The Observation Method for Bridge Scour: Case Histories

Abstract

Bridge scour is the number one cause of bridge collapse in the USA yet the existing guidelines are felt to be excessively conservative. These apparently conflicting statements are due to the fact that most bridge foundations designed before 1987 did not consider scour as part of the design. The Observation Method for Scour (OMS) was developed to address the conservatism inherent in the current procedures by relying significantly on past observations at the bridge. The OMS works in four steps. Step 1 consists of collecting the maximum observed scour depth at the bridge, Zmo. Step 2 consists of finding out what is the biggest flood velocity Vmo that the bridge has been subjected to since its construction. Step 3 answers, by using an extrapolation function, the question: what will be the scour depth Zfut if the bridge is subjected to a major flood velocity Vfut. Step 4 is a comparison between Zfut and the allowable scour depth Zall for the foundation. Eleven bridge scour case histories in Texas and in Massachusetts are presented where the OMS was applied and the results are used to compare predicted and measured values of Zfut for both the OMS and the current FHWA guidelines. The advantages and drawbacks of the OMS are outlined in a final section.