Challenge. Estimate design scour at existing and proposed bridge piers under large, tidal-influenced flow conditions and deep water depths

Solution. INTERA performed hydraulic and scour analyses for the existing and proposed Tacoma Narrows Bridge piers. This site experiences large astronomical spring tides and tidal induced flow velocities. Additionally, the water depths range from 120 feet at the channel piers to greater than 250 feet in the center of the channel. Both existing and proposed piers are large, rectangular in shape, and skewed to the flow. These conditions, combined with the fine sand bed materials made for a challenging scour analysis. INTERA configured a 2-dimensional, depth averaged computer model (RMA-2 model) for the tidal waters encompassing the bridge site, and extending from Puget Sound to Olympia, Washington. The model calibration data included measured water elevation, velocity, and discharge data at and near the bridge site. The model simulated flow conditions for spring tides as well as for the 100-year, and 500-year return interval design conditions. To address the close proximity of the new and existing piers, INTERA designed, conducted, and analyzed physical model local scour tests.