Embedding techniques for assessing debris-induced scour within practice

Abstract

INTRODUCTION Background In the last decades, scientists, engineers and practitioners have become increasingly aware of the risks from debris blockage at bridge piers. Large wood is transported in rivers especially during flood events, in which debris could be entrapped by structures such as bridge piers, and may initiate an accumulation. The localised acceleration of the flow can substantially exacerbate the scour at the base of the pier that would normally occur with the pier alone, and consequently the risk of structural damage or collapse of the structure. Piers that have spread foundations, as most of the UK masonry bridge stock, are more prone to this problem. It was estimated that in the last century approximately more than a third of the bridge failures in the UK caused by scour involved the accumulation of woody debris (Benn, 2013). Similar figures were also observed in the US (Diehl, 1997). In 2015 the University of Exeter was awarded an EPSRC grant for the project Risk Assessment of Masonry Bridges Under Flood Conditions: Hydrodynamic Effects of Debris Blockage and Scour, the primary aim of which was the development of a robust method for estimating the scour depth at a bridge pier with debris accumulations. This research involved a comprehensive experimental investigation at the University of Exeter using a large flume. A total of 46 experiments was carried out. The results from these experiments along with those from others in literature were used to develop a functional relationship for predicting the maximum scour hole depth at a bridge pier. To implement the results from the research within the practice of non-academic partners, the University of Exeter in partnership with Devon County Council (DCC) started a project funded under an EPSRC Impact Accelerator Account (IAA) award. The IAA award focused specifically on trialling the methodology on DCC’s assets and using the knowledge to propose amendments to the current scour assessment practice, as recommended by the Highways England (HE) guidance document BD 97/12. BD 97/12, in its current form, acknowledges the importance of debris accumulations, but does not provide a systematic methodology to assess the effects of debris on scour. This report summarises the findings from the work undertaken as part of the IAA award. In particular, it summarises the proposed amendments to BD 97/12 and illustrates the impact of these changes on scour assessment practice via a number of full-scale case studies. [...]