Protecting Against Bridge Scour: The Leading Cause of Bridge Failure
Bridges are an essential part of a nation’s economy. They are crucial for transporting goods and people to and from major economic hubs. Significant damage to bridges can drastically damage an area’s local economy and cause injury and loss of life. Bridge scour is the leading cause of bridge failure not just in the United States but in the rest of the world as well. Steps must be taken to protect against bridge scour so as to preserve the structural integrity of bridges and to maintain their role as facilitators of transportation, trade, and commerce.To learn more, checkout the infographic below created by the New Jersey Institute of Technology’s Master of Science in Civil Engineering Online degree program.
What is bridge scour?
Bridges connect two distinct landmasses, often those separated by a body of water. Abutments and piers are embedded in the sediments at the bottom of a river, strait or creek to support the weight of a bridge placed over water. Moving water can, through time, remove material surrounding piers and abutments, essentially weakening their ability to support weight.
Bridges in the United States
The U.S. National Bridge Inventory lists 484,500 highway bridges over water in the country. Bridge scouring has, since 1966, resulted in more than 1,500 bridge failures in the U.S., or an average of 30 bridges per year for the past 50 years. A bridge with an unstable abutment or pier foundation is considered scour critical. In a scour evaluation study, 20,904 bridges in the U.S. are currently deemed scour critical. Pennsylvania, Indiana, South Carolina, Oregon, and Tennessee are states with the most scour critical bridges, with Pennsylvania (5,500 bridges) having the most cases.
Types and causes of bridge scour
Bridge scour can be classified into three types. Local scour is the removal of sediment around bridge piers or abutments through the movement of water around obstructions, generating complex flow patterns, faster flow velocities and increased turbulence. Degradation scour occurs with the general lowering of stream channels through the removal of sediment at the bottom of a river, which is in turn due to the river’s natural flow. Contraction scour occurs at the bottom and sides of a creek, river, or stream. The velocity of water increases as the size of the channel it is passing through decreases, which means water moves faster through narrow reaches in stream channels or at bridge crossings, thereby causing more sediment removal.
Storms and Floods
Storms and floods can cause a sudden influx of water. This leads to a corresponding increase in both water volume and velocity, which in turn can lead to significant scouring events and bridge failure. In 1987, the Schoharie Creek Bridge in Fort Hunter, New York collapsed and killed 10 people in vehicles. The collapse was due to excessive scour under pier three, which was caused by changes in the creek bed, insufficient depth of the shallow footings, erodible soils, and insufficient stone riprap. In 1995, stream channel degradation due to a recent flood event caused a combination of contraction scour and local scour that collapsed both the northbound and southbound lanes of the Interstate 5 Bridge in Arroyo Pasajero, California. Seven people died in the collapse.
Following the Schoharie Creek Bridge Failure in 1987, the Federal Highway Administration (FHWA) established a national scour-evaluation program. The program mandated that every bridge over water must be inspected every 2 years. Additionally, bridges rated as scour critical must be inspected every year until it is repaired, replaced, or re-evaluated.
Plan of action
The 2005 National Bridge Inspection Standards (NBIS) made it a requirement to have a Plan of Action (POA) for scour critical bridges to help address the problem. The scour countermeasures outlined in the POA consisted of three general groups: hydraulic, structural, and monitoring. Accelerated replacement is also another strategy, especially for older bridges that are nearing the end of their useful service life.
State Dot Strategies
HEC 18 is the evaluation method used by the FHWA. However, due to the uncertainty and limitations of HEC 18, numerous U.S. states are opting for alternative evaluation methods. One example is the development of a new Scour Evaluation Model by the New Jersey Institute of Technology in conjunction with the FHWA and the New Jersey Department of Transportation (NJDOT). This decision tool evaluates geotechnical, hydrologic, and hydraulic data for a specific bridge. An overall priority rating is then given based on the specific recommended repair, monitoring, or administrative actions.
Monitoring and inspecting bridges after high flows and storms is seen as an alternative solution that can address the problem of having limited funds allocated for the repair and replacement of bridges. Fixed scour monitoring instrumentation is in use, or have been employed, by 32 of 50 U.S. states. There are 120 highway bridge sites in the U.S. that are using, or have used, fixed scour monitors (sonar scour monitoring system). Monitoring can also be a short-term solution until the installation of hydraulic and/or structural countermeasures.
There is an immediate need for better predictions. Floods are the number one natural disaster in the U.S. As rainfall increases due to climate change, the frequency of floods will likely increase in the future. Artificial and natural events can cause changes in water flow that can affect stream beds. This increases the potential for bridge scour and emphasizes the need for better risk assessment and countermeasures. The scarcity of monitoring systems must also be addressed. There are few monitoring systems installed even though monitoring is the most cost-effective approach, one that can prevent expensive bridge failures or at least mitigate the costs of such events.
Climate change can have a significant impact on bridge scouring. Between 1880 and 2013, the ten warmest years have been recorded after 1998. The average global temperature has risen by 1.4 degrees Fahrenheit since 1880. The global average sea level has risen nearly 7″ over the past 100 years and global precipitation is increasing by 2% per degree of global warming. Changes in the Earth’s climate could lead to drastic changes in the frequency and severity of floods and storms while rising sea levels can impact flow dynamics in bodies of water all over the world. These events can have a significant impact on the degree of scouring bridges are subject to.