A Computational Approach to the Study of the Stability of Pier Riprap at the Middle Fork Feather River

A bridge over the Middle Fork Feather River in northern California has avulsed from a channel realignment project constructed at the time the bridge was built and has readopted its historic streambed and flow path. As a result, the flow now enters the bridge at a strong angle and causes excessive backwater and deep scour at one of the piers. The bridge was determined to be scour critical based on the resulting combination of vertical contraction scour and the local pier scour. To mitigate this scour critical condition, a rock mattress consisting of 1 Ton rock over filter fabric was placed around one of the piers in 2011. However, design of the rock mattress did not consider the increase in flow velocity and shear stress under the structure caused by the flow separation below the superstructure from the vertical contraction of the flow. Current design methodologies and scour evaluation procedures do not provide a clear means to analyze when the rocks might become displaced and, hence, further advanced computational mechanics techniques are required to assess the rock stability and, thereby, ascertain the current scour vulnerability of the bridge. From the computational mechanics point of view the analysis of riprap stability can be considered a Fluid Structure interaction (FSI) problem. FSI problems involve solving for the fluid flow force on a solid surface, the response of that solid to the load, and subsequently the change of the flow conditions caused by displacement of the solid. Historically computational fluid dynamics (CFD) software used for solving fluid flows and computational structural mechanics (CSM) software used for solving the deformations and stresses in solid bodies were developing independently. In the recent years the developers of both of these types of software groups have been developing additional solvers needed for FSI problems. However, at the moment there is no integrated software package with both highly robust CSM and CFD solvers needed for general purpose FSI software. The current best practice is to couple highly reliable independent CFD and CSM software in an iterative analysis that requires exchange of the data on the common interfaces between the fluid and the solid structure in small time intervals. That way the best features out of two groups of software can be utilized for FSI purposes. The presentation will cover a methodology for coupling CD-adapco’s STAR-CCM+ CFD software and LSTC’s LS-DYNA CSM software applied to stability analysis of riprap rocks used for armoring a pier at Middle Fork Feather River. STAR-CCM+ is capable of solving flow problems in domains containing solid objects with complex, irregular geometry in relative motion along arbitrary paths through the fluid domain. Mesh motion and mesh morphing techniques were implemented in it for handling arbitrary motions of the objects. LS-DYNA software is a general purpose finite element program capable of simulating highly non-linear real world problems in structural mechanics including changing boundary conditions (such as contact forces between rocks that change over time), large displacements, large deformations, and non-linear material property relations. The bathymetry of the Middle Fork Feather River was obtained from a sonar scan of the bridge site. It was numerically enhanced and transformed to CAD format and imported to CFD software as the initial geometry of the numerical model. A rock shape has been laser scanned to represent the real riprap elements. Several coupled simulations have been performed with varying flow conditions to identify failure conditions for the riprap. The presentation will include the results of the analysis and the implications for rip rap installation design and sizing