Tidal Bridge Scour in a Coastal River Environment: Case Study
Summary (3 min read)
- AECOM was hired by Flat Iron -United Joint Venture 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 .
- 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.
- The predicted velocity and depth information from these events were used to calculate scour depths at the bridge and assist in designing the bridge substructure units to withstand scour.
- The analysis consisted of three subtasks.
- The second subtask was obtaining various historical bathymetric surveys conducted at the US 17 Bridge crossing, located about 1.6 km downstream of the Washington Bypass Bridge, and calculating the vertical changes of the river bottom.
- In order to estimate the site-specific detailed hydrodynamic characteristics at the Washington Bypass Bridge it was necessary to construct a 2-dimensional hydrodynamic model.
- Hydrodynamic modeling was accomplished using the Surface Water Modeling System (SMS) in conjunction with RMA-2.
- The model domain was defined considering the area of interest, location of available data sources, and the limitation on computational resources.
- The area of interest was confined to the vicinity of the bridge crossing.
- 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.
- The model domain was meshed using triangular and rectangular elements.
- The approximate number of elements in the meshes used ranges from 9,000 to 23,000.
Calibration and Verification
- The RMA-2 model is calibrated primarily with two (2) parameters: the Peelet Number (Pe) and the Manning Roughness Coefficient (n) , and was perfonned in two stages.
- The channel calibration of the model was perfonned with the available data and the data sets collected during the project's monitoring period.
- Input variables for the model included WSE from the Pamlico Gage and model elements account for storage in watershed (which were used beyond the Tranter's Creek and Grimesland boundaries).
- Verification of the model was performed in two (2) ways .
- Second, for high flow conditions USGS ADCP velocity transects data at the maximum velocity conditions during Hurricane Floyd at 4S meters upstream of US 17 were compared to model predictions.
- Once the model was calibrated and verified, two different scenarios and two different return intervals totaling four (4) flow cases were developed: a rain-inducedflow scenario and a storm-surge scenario, for 100-and SOO-year storms.
Rain Induced Flow Scenario
- As seen during Hurricane Floyd, a hurricane causing significant rainfall over an already saturated ground can cause significant flooding in Tar/Pamlico watershed.
- Another observation made during Floyd was the duration of the peak flow.
- The duration is important for scour in cohesive sediment (such as this case), as equilibrium scour conditions require time to develop.
- FEMA FIS (Reference 2) was used to determine peak flow values for the model boundaries.
- Only high flows (i.e. >283 cms) in ebb direction were considered while generating the relationship.
Hurricane Surge Scenario
- Upon establishment of the typical tide, design hurricane surges were developed using the Pooled Fund Study (Reference 5) that was coordinated by the SCDOT and Boundary Conditions for Bridge Scour Analysis (Reference 6) by the NCDOT.
- The same data sources used to create the calibrated model (i.e. shoreline, bathymetry, topography, etc.) were also used to extend the model.
- Due to the large floodplain-to-channel ratio in the area (>8) , most of the numerical model cells were dry except for a short duration during the entire simulation.
- The deviation was observed to be 8 degrees during ebb times (i.e. rain-induced-flow scenario) and 20 degrees during flood times (flood periods produce peak velocity/depth combinations).
- Figure 3 shows peak velocity magnitude and direction for the 100-year stormsurge scenario.
- Scour evaluation and calculations were performed in accordance with the guidelines set forth in the most current editions of the Federal Highway Administration's Hydraulic Engineering Circular (HEC) HEC-20 3 rd Edition (Stream Stability at Highway Structures), HEC-18 4th Edition (Evaluating Scour at Bridges), and HEC-25 151 Edition (Tidal Hydrology, Hydraulics and Scour at Bridges).
- The hydraulic data required for the scour analysis were extracted from the 2-D model, the soil data were supplied by Mactec Inc., and the preliminary bridge geometry was taken from AECOM' s preliminary bridge design plans.
- All equation constants and coefficients used in the analysis were taken from literature.
- The following four components of total scour were calculated: aggradation and degradation, general scour, local scour, and lateral stream migration .
- Based on the results of the 2D modeling of peak flows caused by rainfall and peak flows due to storm surge hurricane surge conditions (landward flow) produced the worst-case design scour conditions at the proposed bridge location.
Analysis of Long-Term Bed Elevation Change
- The analysis consisted of three subtasks.
- The third subtask was a channel stability assessment performed by riverine geomorphology expert Prof.
- Stanley Riggs of Eastern Carolina University, who has specific knowledge of the project site.
- The historical aerial photograph analysis was performed by digitization of the shoreline in the vicinity of the proposed bridge crossing and then the subsequent observation of the resulting pattern and rate of the channel movement.
- Considering the current rate of change, significant degradation is not expected.
Scour at Abutments
- Per the preliminary bridge design plans, the abutments were determined to be outside of the flood elevation, and were not analyzed further for scour.
Computation of the Magnitude of Local Scour at Piers
- Local scour at the proposed Washington Bypass Bridge was computed for hurricane storm surges with recurrence intervals of 100 years and 500 years using the CSU equation as presented in HEC-IS.
- Bents on the overbank which lie outside the potential 30 meters of channel migration should be designed to withstand the smaller calculated scour depths in these areas .
- .5,4.3) 4 (1.8, 5.5) Adequate data was a key in preparing a comprehensive scour analysis.
- Especially, the importance of long-term ADCP data and how it can improve the confidence level of sophisticated hydrodynamic model simulations became apparent.
- By limiting complex scour equations to simple scour equation, over estimation of scour at the extreme values could be minimized.
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