Tacoma Narrows Bridge Hydraulic Analysis

Hydraulic and scour analyses were conducted to assure that the caissons for the existing bridge and new bridge foundations are adequate. The Tacoma Narrows Bridge is located in a unique hydraulic setting with daily tidal swings of 4–5m (12–17 ft) between high and low tides result in currents reaching 3.7 m/sec (7 knots). Large caissons in 38m (125 ft) deep water at these high velocities can produce significant scour. The caissons are skewed to the flow which also adds to the scour potential. However, the scour depths of observed scour holes that had occurred after 63-years of tidal flows for the existing structure are about 50% of the predicted scour depth using HEC-18 methods. To control the foundation costs, the hydraulic analysis included physical modeling, one and two dimensional hydraulic modeling and various site measurements. Site data was obtained to establish bed elevations, 3D velocity vector data to calibrate the hydraulic models and video observations to determine the channel bed material properties. Physical modeling used the large flumes at Colorado State University and new methods developed by D. Max Sheppard from Florida State University. This project was undertaken through a Design/Build contract and resulted in a need to determine the scour depth in a very short time frame and to reduce the depth of scour to the minimum reasonable amount. Additionally, approval of the methods and findings needed to be obtained quickly. Because of the size and complexity of the analysis, close coordination between the designers, construction engineers and project oversight reviewers was maintained throughout the design effort. The design effort between initiation of studies and determination of the scour depth took only 5 months and included physical and two dimensional hydraulic modeling. The analysis also showed a potential reduction of the scour potential from HEC-18 methods of about 10 meters (36 ft).