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A Two-DIMENSIONAL NUMERICAL MODEL STUDY OF CLEAR-WATER SCOUR AT A BRIDGE CONTRACTION WITH A COHESIVE BED

Recently, Guven et al. (2001,2002) presented a one-dimensional approach for modeling time-dependent clear-water contraction scour in a cohesive soil, where the scour rate for the soil is described by an erosion function. This report extends that method by using a two-dimensional model to calculate scour. The output from the twodimensional Finite Element Surface Water Modeling System (FESWMS) model in the, Surfacewater Modeling System (SMS) software provides sufficient information to I calculate scour over time at desired time steps for the entire two-dimensional domain. For this study, a 4: 1 bridge contraction with a beveled abutment geometry was investigated. The model output (velocity, depth, and water surface) and the SMS data calculator, which allows the user to calculate new data sets from existing ones, are used to calculate time-dependent scour based on the erosion function at each coordinate in the model. The erosion function describes the erosion rate for a given soil. For the timedependent method, the bed elevation is then updated at each node in the model, and the streambed is set at a new elevation so that the model can be run again to obtain new output. While it is possible to change the flow rate for each time step, the flow rate was kept constant for this study. An ultimate scour depth (referred to in Guven et al. (2002) as "maximum scour depth") can also be calculated based on the unit flow rate and other model parameters at a particular time step. This report covers the methodology and results for calculating time-dependent as well as ultimate scour using a two-dimensional [mite element computer model.