Harris County (Fig. 3) was chosen as the area of study for this project. The three phases described in the methodology above was used for this area. Six bridges were determined to be scour susceptible (Fig. 6).

1
Identifying Bridge Scour Susceptibility A
GIS-based Approach Cassandra J. Rutherford,
E.I.T.Master of Science Candidate in Civil
Engineering

Scour has been shown to be a leading cause of
bridge failure. Due to an increase in bridge
failure frequency, the Federal Highway
Administration (FHWA) requires that all state
highway agencies evaluate bridges on the Federal
Aid System for susceptibility to scour-related
failure. Scour occurs when streambed material
around bridge foundations is eroded. Due to the
large number of bridges and their spatial
distribution, a robust method of analysis is
necessary to identify potential high-risk areas.
Hydrologic, transportation, soil and bridge
geometry data are integrated into a geographical
information system (GIS) to identify structures
that are susceptible to scour. HEC-GeoRAS is
used as a pre-processor for the HEC-RAS model.
Using HEC-RAS, the total scour at a bridge
cross-section due to contraction scour, pier
scour, and abutment scour is modeled to verify
the susceptibility prediction. Identification of
bridge sites where scour is a potential problem
enables implementation of countermeasures to
prevent further sediment transport that may lead
to failure.
Abstract
Two bridges were selected from the six scour
susceptible bridges (Fig. 6) for validation.
HEC-GeoRAS and HEC-RAS were used for the
validation phase. The Buffalo Bayou was
chosen as the validation area based on the
availability of the Triangular Irregular Network
(TIN) data (Fig. 7). Contours are created to
allow ArcView to refresh more quickly (Fig. 8).
The HEC-GeoRAS themes were created (Fig. 9)
and imported into HEC-RAS (Fig. 10). Finally,
scour calculations were ran to verify the scour
susceptibility of the bridges selected in the
Buffalo Bayou.
Validation
Methodology
The methodology for this project is
shown in Fig. 2. In the Data Processing phase,
data collected was in tabular format and
converted into shapefiles (Fig. 4). ArcView 3.2
was used to determine scour susceptible bridges
in the Analysis phase. The flow direction was
determined using PrePro, based on the digital
elevation model (DEM), allowing for bridges
containing upstream gages to be selected. To
determine if the bridges were located in highly
erosive soils, highly erosive soils were
dissolved together based on the erodibility
factor from the SSURGO shapefile (Fig. 5). To
determined bridges with poor channel protection,
bridges were selected that had a channel
protection factor less than 6.
Analysis
Validation
Data Processing
Fig. 6. Scour Susceptible Bridges
Fig. 2. Methodology
Bridge scour is the erosion of streambed
soils and sediment that provide support for
bridge foundations. Bridge scour is a
nation-wide problem of the 832 bridge failures
since 1950, 60 were related to scour. Annually,
50 million is spent in Federal aid for
scour related bridge failures and repairs. In
the last 15 years, three bridge failures have
resulted in the loss of 25 lives. The Texas
Department of Transportation (TxDOT) maintains
over 49,186 bridges of which 40,814 bridges are
over waterways (Fig. 1). Currently, 688 bridges
have been determined to be scour critical. Scour
critical bridges are defined as having structural
integrity potentially jeopardized due to scour.
Bridge scour has been evaluated based on many
different methodologies, but the most common
approach is the use of HEC-18. HEC-18 provides
for a detailed scour analysis, referred to as
Level II Analysis. Scour susceptible bridges are
bridges that have the likelihood to be vulnerable
to scour. Identifying scour susceptibility
allows DOTs to efficiently direct resources to
those bridges rather than spending time and money
analyzing bridges that are at low risk for scour.
Due to the large number of bridges, their
spatial distribution, wide variety of data
required and cost of performing Level II
analysis, a robust method of analysis is
required. A geographical information system (GIS)
is an effective means for spatially storing the
database, analyzing the data and presenting the
results.
Bridge Scour
Fig. 8. Buffalo Bayou Contours
Fig. 7. Buffalo Bayou TIN
Fig. 1. Texas Bridges
Harris County (Fig. 3) was chosen as the
area of study for this project. The three phases
described in the methodology above was used for
this area. Six bridges were determined to be
scour susceptible (Fig. 6).
Harris County Case Study
Fig. 9. HEC-GeoRAS themes
Fig. 10. HEC-RAS geometry
Conclusions based on the chosen area of
research are that prioritization of bridges
susceptible to scour can be accomplished using a
GIS scheme. Although this method does not
provide actual scour calculations and
measurements, it does allow for bridges that are
at risk for scour to be identified and allows for
DOTs in charge of maintenance to allocate
resources to these scour susceptible bridges.
Future work includes the automation of GIS method
discussed in this project and continue to work
for more accurate results using the HEC-RAS scour
calculations.
Conclusions
Fig. 3. Harris County
Fig. 4. Bridges and Gages
Fig. 5. Erodible Soils
CVEN 689 Applications of GIS to Civil Engineering
Instructor Francisco Olivera, Department of
Civil Engineering April 29, 2003