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).

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.

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.

Identifying Bridge Scour Susceptibility: A GIS-based Approach

Cassandra J. Rutherford, E.I.T.Master of Science Candidate in Civil Engineering

HEC-GeoRAS

HEC-GeoRAS

Fig. 2. Methodology

Analysis ValidationScour 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.

AbstractTwo 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

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.

Bridge Scour

Harris County Case Study

CVEN 689 Applications of GIS to Civil Engineering Instructor: Francisco Olivera, Department of Civil Engineering

April 29, 2003

Methodology

Conclusions

Data Processing

Fig. 1. Texas Bridges

Fig. 3. Harris County Fig. 4. Bridges and Gages Fig. 5. Erodible Soils

Fig. 6. Scour Susceptible Bridges

Fig. 7. Buffalo Bayou TIN Fig. 8. Buffalo Bayou Contours

Fig. 9. HEC-GeoRAS themes Fig. 10. HEC-RAS geometry