Linking Flood-Related Damage to Bridges and Stream Geomorphic Conditions in Vermont

Abstract:

Bridge scour design predicts the depth of erosion given conditions of the streambed, geometry and flow. The depth of the scour hole is calculated empirically, while the width is assumed to be a percentage of the estimated depth. In Vermont, bridge damage resulting from the 2011 Tropical Storm Irene showed high numbers of stream widening and outflanking at bridges rather than vertical scour holes. The outflanking of bridges is not represented in current bridge scour design, where riprap is typically recommended to prevent horizontal stream migration. This study evaluates whether stream geomorphic information can be used to determine conditions that resulted in outflanking. Post flood inspection photographs are reviewed to identify cases of outflanking, and data from the National Bridge Inventory (NBI) and the Vermont Agency of Natural Resources’ (VTANR) Rapid Geomorphic Assessment (RGA) are used to determine the specific site and stream characteristics at the bridges. When applicable, bridge design plans and HEC-RAS modeling are used to compare scour depth calculations to evidence of damage from site photographs. Geomorphic assessments are used to determine if there are site differences that resulted in outflanking rather than traditional vertical scour. The damage from outflanking erosion can be quantified and modeled similar to the process of calculating scour depths, aiding in the prediction of outflanking, and improving the ability to design against it.