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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Chapter 9

SCOUR MONITORING AND INSTRUMENTATION

9.1 INTRODUCTION

There are many scour critical bridges on spread footings or shallow piles in the United States and a large number of bridges with unknown foundation conditions (Lagasse et al. 1995). With limited funds available, these bridges cannot all be replaced or repaired. Therefore, they must be monitored and inspected following high flows. During a flood, scour is generally not visible and during the falling stage of a flood, scour holes generally fill in. Visual monitoring during a flood and inspection after a flood cannot fully determine that a bridge is safe. Instruments to measure or monitor maximum scour would resolve this uncertainty. As introduced in Chapter 2 (Section 2.3), monitoring as a countermeasure for a scour critical bridge involves two basic categories of instruments: portable instruments and fixed instruments.

Whether to use fixed or portable instruments in a scour monitoring program depends on many different factors. Unfortunately, there is not one type of instrument that works in every situation encountered in the field. Each instrument has advantages and limitations that influence when and where they should be used. The idea of a toolbox, with various instruments that can be used under specific conditions, best illustrates the strategy to use when trying to select instrumentation for a scour monitoring program. Specific factors to consider include the frequency of data collection desired, the physical conditions at the bridge and stream channel, and traffic safety issues.

Fixed instrumentation is used when frequent measurements or regular, ongoing monitoring (e.g., weekly, daily, or continuous) are required. Portable instruments would be preferred when only occasional measurements are required, such as after a major flood, or when many different bridges must be monitored on a relatively infrequent basis. The physical conditions at the bridge, such as height off the water and type of superstructure, can influence the decision to use fixed or portable equipment. For example, bridges that are very high off the water, or that have large deck overhang or projecting geometries, would complicate portable measurements from the bridge deck. Making portable measurements from a boat assumes that a boat ramp is located near the bridge, and/or there are no issues with limited clearance under the bridge that would prohibit safe passage of a boat. Bridges with large spread footings or pile caps, or those in very deep water can complicate the installation of some types of fixed instruments. Stream channel characteristics include sediment and debris loading, air entrainment, ice accumulation, or high velocity flow, all of which can adversely influence various measurement sensors used in fixed or portable instruments. Traffic safety issues include the need for traffic control or lane closures when either installing or servicing fixed instruments, or attempting to make a portable measurement from the bridge deck.

It is apparent that the selection of the instrument category (fixed or portable) and the specific instrument types to be used in a monitoring plan is not always straightforward. In some situations there is no clearly definable plan that will be successful, and the monitoring plan is developed knowing that the equipment may not always work as well as might be desired. Ultimately, the selection of any type of instrumentation must be based on a clear understanding of its advantages and limitations, and in consideration of the conditions that exist at the bridge and in the channel.

To improve the state-of-practice when adopting fixed instrumentation as a countermeasure, the Transportation Research Board (TRB) under the National Cooperative Highway Research Program (NCHRP) completed NCHRP Project 21-3 "Instrumentation for Measuring Scour at Bridge Piers and Abutments" in 1997 (Lagasse et al. 1997, Schall et al. 1997a, 1997b). NCHRP Project 21-7, "Portable Scour Monitoring Equipment," was completed in 2004 and developed an articulated arm truck as a platform for deploying a variety of portable scour monitoring instruments (Schall and Price 2004). In addition, to facilitate the technology transfer of instrumentation-related research to the highway industry, particularly those in inspection and maintenance operations, the Federal Highway Administration (FHWA) developed a Demonstration Project (DP97) on scour monitoring and instrumentation. The purpose of Demonstration Project 97 was to promote the use of new and innovative equipment, both fixed and portable, to measure scour, monitor changes in scour over time, detect the extent of past scour, and serve as countermeasures (FHWA 1998, Ginsberg and Schall 1998). This chapter provides information on the use of portable and fixed instrumentation for scour monitoring. The portable instrument discussion includes lessons learned from NCHRP 21-7 and concepts developed during DP97. The fixed instrument discussion includes results from NCHRP Project 21-3 and highlights fixed instrument installations completed in the last ten years. Information on implementation and experience of several State DOTs with scour monitoring instrumentation is also summarized.

