Applying mobile boundary water surface profile models to coarse-bedded bridge crossings

Abstract

This thesis presents general processes for applying mobile boundary models at coarse-bedded bridge crossings. To develop these processes, the research uses a series of case studies involving highway bridges that span coarse-bedded rivers (gravel, cobble beds). From the case studies, the thesis presents seven tables and charts describing the overall processes and criteria for applying mobile boundary models at coarse-bedded bridge crossings. These include the preferred physical attributes for using a mobile boundary model at a bridge site, the required fixed boundary analysis, an evaluation of the dominating morphological feature at the crossing (e.g., bridge opening), a mobile boundary model cost analysis, and the requirements for performing a mobile boundary analysis. The presented mobile boundary analysis also includes recommended values for various operational parameters. These parameters include time step, active layer thickness, sediment transport equation, and stream bed gradation.The research shows that mobile boundary models will produce lower bridge backwater elevations (water surface profiles) than those that fixed boundary models produce. This occurs when the bridge opening dominates the hydraulics at the crossing. To test for bridge dominance, the Froude value, Fu, upstream of the bridge site should not exceed the Froude value, Fb, at the bridge site.The research also shows that values of hydraulic parameters (e.g., bridge scour) from fixed boundary models are useful in obtaining a broad estimate of the reduction in backwater that a mobile boundary model might produce. The statistical relationship between fixed boundary hydraulic values and mobile boundary backwater elevations, however, is weak. This relationship, nevertheless, is qualitatively strong enough to indicate when to use a mobile boundary model.Finally, the research uses additional selected case studies to evaluate the water surface profile accuracy of mobile boundary models versus fixed boundary models. These case studies show that the mobile boundary model will produce a more accurate water surface profile than a fixed boundary model. However, in some instances the added accuracy of a mobile model may not produce backwater curves with more practical use than a fixed model in the hydraulic design of a bridge.