Analysis of bridge and stream conditions of over 300 Vermont bridges damaged in Tropical Storm Irene

Abstract

The 2011 Tropical Storm Irene deposited 100–200 mm of rain in Vermont with a rainfall recurrence interval for a 12-hour storm exceeding 500 years in some areas. This single hurricane-related event damaged over 300 bridges. The wide range of damage prompted a networkwide analysis of flood, scour, stream and structural conditions. A first step was the assembly of a unique data set containing information on 326 damaged bridges, 1936 undamaged bridges and the surrounding stream conditions. Descriptions of the damage appear as case studies that include pre-storm bridge and stream geomorphology conditions. The assembled and georeferenced data include rainfall, damage type and extent, estimated and actual repair costs, bridge characteristics, bridge ratings and stream geomorphic assessments from a number of sources. The analyses identified significant features of bridge vulnerability under extreme floods. The bridge age and rating assessment characteristics, such as substructure, channel and structural adequacy ratings, followed by scour, waterway adequacy and sufficiency ratings, correlated strongly to damage. The stream geomorphic features have promise to supplement future bridge rating systems and in identifying hydraulic vulnerability of bridges. Empirical fragility curves relating probability of meeting or exceeding different bridge damage levels based on channel and waterway adequacy ratings are also presented.