9.2 PORTABLE INSTRUMENTATION

9.2.1 Components of a Portable Instrument System

Portable instrumentation is typically used when a fixed instrument has not been installed at a bridge; however, portable instruments are also useful when it is necessary to supplement fixed instrument data at other locations along the bridge. Physical probing has been used for many years as the primary method for portable scour monitoring by many DOTs. More recently, sonar has seen increased use, in part due to the technology transfer provided through FHWA's Demonstration Project 97 (FHWA 1998). The use of these methods during low-flow conditions has been very successful, for example during the 2-year inspection cycle; however, their success during flood conditions, when the worst scour often occurs, has been more limited. When appropriate, portable instrumentation is an important part of a scour monitoring program.

A portable scour measuring system typically includes four components (Mueller and Landers 1999):

  1. Instrument for making the measurement
  2. System for deploying the instrument(s)
  3. Method to identify and record the horizontal position of the measurement
  4. Data-storage device
9.2.2 Instrument for Making the Measurement

A wide variety of instruments have been used for making portable scour measurements. In general, the methods for making a portable scour measurement can be classified as:

  1. Physical probing
  2. Sonar
  3. Geophysical

Physical Probes. Physical probes refer to any type of device that extends the reach of the inspector, the most common being sounding poles and sounding weights. Sounding poles are long poles used to probe the bottom (Figure 9.1). Sounding weights, sometimes referred to as lead lines, are typically a torpedo shaped weight suspended by a measurement cable (Figure 9.2). This category of device can be used from the bridge or from a boat. An engineer diver with a probe bar is another example of physical probing. Physical probes only collect discrete data (not a continuous profile), and can be limited by large depth and velocity (e.g., during flood flow condition) or debris and/or ice accumulation. Advantages of physical probing include not being affected by air entrainment or high sediment loads, and it can be effective in fast, shallow water.

Sonar. Sonar instruments (also called echo sounders, fathometers or acoustic depth sounders) measure the elapsed time that an acoustic pulse takes to travel from a generating transducer to the channel bottom and back (FHWA 1998). Sonar is an acronym for S O und NA vigation and Ranging that was developed largely during World War II. However, early sonar systems were used during World War I to find both submarines and icebergs and called ASDICs (named for the Antisubmarine Detection Investigation Committee). As technology has improved in recent years better methods of transmitting and receiving sonar and processing the signal have developed, including the use of digital signal processing (DSP). The issues of transducer frequency (typically around 200 kHz) and beam width are important considerations in the use of sonar for scour monitoring work. Additionally, sonar may be adversely impacted by high sediment loads or air entrainment.

Applications of single beam sonar range from fish finders to precision survey-grade hydrographic survey fathometers. Low-cost fish-finder type sonar instruments have been widely used for bridge scour investigations (Figure 9.3) with a tethered float to deploy the transducer. Float platforms have included kneeboards (Figure 9.4) and pontoon-style floats (Figure 9.5).

Other types of sonar, such as side scan, multi-beam and scanning sonar, are specialized applications of basic sonar theory. These devices are commonly used for oceanographic and hydrographic survey work, but have not been widely utilized for portable scour monitoring. Side scan sonar transmits a specially shaped acoustic beam to either side of the support craft. These applications often deploy the transducer in a towfish, normally positioned behind and below the surface vessel.

While side scan sonar is one of the most accurate systems for imaging large areas of channel bottom, most side scan systems do not provide depth information. Multi-beam systems provide a fan-shaped coverage similar to side scan, but output depths rather than images. Additionally, multi-beam systems are typically attached to the surface vessel, rather than being towed. Scanning sonar works by rotation of the transducer assembly or sonar "head," emitting a beam while the head moves in an arc. Since the scanning is accomplished by moving the transducer, rather than towing, it can be used from a fixed, stationary position. Scanning sonar is often used as a forward looking sonar for navigation, collision avoidance and target delineation.

The Sonar Scour Vision system was developed by American Inland Divers, Inc (AIDI) using a rotating and sweeping 675 Khz high resolution sonar (Barksdale 1994). The transducer is mounted in a relatively large hydrodynamic submersible, or fish, that creates a downward force adequate to submerge the transducer in velocities exceeding 20 ft/s (6 m/s) (Figure 9.6). Given the forces created, the fish must be suspended from a crane or boom truck on the bridge. From a single point of survey, the system can survey up to 328 ft (100 m) radially. Data collected along the face of the bridge can be merged into a real-time 3-dimensional image with a range of 295 ft (90 m) both upstream and downstream of the bridge.

