SAAD Consultants as a subcontractor performes bridge scour analysis for five (5) prime contractors. The following is an example of a typical bridge scour project. The Maryland State Highway Administration is proposing the replacement of the bridge carrying MD 351 over Tuscarora Creek in Frederick County, MD. The photo shows the existing bridge. The purpose of this study is to estimate the depth of contracting and abutment scour at the proposed bridge. The discharges used in the hydraulic modeling and scour analyses were calculated using data and procedures included in ArcView-GISHydro2000 software of the Structure Hydrology and Hydraulics Unit of the Maryland State Highway Administration. The drainage area at the bridge shown on the figure below is approximately 1.9 sq.mi.
The HEC-RAS 3.11 hydraulic model for the scour analyses was constructed using surveyed cross-sections and bridge data. The computations were initiated using the Slope Area method, assuming a slope was obtained from the topographic map and from the surveyed water surface. The roughness coefficients Manning “n” for the channel and the left and right overbanks were estimated from field observations.
Because there was a possibility that the maximum scour depth would have occured as a result of allowing a maximum flow to go under the bridge, rather than roadway overflow, a model was developed for this scenario.
A trial and error procedure was used to determine the amount of flow that fit this condition (Incipient Overtopping Flow). The hydraulic model of the reach with the proposed bridge in place for the scour analysis used this no-overflow flow as well as the 100- and 500-year discharges.
ABSCOUR computer program, developed by the Structure Hydrology and Hydraulics Unit of the Maryland State Highway Administration, was used to perform the scour analysis in accordance with the Federal Highway Administration (FHWA) HEC-18.
The calculations were performed for different values of the soil, D50 representing different layers. The figure above shows the bridge cross-section and the scour depth for the 100-yr flood.
