Overview
Scour, as defined by the USDA Forest Service, is the erosion or removal of stream bed or bank material from bridge foundations due to flowing water. It has been identified as the leading cause of bridge damage or failure in the United States. There are several different forms of scour including aggradation, degradation, contraction scour, and abutment scour. Each of these poses a significant threat to the integrity of both new and old bridges.
One of the most notorious bridge collapses in US history occurred over Schoharie Creek, NY in 1987. Rainfall combined with melting snow increased water flow around bridge foundations carrying highway I-90. Sediment washed away under the footing of one pier frame, which triggered the collapse that took 10 lives. It was later determined that this disaster could have been avoided if proper inspection would have occurred.
The 1987 New York bridge collapse prompted the need for new guidelines. Since 1988, the US Federal Highway Administration has been working in conjunction with the USDA Forest Service to revise the standards for bridge design, evaluation, and inspection while also identifying potential problem areas due to scour. Despite this effort, there is still a relatively high level of uncertainty regarding the threat of scour. To compensate for this uncertainty, scour depth is typically over-estimated. This however, proves to be very costly for bridge construction.
New systems have emerged to monitor scour and predict potential threats to bridges. These systems may help reduce the cost of new bridge construction without sacrificing safety.
Selecting a Sensor
Two measurement parameters are commonly used monitor the effects of scour: distance-to-bottom and change in water level. A sonar altimeter can be used to provide the distance-to-bottom measurement, and a pressure or radar level sensor is commonly selected for measuring water level fluctuation. The strategic location of these instruments, combined with a remote data collection and alarming system, can provide a comprehensive bridge scour monitoring solution.
Precision sonar altimeters are typically fixed to a mounting arm off a bridge structure to provide accurate height off the riverbed. The Tritech Micron EchoSounder uses digital sonar technology to track the scour. The units output a digital NMEA 0183 output for interfacing with iSIC or SDL data loggers.
A pressure sensor measures the combined pressure exerted by the atmosphere and the head of water above it. The vented sensor automatically corrects for changes in barometric pressure to ensure true water level measurements. The sensor is fixed below the minimum expected water level, and a cable with a vent tube sends signals to the connected data logger. The pressure sensor is relatively cost-effective to install and maintain.
Alternatively, radar water level sensors may be used as a non-contact monitoring solution. Radar sensors mount above the water and measure the time of travel for a radar pulse to reflect and return to the sensor. Two antennas transmit pulses toward the water surface through a Teflon face plate. The transmitting antenna transmits short radar pulses, and the receiving antenna receives the pulses reflected from the water surface. The time delay from transmission to receipt is proportional to the distance between sensor and water surface.
Data Logging and Telemetry
For a complete bridge scour monitoring system, a NexSens iSIC or SDL data logger is used for data collection and transmission. The terrestrial iSIC data loggers can be pole mounted or fastened on the top of a bridge pier. The SDL submersible data loggers offer a better solution when there is an elevated risk or flooding or if the monitoring system is located in a vandal-prone area. NexSens solar panels provide a continuous charge to the battery to keep the system powered without the need to continuously swap batteries. When configured with telemetry, data can be transmitted in real-time from the bridge site to a project computer, eliminating the need for periodic site visits to upload data.
Two factors help determine the best telemetry method - site conditions and distance to the project computer. When the project computer can be located within a few miles (line-of-site) or few hundred feet (non-line-of-site), license free spread spectrum radio telemetry is the best choice. If the project site is remotely located relative to the project computer, cellular or satellite telemetry becomes the best option. Options are also available for landline phone or Ethernet telemetry if the site has access to a landline or Ethernet hub.
Alarm Notification
NexSens bridge scour monitoring systems offer two types of alarms; software and data logger. Software alarms are used to notify persons via SMS text messaging or email of parameters exceeding pre-defined limits. NexSens iChart Software sends the alarm when it receives data from the data logger. Data logger alarms are used to change the functionality of the data logger based on parameter inputs, such as changing sample and log intervals based on a particular logged reading.
Once the riverbed alarm limit is determined, the value is entered into NexSens iChart Software. When the river bed depth exceeds this pre-defined range from scour, a text message or email is automatically sent to a pre-defined list of contacts to take appropriate actions. Typically, this involves closing off the bridge for inspection.
Managing Data
Riverbed depth and water level data is logged and transmitted at a user-defined interval (minimum 1 minute) back to the project computer running iChart Software. iChart is a user-friendly Windows-based software package that serves as the centralized interface and database for all incoming data. A single Software license can manage a virtually unlimited number of bridge scour sites. The software allows users to generate customized reports with data from all systems in an iChart database. Reports can be converted to PDF, exported to Microsoft Excel, sent to interested parties via e-mail, uploaded to a web server, and more.
NexSens WQData is an optional web-based data management solution for bridge scour monitoring systems and other environmental data. The modular datacenter interface can be easily customized to include project-specific themes and information. WQData allows users to access to bridge scour data from any web browser.
System Maintenance
The bridge scour monitoring components are nearly maintenance-free. The sensors do not require calibration, and there are no parts that need replacing regularly. Periodic site visits are helpful based on local conditions for preventative maintenance. When visiting the site, the user should check the sensors for dirt or fouling on the sensor. If fouling exists, the sensor can be cleaned using a gentle cleaner and soft sponge. If using non-contact radar level sensors, the user should also check for obstructions in the measurement beam, including tree branches that could distort the measurement.
Rental Equipment and On-Site Services
Bridge scour monitoring systems are most commonly purchased and installed for long-term or permanent applications; however, there are commonly short-term scour measurement and alert applications that make it cost-prohibitive. NexSens Technology offers both standalone and real-time cellular telemetry bridge scour systems with weekly and monthly rental rates to meet project requirements.
To ensure that customers can begin using their bridge scour monitoring system as quickly and efficiently as possible, our application engineers are available for training and technical support, both by phone and in person. Our company offers both extensive field experience and a wide array of deployment hardware to facilitate seamless implementation of the project. Contact our application engineering team for your next bridge scour monitoring project.