Photograph of workers standing on a river bridge using a long sounding pole to probe the river bottom
Figure 9.1 Sounding pole measurement.

Photograph showing a streamlined finned ellipsoid heavy lead-line sounding weight.
Figure 9.2. Lead-line sounding weight.

Photograph of worker standing on a river bridge holding the support rod of a portable sonar. The instrumentation is carried hands free at waist level by a light harness for viewing real time data.
Figure 9.3. Portable sonar in use.

Photograph of a worker on a bridge holding a tethered kneeboard float used to support a suspended sonar device.
Figure 9.4. Kneeboard float.

Photograph of a PVC pontoon float with attached sonar transducer.
Figure 9.5. Pontoon float.

Photograph of American Inland Divers hydrodynamic submersible suspended at the water surface over the edge of a river bridge. The flotation buoy above the submersible and the cable link are clearly seen.  Photograph, Barksdale 1994
Figure 9.6. AIDI system (Barksdale 1994).

Geophysical. Surface geophysical instruments are based on wave propagation and reflection measurements. A signal transmitted into the water is reflected back by interfaces between materials with different physical properties. A primary difference between sonar and geophysical techniques is that geophysical methods provide sub-bottom, while sonar can only "see" the water-soil interface and is not able to penetrate the sediment layer. The main difference between different geophysical techniques are the types of signals transmitted and the physical property changes that cause reflections. A seismic instrument uses acoustic signals, similar to sonar, but at a lower frequency (typically 2-16 kHz). Like sonar, seismic signals can be scattered by air bubbles and high sediment concentrations. A ground penetrating radar (GPR) instrument uses electromagnetic signals (typically 60-300 mHz), and reflections are caused by interfaces between materials with different electrical properties. In general, GPR will penetrate resistive materials and not conductive materials. Therefore, it does not work well in dense, moist clays, or saltwater conditions.

The best application of geophysical technology in scour monitoring may be as a forensic evaluation tool, used after the flood during lower flow conditions to locate scour holes and areas of infilling. In general, the cost and complexity of the equipment and interpretation of the data are limiting factors for widespread use and application as a portable scour monitoring device. These issues have moderated as newer, lower cost GPR devices with computerized data processing capabilities have been developed. However, GPR may still be limited by cost and complexity, and often the need for bore hole data and accurate bridge plan information to properly calibrate and interpret the results.

9.2.3 System for Deploying the Instrument

The system for deploying the scour instrument is a critical component in a successful portable scour measurement system. In practical application, particularly under flood flow conditions, the inability to properly position the instrument is often the limiting factor in making a good measurement. The use of different measurement technologies from different deployment platforms can produce a wide variety of alternative measurement approaches.

Deployment methods for portable instruments can be divided into two primary categories:

  1. From the bridge deck
  2. From the water surface

Bridge Deck Deployment. Bridge deck deployment can be defined by two categories, non-floating and floating. Non-floating systems generally involved standard stream gaging equipment and procedures, including the use of various equipment cranes and sounding weights for positioning a sensor in the water. This category could also include devices that use a probe or arm with the scour measurement device attached to the end. Probes or arms include things as simple as an extendable pole or rod (such as a painter's pole), to a remotely controlled articulated arm. Hand held probes or arms are not generally useable at flood flow conditions.

A prototype articulated arm to position a sonar transducer was developed under an FHWA research project (Bath 1999). An onboard computer calculated the position of the transducer based on the angle of the boom and the distance between the boom pivot and transducer. Additionally, the system could calculate the position of the boom pivot relative to a known position on the bridge deck. The system was mounted on a trailer for transport and could be used on bridge decks from 16 - 50 ft (5 - 15 m) above the water surface (Figure 9.7). Field testing during the 1994 floods in Georgia indicated that a truck mounted system would provide better maneuverability, and that a submersible head or the ability to raise the boom pivot was necessary to allow data collection at bridges with low clearance (less than 16 ft (5 m)).

Photograph of prototype trailer mounted articulated arm for positioning a sonar transducer from a bridge.
Figure 9.7. FHWA articulated arm in use.

