulting seedling jams had a typical half-cone shape and generally grew continuously over time, with observed scour increasing with the size of the debris jam. The scour in the presence of a dynamic debris jam could have depth up to twice and volume up to eight times that of a pier without a debris jam. In addition, the dynamic debris jam also induced additional hydraulic head across the cylinder pier, which correlated with the size of debris jam and Froude number. Scour around a bridge pier is the removal of sediment from the pier foundation by the surrounding flow. This can destabilize the bridge infrastructure, leading to potential failure. The accumulation of wood debris on a bridge pier leads to the formation of a debris jam that can increase scour. Most previous studies measured scour induced by a fixed debris jam geometry of constant size, with no temporal variation. However, in real situations, debris will gradually and often rapidly, especially during floods, accumulate over time around a bridge pier, and the associated scour increases as the debris jam increases in size. This paper presents an experimental study of the temporal evolution of debris accumulation on the model of a bridge pier and the associated progress of scour as a debris jam develops. In contrast to most previous studies that used wooden dowels, the debris is modeled using real seedling trees, which have roughness characteristics more akin to real-world debris. The resulting debris jams are relatively stable due to the interlocking of debris pieces. It was found that the instantaneous scour depth increased in the presence of the developing debris jam and that it was proportional to how much the debris jam blocked the flow. Interestingly, the plan area of the scour hole increased more than that occurring in the absence of the debris; the scour hole volume is hence much greater when a debris jam is present.