Abstract
As part of an innovative design/build project a data-rich hydraulics and scour analysis was performed for the proposed 5 kilometer long Washington Bypass Bridge over the Tar River in Washington, NC. This analysis featured: 1) a current and stage monitoring program, 2) historical aerial photograph analysis, 3) extensive long term bed elevation study, 4) debris scour evaluation, 5) variable skew angles due to spatial and temporal changes in flow characteristics, 6) complex pier analysis, and 7) use of a two-dimensional (2D) hydrodynamic model (TABS RMA-2) to evaluate riverine flooding and hurricane surge hydraulics causing extensive wetting-and-drying. The model was subjected to a comprehensive two-step model calibration/verification process. Low-flow conditions were calibrated and verified using project-collected Acoustic Doppler Current Profiling (ADCP) and tidal gage monitoring. Boat-mounted ADCP measurements collected by USGS during hurricane surge (Hurricane Dennis) and rain-induced flooding (Hurricane Floyd) were used for high-flow calibration and verification. This was a rare opportunity for a bridge designer to be able to evaluate scour using a sophisticated hydrodynamic model that was calibrated with field data collected during an event that represented the design conditions. Two-dimensional hydrodynamic modeling was used to simulate complex hydraulics of the project site located at the end of the 8,300 square kilometer watershed which is tidally influenced. Due to the size of the upstream drainage area and the proximity to the open ocean both rain-induced-flow and storm-surge scenarios were considered.
Highlights
AECOM was hired by Flat Iron - United Joint Venture (FLUNN) to perform engineering services relating to the hydraulics and scour analysis for the Washington Bypass Bridge (BIN 353 -US17 over the Tar River) in Washington, NC (Figure I)
As part of this study, AECOM designed and oversaw the required data collection performed by Ocean Surveys, Inc. (OSI) which was used for the calibration and verification of the hydrodynamic model
In order to obtain an adequate solution at the area of interest, model boundaries were established distant from the area of interest and where USGS station locations were available as data sources
Summary
AECOM was hired by Flat Iron - United Joint Venture (FLUNN) to perform engineering services relating to the hydraulics and scour analysis for the Washington Bypass Bridge (BIN 353 -US17 over the Tar River) in Washington, NC (Figure I). Under this agreement, AECOM developed a 2-dimensional hydrodynamic model (RMA-2) to evaluate the flow depth and velocity for the 100-, and 500-year storm events. As part of this study, AECOM designed and oversaw the required data collection performed by Ocean Surveys, Inc. The data collection occurred during a Spring Tide and involved three (3) tide gages, one (I) "upward looking" Acoustic Doppler Current Profiler (ADCP) , and ADCP transects of cross sections at and either side of the bridge.
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