NCHRP Project 21-7 (Schall and Price 2004) resulted in a truck mounted articulated arm to facilitate portable scour measurements during flood conditions (Figure 9.8). The truck was designed to operate in high velocity flow while providing accurate positioning information and efficient data collection procedures. These measurements can be completed from a variety of bridge geometries including limited clearance, overhanging geometry, and high bridges. Scour measurement can be by a streamlined sonar probe, sounding weights, kneeboards, or physical probing. A dual winch system was developed to facilitate cable suspended operations. Crane location is tracked by a variety of sensors installed on the articulated arm, and by a survey wheel on the back of the truck. Data loggers manage data, and a laptop is used for data reduction. Sonar data and positioning data collected at the end of the crane are transmitted by a wireless link that eliminates any wires from the water surface to the truck (Figure 9.9).

Photograph of a truck mounted articulated mechanical arm for portable scour measurements. From Schall and Price 2004
Figure 9.8. NCHRP 21-3 articulated arm truck (Schall and Price 2004).

Photograph of an articulated arm truck on a modern concrete river bridge with the arm in use. The arm is reaching from the back of the truck to the water surface next to a riprapped concrete pier.
Figure 9.9. Sonar instrument deployed from articulated arm truck.

Float based systems permit measurement beneath the bridge and along side the bridge piers. Tethered floats are a low-cost approach that have been used with some success during flood flow conditions. A variety of float designs have been proposed and used to varying degrees for scour measurements, typically to deploy a sonar transducer. Common designs include foam boards, PVC pontoon configurations, spherical floats, water skis and kneeboards (FHWA 1998). The size of the float is important to stability in fast moving, turbulent water.

Floating or non-floating systems can be also be deployed from a bridge inspection truck, an approach that is particularly useful when the bridge is high off the water. For example, bridges that are greater than 50 ft (15 m) off the water are typically not accessible from the bridge deck without using this approach.

Water Surface Deployment. Water surface deployment typically involves a manned boat, however, safety issues under flood conditions have suggested the use of unmanned vessels. The use of manned boats generally requires adequate clearance under the bridge and nearby launch facilities. This can be a problem at flood conditions when the river stage may approach or submerge the bridge low chord, and/or boat ramps may be underwater. Smaller boats may be easier to launch, but safety at high flow conditions may dictate use of a larger boat, further complicating these problems.

When clearance is not an issue, the current and turbulence in the bridge opening may be avoided using one of the tethered floating or nonfloating methods described above from a boat positioned upstream of the bridge. For example, a pontoon or kneeboard float with a sonar transducer could be maneuvered into position from a boat holding position upstream of the bridge, thereby avoiding the current and turbulence problems at the bridge itself.

The safety, launching and clearance issues have suggested that an unmanned or remote control boat might be a viable alternative. A prototype unmanned boat using a small flat bottom jon boat and an 8 hp outboard motor with remote controls (Figure 9.10) was successfully tested during six flood events (Mueller and Landers 1999).

Photograph of a small aluminum remote controlled jon boat with outboard motor and mounted sensors
Figure 9.10. Unmanned, remote control boat.

9.2.4 Positioning Information

In order to evaluate the potential risk associated with a measured scour depth it is necessary to know the location of the measurement, particularly relative to the bridge foundation. Location measurements can range from approximate methods, such as "3 ft (1 m) upstream of pier 3," to precise locations based on standard land and hydrographic surveying technology.

The most significant advancement for portable scour measurement positioning may be in the use of Global Positioning Systems (GPS). GPS is a positioning system based on a constellation of satellites orbiting the earth. An advantage of GPS over traditional land-based surveying techniques is that line-of-sight between control points is not necessary. A GPS survey can be completed between control points without having to traverse or even see the other point. GPS also works at night and during inclement weather, which could be a real advantage for scour monitoring during flood conditions. The most significant disadvantage of GPS is the inability to get a measurement in locations where overhead obstructions exist, such as tree canopy or bridge decks. However, GPS measurements up to the bridge face, without venturing under the bridge, have been successful.

