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Bridge Scour Studies
MD140 Crossing over West Branch Tributary
Maryland State Highway Administration
SAAD Consultants offers subcontractor support in performing bridge scour studies for
five (5) prime contractors of the Maryland State Highway Administration. The following is an example of a typical bridge
scour project. The Maryland State Highway Administration proposed the
replacement of the bridge carrying MD140 over West Branch Tributary in
Carroll County, MD. The photo to the left shows the existing bridge.
The purpose of this study was two-fold: 1) to determine whether the proposed
bridge would have an impact on the water surface elevations, and 2) to
estimate the scour depths at the abutments and pier of 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 is approximately 1.10 sq.mi. The delineated drainage
area by GISHydro2000 generally follows the contour lines shown on
the 1:24000 USGS Topo map.
The HEC-RAS 3.0 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. The roughness coefficients Manning "n", for
the channel and the left and right overbanks, were estimated from
field observations. The reach was modeled twice with the existing
and proposed bridges in place to make the water surface elevation
comparison. The hydraulic model of the reach with the proposed bridge
in place for the scour analysis used the 100- and 500-year discharges.
The water surface elevations and top width comparisons for the existing
and proposed bridges were made. The comparisons suggested that the
change of the water surface elevation and the top width at the upstream face of the bridge was negligible.
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.
Practically, the 100- and 500 yr floods will not affect the bridge
abutments. The calculations were performed for different values of
the soil, D50 representing different layers. Pier scour
analysis was performed using soil data from a boring located at the
upstream face of the bridge.
The figures above show the upstream
bridge cross-section and the scour depths for 100-yr and for 500-yr
floods respectively.
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