Numerical Model Evaluations of Cumulative Contraction Scour at a Bridge Site with Cohesive Soils

Abstract

The objective of this study was to perform 2-Dimensional numerical contraction scour calculations at an actual bridge site with cohesive soils. The 2-D model used in the analysis was the Finite Element Surface Water Modeling System (FESWMS) computer program provided within the Surface Water Modeling Software (SMS). The study presents the development of the model, the 2-D numerical transient contraction scour calculation method, and a comparison of the 2-D contraction sour calculation results from this study with 1-D numerical model results for the same bridge site presented in Curry et al. (2003). The 2-D calculations are performed using velocity distributions and water depths obtained from the FESWMS model along with the cohesive soil properties of the bridge site presented in Curry et al.(2003). The cohesive soil properties used in the calculations are the critical shear stress (tc), and the initial erodibility (Si) that were btained from soil tests using the Erosion Function Apparatus described in Crim (2003). The model’s output of water velocities and water depths that vary in two perpendicular horizontal planes provide the rest of the data needed for the calculations of the 2-D distribution of scour in the contraction at the bridge site. This allows for the creations of plan views of the predicted contraction scour, which is one of the primary advantages of the 2-D model. The outputs presented in this thesis include plan views, cross-sectional views, and longitudinal views of cumulative contraction scour for 5, 10, 50, and 100 days. The results are representative of how scour develops overtime using 2-dimensional calculations. The Choctawhatchee River bridge site near Newton, AL is the focused site of this study. This study provides detailed information on how to set up a 2-dimensional numerical analysis of contraction scour using FESWMS within SMS. The process for calculating cumulative contraction scour at a bridge site with cohesive soil presented in this study is one possible way for predicting scour. The actual application of this method in the field of scour prevention and the development of new methods in bridge foundation designs is still to be seen. More research and experimentation needs to be done along with publications that promote these methods to the engineering field. This is a continuation of ongoing research in the field of scour prediction in cohesive beds, and presents methods and data that can be used for future research .