9.2.5 Data Storage Devices

Portable scour monitoring data are typically manually recorded in a field book, however, there has been a growing interest in more automated data collection using various data storage devices. Available data storage devices include hydrometeorological data loggers, laptop computers and more recently palm computers and organizers. Data loggers provide a compact storage device, however, they are generally not very user friendly with each company typically having a unique programming language and approach. In field applications, laptop computers are bulky and need to be ruggedized to survive the rain, dirt and dust of a field environment. Palm computers and organizers may have an application as their capability and user-friendliness continue to improve. The advantage of laptop computers and palm computers is the ability to integrate data reduction software, such as plotting or topographic mapping programs to display the results, often in real time mode while the data collection occurs.

9.3 FIXED INSTRUMENTATION

9.3.1 NCHRP Project 21-3

The basic objective of NCHRP Project 21-3 was to develop, test, and evaluate fixed instrumentation that would be both technically and economically feasible for use in measuring or monitoring maximum scour depth at bridge piers and abutments (Lagasse et al. 1997). The scour measuring or monitoring device(s) were required to meet the following mandatory criteria.

Mandatory Criteria

  • Capability for installation on or near a bridge pier or abutment
  • Ability to measure maximum scour depth within an accuracy of ± 1 ft (± 0.3 m)
  • Ability to obtain scour depth readings from above the water or from a remote site
  • Operable during storm and flood conditions

Where possible, the devices should meet the following desirable criteria:

Desirable Criteria

  • Capability to be installed on most existing bridges or during construction of new bridges
  • Capability to operate in a range of flow conditions
  • Capability to withstand ice and debris
  • Relatively low cost
  • Vandal resistant
  • Operable and maintainable by highway maintenance personnel

Since the mandatory criteria required that the instruments be capable of installation on or near a bridge pier or abutment, the research was limited to fixed instruments only. While the research was conducted in phases, a final project report was prepared to integrate and summarize the findings, interpretation, conclusions and recommendations for the total research effort (Lagasse et al. 1997). A separate Installation, Operation, and Fabrication Manual was developed for both the magnetic sliding collar device and low-cost sonic instrument system that resulted from this research (Schall et al. 1997a, 1997b).

9.3.2 Scour Measurement

Although a vast literature exists relating to bridge scour, relatively few reports deal specifically with instrumentation. The final report for NCHRP Project 21-3 includes an extensive bibliography on equipment for scour measurement and monitoring (Lagasse et al. 1997). A detailed survey of the evolution of scour measuring instrumentation was presented at the Transportation Research Board Third Bridge Engineering Conference in 1991 (Lagasse et al. 1991). This section summarizes the development of scour measuring equipment and techniques that had particular relevance to the instrumentation developed under the NCHRP project.

Major advances in instrumentation such as sonar, sonic sounders, electronic positioning equipment, and radar occurred during World War II. By the mid 1950s, many devices became commercially available and were introduced into scientific studies of rivers. A dual channel sonic stream monitor was used in the 1960s to study alluvial channel bed configurations and the scour and fill associated with migrating sand waves. Commercial sonic sounders became available about the same time and soon were used extensively in hydrographic surveys.

In the 1970s, many scour studies were undertaken in New Zealand. One of the instruments used in the field to measure maximum scour depth at bridge piers was called the "Scubamouse." The device consists of a vertical pipe buried or driven into the streambed in front of the bridge pier around which is placed a horseshoe-shaped collar that initially rests on the streambed. The collar slides down the pipe and sinks to the bottom of the scour hole as scour progresses during a flood. The position of the collar is determined by sending a detector down the inside of the pipe after the flood. Earlier models involved a metal detector inside a PVC pipe, but the pipe was sometimes damaged by debris, so the current models use a steel pipe, a radioactive collar, and a radiation detector inside the pipe. This device has been installed on many bridges in New Zealand (Melville et al. 1989).

In the United Kingdom, Hydraulic Research Limited of Wallingford has developed and deployed a buried rod instrument system to monitor bed scour during flood events (Waters 1994). This 'Tell Tail' scour monitoring system is based on omni-directional motion sensors, buried in the river or sea bed adjacent to the structure. The sensors are mounted on flexible 'tails' and are connected to the water surface via protected cables. Under normal flow conditions, the detectors remain buried and do not move. When a scour hole develops, the sensors are exposed and transmit alarm signals to the surface.

