Environmental Engineering and Management Journal May 2012, Vol.11, No. 5, 975-989 http://omicron.ch.tuiasi.ro/EEMJ/ “Gheorghe Asachi” Technical University of Iasi, Romania BRIDGE PIER SCOUR: A REVIEW OF PROCESSES, MEASUREMENTS AND ESTIMATES Luigia Brandimarte  , Paolo Paron, Giuliano Di Baldassarre UNESCO-IHE, Institute for Water Education, Westvest 7, 2601 DA Delft, The Netherlands Abstract Scouring of piers and abutment has been recognized as the main cause of damage and failure of bridges over waterways. The scientific community has produced a number of studies addressing the complex characteristics of the scour process and has provided engineers with several techniques for the estimate of the maximum expected scour depth at a bridge site. Nevertheless, the prediction of scour depths is affected by many sources of uncertainty, such as observation uncertainty, parameter uncertainty, and structural uncertainty. Only a few studies have recently tried to estimate the uncertainty associated to the scour depth prediction. This paper offers a broad review of the main aspects to be taken into account when analyzing bridge pier scour: 1) processes: to better understand the dynamics triggering pier scour, an analysis of the type of scour occurring at bridge piers, the most influencing factors, failure mechanisms and local pier scour dynamics is carried out; 2) measurements: one of the main difficulties faced in the real world practice is scour data collection; this session reviews the latest techniques available for the measurements of the scour depth at bridge piers; 3) estimates: this session critically reviews different approaches the scientific literature has offered for the estimate of the maximum local scour depth and discusses the difficulty to address uncertainty in the estimates. This review is meant to be a useful reference for scientists and technicians dealing with the bridge pier scour issue. Key words: bridge failure, pier scour, scour dynamics, scour measurements, scour estimates Received: October, 2011; Revised final: April, 2011; Accepted: May, 2011  Author to whom all correspondence should be addressed: e-mail: l.brandimarte@unesco-ihe.org; Phone:+31 15 215 1869; Fax:+31 15 212 29 21 1. Introduction Bridges have always been a big challenge for engineers and builders, both in their design stage and in maintaining their stability and function over time. Indeed, for the Ancient Romans, these fascinating architectural structures that allowed man to overcame natural impediments were considered not just a technical and engineering masterpiece but, because of the huge effort that it took to erect them, also a sort of “architectural miracle” (Brandimarte and Woldeyes, 2012). Furthermore, the engineers who built them were considered a type of High Priest (the word Pontifex comes from the Latin Pontem Facere that means “to build bridges”), acting as mediators between Gods and the believers. Any kind of structure in river channels induces a forced interaction between the structure itself and the natural river flow. Quite generally, a crossing bridge, with pier and abutments in the river bed and banks, represents an alteration of the natural geometry of the river section and, thereby, creates an obstacle for the river flow that, as it approaches the bridge, has to change its own natural pattern; furthermore, because of the modified flow conditions at the bridge crossing, the streamflow acquires a strong erosive power. As a consequence, in the subcritical flow conditions that are usually encountered in river channels, the resulting increase in flow velocity and erosive power of the streamflow creates conditions that endangers the stability of bridge foundations.