Case Study: Bed Stability around the East Caisson of the Tacoma Narrows Bridge

A study on the hydraulic and sediment conditions at the Tacoma Narrows Bridge, in Washington State, was carried out to examine the stability of the bed material around the bridge caissons. Specifically, this was conducted around the east caisson, where the maximum velocities around either of the two caissons are experienced. This was performed for the peak tidal exchange event of May 27th to 28th, 2002. During this max flow event, multibeam surveyed bathymetry and threedimensional acoustic doppler current profiler velocity data were collected around the east caisson in the course of both the flood and ebb. The surface of the bed material surrounding the east caisson was videotaped during the slack conditions following the yearly maximum flow event, and used to determine the particle size distribution and spatial arrangement of those distributions around the caisson. This was done by lowering a submersible video camera and appropriate lighting to the bottom of the Narrows, a depth of approximately 45  m. Flow and sediment observations were coupled to determine the commencement of sediment motion for different size classes of sediment. Two methods were utilized to calculate friction velocity in order to assess the stability of different bed particle size fractions during these high flow conditions. Friction velocity was first calculated from measured velocity profiles at various locations around the east caisson. The second method was based on the concept of a free stream power-law expression for depth-averaged velocity. Stability was then examined using the critical shear stress concept and captured video data of the bed. General results showed the particles <=30  mm in diameter were in motion during the flood and ebb. The work is here presented as a case study because of the unique largescale flow conditions that are present around the east caisson of the Tacoma Narrows Bridge.