In the early 1990s, there were no accepted methods or off-the-shelf equipment for collecting scour data in the United States. In part, this was because there had been no coordinated long-term effort to study scour processes. Also, most scour studies were site-specific and the equipment and techniques that were used were tailored to the geometry of the site and its hydrology and hydraulic conditions. The Brisco MonitorTM, a sounding rod device, was available, but had not been tested extensively in the field.

Scour studies in the United States were carried out with a great variety of portable equipment and techniques, and, through the U.S. Geological Survey (USGS) National Scour Study, conducted in cooperation with the Federal Highway Administration (FHWA), efforts were made to standardize the collection of scour data (Landers and Trent 1991). Techniques for determining the extent of local scour include the use of divers and visual inspection, direct measures of scour with mechanical and electronic devices, and indirect observations using ground-penetrating radar and other geophysical techniques (Gorin and Haeni 1989).

In the early 1990s, the USGS investigated the use of fixed instruments for scour measurement at a new bridge on U.S. Highway 101 across Alsea Bay near Walport, Oregon. Depth soundings were made using commercially available sonic sounders. The transducers for sounding were mounted on brackets attached to the piers and pointed out slightly to avoid interference from the side of the pier. The system worked well, but the installation was not subject to debris, ice, or air entrainment from highly turbulent flows (Crumrine et al. 1996).

The initial literature search on scour instrumentation in 1990 revealed, and a resurvey of technology in 1994 confirmed, that fixed scour-measuring and -monitoring instruments can be grouped into four broad categories:

  1. Sounding rods - manual or mechanical device (rod) to probe streambed
  2. Buried or driven rods - device with sensors on a vertical support, placed or driven into streambed
  3. Fathometers - commercially available sonic depth finder
  4. Other Buried Devices - active or inert buried sensor (e.g., buried transmitter)

As a result of the literature review a laboratory testing program was designed to test at least one device from each category and to select devices for field testing that would have the greatest potential for meeting mandatory and desirable criteria.

9.3.3 Summary of NCHRP Project 21-3 Results

No single methodology or instrument can be utilized to solve the scour monitoring problems for all situations encountered in the field. Considering the wide range of operating conditions necessary, environmental hazards such as debris and ice, and the variety of stream types and bridge geometries encountered in the field, it is obvious that several instrument systems using different approaches to detecting scour will be required.

Under NCHRP Project 21-3, a variety of scour measuring and monitoring methods were tested in the laboratory and in the field, including sounding rods, driven rod devices, fathometers, and buried devices. Two instrument systems, a low-cost bridge deck (above water) serviceable fathometer and a magnetic sliding collar device using a driven rod approach were installed under a wide range of bridge substructure geometry, flow, and geomorphic conditions. Both instrument systems met all of the mandatory criteria and most of the desirable criteria established for the project and were considered fully field deployable in 1997.

The Installation, Operation, Fabrication Manuals for the low-cost sonic system and magnetic sliding collar devices (Schall et al. 1997a, 1997b) provide complete instrument documentation, including specifications and assembly drawings. That information, together with the findings, appraisal, and applications information of the final report (Lagasse et al. 1997), provide a potential user of a scour monitoring device complete guidance on selection, installation, operation, maintenance, and if desired, fabrication of two effective systems, one of which could meet the need for a fixed scour instrument at most sites in the field.

Of the devices tested extensively in the field, the low-cost sonic system and the manual-readout sliding collar device are both vulnerable to ice and debris; however, both proved to be surprisingly resistant to damage from debris or ice impact at field test sites. The sonic system can be rendered inoperative by the accumulation of debris, and presumably ice, between the transducer face and streambed. The manual-readout sliding collar requires an extension conduit, generally up the front face of a pier, which can be susceptible to debris or ice impact damage unless the extension can be firmly anchored to a substructure element. From this perspective, the automated sliding collar device has the distinct advantage of having a configuration which places most of the device below the streambed, and therefore, less vulnerable to ice or debris. The connecting cable from the device to a data logger on the bridge deck can be routed through a buried conduit and up the downstream face of a bridge pier or abutment where it is much less vulnerable to damage. An overview of these and other operational instrument systems is provided in the next section.

