Transcript Identifying Bridge Scour Susceptibility: A GIS

Identifying Bridge
Scour Susceptibility:
A GIS-based Approach
Cassandra Rutherford
Master of Science Candidate
Department of Civil Engineering
Project Objective
To develop a GIS-based method to
evaluate the potential for scour at
bridges and to identify those
structures that are scour
susceptible.
Presentation Overview
Bridge Scour
Case Study
GIS-based Approach
Progress
Available Data
Future Work
Methodology
Questions
Bridge Scour
Bridge scour is the
erosion of streambed
soils and sediment
that provide support
for bridge foundations.
Bridge Scour
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The primary variables that effect scour:
Bed materials
Channel protection
Channel and bridge geometry
Flow hydraulics
Bridge Scour
• Of the 832 bridge failures since 1950,
60% were related to scour.
• $50 million spent annually in Federal
aid for scour related bridge failures and
repairs.
• 3 bridges failures have resulted in the
loss of 25 lives in the last 15 years.
Bridge Scour
Over the 49,186
bridges in Texas . . .
• 40,814 bridges –
over waterways
• 688 bridges –
scour critical
Bridge Scour
Scour critical bridges – structural integrity
potentially jeopardized due
to scour. Requires Level II Analysis.
Level II Analysis – developed in HEC 18
requires on site investigation, bridge
foundation information, modeling of stream
Bridge Scour
Scour susceptible bridges – likelihood that
bridge may be vulnerable to scour.
Current methods require large database of
analyzed bridges for comparisons, or
envelope curves based on historical scour
measurements.
GIS-based Approach
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. GIS is an effective means for
spatially storing the database, analyzing the
data and presenting the results.
Prioritization of bridges susceptible to scour
can be more easily accomplished using a
GIS scheme than previous methods.
General Methodology
Data
Processing
Analysis
Validation
HEC
GeoRas
HEC
GeoRas
Available Data
USGS Harris County
Surface-Water Data
- daily stream flow and measurements
provides gage location, channel width,
cross section area, velocity, gage
height, discharge and drainage area.
Available Data
Federal Highway Administration
National Bridge Inventory (NBI)
Database
- electronic database coded by DOTs
provides data on bridges such as structural
type, number of spans, geometry, length,
width, channel protection, etc.
Available Data
USDA – NRCS
SSURGO Soil Data
- shapefile of county along with multiple
tables providing soil information such
as classification, hydrologic group,
sieve passing percents, soil erodibility
factor, etc.
General Methodology
Data
Processing
Analysis
Validation
HEC
GeoRas
HEC
GeoRas
Case Study: Harris County
Case Study: Harris County
Datasets Used:
Texas Bridge Database (NBI)
USGS DEM (1:24,000) Harris County (TNRIS)
Harris County data files
Texas Counties shapefile (ESRI)
Reach Files Version 3 (EPA)
Harris County Soils shapefile (SSURGO)
Harris County Stream gages (USGS)
TIN Data for Buffalo Bayou (Dodson)
Case Study: Harris County
Case Study: Harris County
Case Study: Harris County
Erodibility
Case Study: Harris County
Bridges with Erosive Soils
Case Study: Harris County
Channel Protection
N
Not over water
9
None needed
8
Well vegetated
7
Minor damage
6
Slumping
5
Eroded
4
Extensive erosion
3
Protection failing
2
State of collapse
1
Bridge closed
< 6
Case Study: Harris County
Scour Susceptible Bridges
Case Study: Buffalo Bayou
Selected 2 Bridges on Buffalo Bayou
to model total scour in HEC RAS
General Methodology
Data
Processing
Analysis
Validation
HEC
GeoRas
HEC
GeoRas
HEC GeoRAS
Used to develop the
spatial data required
to generate a HECRAS import file with a
defined 3-D stream
network and 3-D
cross sections
HEC GeoRAS
Buffalo Bayou
Triangular Irregular
Network (TIN) data is
required. Contours
are created to allow
ArcView to refresh
more quickly.
HEC GeoRAS
The following themes must be created:
Stream Centerline
Main Channel Banks
Flow Path Centerline
Cross-Sectional Cut Lines
and Land Use
General Methodology
Data
Processing
Analysis
Validation
HEC
GeoRas
HEC
GeoRas
HEC RAS
An accurate hydraulic models of river
reach containing bridge must be
developed before scour calculations
can be performed.
HEC RAS
Scour calculations
in HEC RAS require:
Depth at bridge
Velocity
Number of piers
General Methodology
Data
Processing
Analysis
Validation
HEC
GeoRas
HEC
GeoRas
HEC GeoRAS
Scour calculations
results can be
exported back
into ArcView.
General Methodology
Data
Processing
Analysis
Validation
HEC
GeoRas
HEC
GeoRas
Current Progress
• Text files correctly delimited and
column headers properly determined
• All non-digital data made into point
shapefiles
• Data correctly projected and combined
in one project
• Used method to select 6 scour
susceptible bridges
• Began initial HEC-GeoRAS and HEC
RAS modeling
Future Work
• Generate HEC GeoRAS themes
• Validate method by modeling 2 bridges
and calculating the total scour using
HEC RAS
• Determine accuracy of the method
• Automate GIS method
Questions?