Effects of Debris on Bridge Pier Scour

Waterborne debris (or drift), composed primarily of tree trunks and limbs, often accumulates on bridges during flood events. Debris accumulations can obstruct, constrict, or redirect flow through bridge openings resulting in flooding, damaging loads, or increased scour at bridge foundations. The size and shape of debris accumulations vary widely, ranging from a small cluster of debris on a bridge pier to a near complete blockage of a bridge waterway opening. The effects of debris accumulation can vary from minor flow constrictions to severe flow contraction resulting in significant bridge foundation scour. Currently, there is only limited guidance available on which to base critical public safety decisions during flooding on debris-prone rivers. There is a need for accurate methods of quantifying the effects of debris on scour at bridge-pier foundations for use in the design, operation, and maintenance of highway bridges. This paper provides a preview of the results expected from National Cooperative Highway Research Program (NCHRP) Project 24-26. The project was started in July 2004 and is scheduled for completion in December 2007. It is expected that this research project will produce results on two related problems: (1) predicting the accumulation characteristics of debris from potentially widely varying source areas, in rivers with different geomorphic characteristics, and on bridges with a variety of substructure geometries, and (2) developing improved methods for quantifying the depth and extent of scour at bridge piers considering both the accumulation variables and the range of hydraulic and geomorphic factors involved.