9.3.4 Operational Fixed Instrument Systems

A scour monitoring system at a bridge may be comprised of one or more types of fixed instruments. The various devices are either mounted on the bridge or installed in the streambed in the vicinity of the bridge. Remote units with data loggers may be installed so that the scour measuring device transmits data to the unit. The data from any of these fixed instruments may be downloaded manually at the site, or it can be telemetered to another location. The early scour monitoring devices measured streambed elevations using simple on-site manually read units. The more recent installations use remote technology for data retrieval. Each bridge may have one or more remote sensor units that transmit data to a master unit on or near the bridge (Figure 9.11). The scour monitoring data is then transmitted from the master unit to a central office for data analysis. Remote technology transmits data by modem using cellular or landline telephones, or by satellite. Recent installations include systems that post the data on the Internet so that authorized persons may access the data from any location using a computer with Internet access.

Photograph of wall mounted metal enclosures and the interior data logger instrumentation
Figure 9.11. Master station with data logger for use with any of the fixed instruments.

Sonars . The sonar scour monitors are mounted onto the pier or abutment face (Figures 9.12 and 13) to take streambed measurements. Currently new sonar monitors range from fish finders to smart sonar transducers, both of which are commercially available. The sonar transducer is connected to the sonar instrument or directly to a data logger. The sonar instrument measures distance based on the travel time of a sound wave through water to the stream bed and back to the transducer. The data logger controls the sonar system operation and data collection functions and can be programmed to take measurements at prescribed intervals. Sonar sensors normally take a rapid series of measurements and use an averaging scheme to determine the distance from the sonar transducer to the streambed. These instruments can track both the scour and refill (deposition) processes. The early sonar monitors used off-the-shelf "fish finders."

Sketch of bridge mounted above-water serviceable low-cost fathometer system. The solar instrument enclosure is shown mounted on the upstream bridge rail and the transducer mounted to the upstream face of the pier just above water level Sketch from Schall et al. 1997b
Figure 9.12. Above-water serviceable low-cost fathometer system (Schall et al. 1997b).

Photograph of bridge mounted above-water serviceable low-cost fathometer system. The solar instrument enclosure is shown mounted on the upstream bridge beam, the remote station is mounted high on the side of the pier and the transducer mounted to the upstream face of the pier just above water level
Figure 9.13. Sonar scour monitor installation including remote station and solar panel.

Magnetic Sliding Collars . Magnetic sliding collars (Figures 9.14, 9.15, and 9.16) are rods or masts that are attached to the face of a pier or abutment and driven or augered into the streambed. A collar with magnetic elements is placed on the streambed around the rod. If the streambed erodes, the collar moves or slides down the rod into the scour hole. The depth of the collar provides information on the scour that has occurred at that particular location.

The early version of the sliding magnetic collar used a battery operated manual probe that was inserted down from the top and a buzzer sounded when the probe tip sensed the level of the magnetics on the collar (Figure 9.14). More recent instruments have a series of magnetically activated switches inserted in the rod at known distances. Magnets on the steel collar come into proximity with the switches as the collar slides into the scour hole. The switches close sequentially as the collar slides by, and their position is sensed by the electronics (Figures 9.15 and 9.16). A data logger reads the level of the collar via the auto probe and tracks scour activity. Sonar scour monitors may be used to provide the infill scour process at a bridge, whereas magnetic sliding collars can only be used to monitor the maximum scour depth.

Float-out Devices. Buried devices may be active or inert buried sensors or transmitters. Float-out devices (Figure 9.17) are buried transmitters. This device consists of a radio transmitter buried in the channel bed at pre-determined depth(s). If the scour reaches that particular depth, the float-out device floats to the stream surface and an onboard transmitter is activated. It transmits the float-out device's digital identification number with a radio signal. The signal is detected by a receiver in an instrument shelter on or near the bridge. The receiver listens continuously for signals emitted by an activated float-out device. A decoded interface decodes the activated float-out device's unique digital identification number that will determine where the scour has occurred. A data logger controls and logs all activity of the scour monitor. These devices are particularly easy to install in dry riverbeds, during the installation of an armoring countermeasure such as riprap, and during the construction of a new bridge. The float-out sensor is a small low-powered digital electronics position sensor and transmitter. The electronics draws zero current from a lithium battery which provides a 9-year life expectancy when in the inactive state buried in the streambed.

Sketch of bridge mounted manual read out magnetic sliding collar device for detecting scour. The hollow conduit for lowering the manual probe in is mounted to the pier and runs from the bridge rail to below the streambed. The conduit has a removable and lockable cap at the bridge rail. The bottom section of conduit is in front of the pier footing at the location of maximum scour and extends below the stream bed and the bottom of the footing. The magnetic sliding collar is around the conduit and is initially positioned on the stream bed. Sketch from Schall et al. 1997b
Figure 9.14. Manual read out magnetic sliding collar device (Schall et al. 1997a).

Sketch of bridge mounted automated read out magnetic sliding collar device for detecting pier scour. The electrical cable to the magnetic sliding collar is enclosed in flexible 1 inch UV resistant rubber hose conduit and mounted to the pier. It runs from the instrument enclosure on the bridge rail to the top of the instrumentation located just above the streambed. The rod for the magnetic sliding collar is positioned just upstream of the pier footing at location of maximum scour and extends a distance below the stream bed. The magnetic sliding collar is shown on top of the stream bed.
Figure 9.15. Automated read out magnetic sliding collar system (Schall et al. 1997a).

Photograph of magnetic sliding collar, vertical guide rod and connecting conduit on streambed next to a bridge pier- part of an automated read our system for detecting scour
Figure 9.16. Detail of magnetic sliding collar on the streambed.

Photograph of two color coded PVC float out devices
Figure 9.17. Float-out devices prior to installation. Color coded and numbered for identification purposes.

Tilt Sensors. Tilt sensors (Figure 9.18) measure movements and rotations of the bridge itself. An X, Y tilt sensor or clinometer monitors the bridge position. Should the bridge be subject to scour causing one of the support piers or abutments to settle, one of the tilt sensors would detect the change. A pair of clinometers is installed on the bridge piers or abutments (Figure 9.19). One tilt meter senses rotation parallel to the direction of traffic (the longitudinal direction of the bridge), while the other senses rotation perpendicular to traffic (usually parallel with the stream flow).

Photograph of Tilt meter equipment fixed to the side of a California bridge. The solar panel and the data lodger are mounted on a steel support outside of the bridge parapet.
Figure 9.18. Tilt meter on California bridge.

Photograph of an open tilt meter enclosure box fixed to the side of a bridge. Inside two clinometers can be seen mounted at 90 degrees to each other - transverse and longitudinal to the bridge
Figure 9.19. Detail of tilt meter.

Time Domain Reflectometers. In Time Domain Reflectometry (TDR) an electromagnetic pulse is sent down two parallel pipes that are buried vertically in the streambed (Figure 9.20) (Zabilansky 2002). When the pulse encounters a change in the boundary conditions (i.e., the soil-water interface), a portion of the pulse's energy is reflected back to the source from the boundary. The remainder of the pulse's energy propagates through the boundary until another boundary condition (or the end of the probe) causes part or all of the energy to be reflected back to the source. By monitoring the round-trip travel time of a pulse in real time, the distance to the respective boundaries can be calculated and this provides information on any changes in streambed elevation. Monitoring travel time in real time allows the processes affecting sediment transport to be correlated with the change in bed elevation. Using this procedure, the effects of hydraulic and ice conditions on the erosion of the riverbed can be documented.

Sketch of a time domain reflectometry probe installed vertically into the riverbed. The top is above the riverbed and the anchor length is shown below the sensing length. The cable to shore link for the instrumentation is shown exiting the probe at the bottom of the buried sensing length. A representative signal wave is indicated vertically next to the probe and the location of the difference in amplitude indicates the riverbed elevation. Sketch courtesy of USACE Cold Regions Research and Engineering
Figure 9.20. Time domain reflectometry probe.

(Courtesy of USACE Cold Regions Research and Engineering Laboratory)

Sounding Rods . Sounding-rod or falling-rod instruments are manual or mechanical (automated) gravity based physical probes (Figure 9.21). As the streambed scours, the rod, with its foot resting on the streambed, drops following the streambed and causing the system counter to record the change. The foot must be of sufficient size to prevent penetration into the streambed caused by the weight of the rod and the vibration of the rod from flowing water. These devices are susceptible to streambed surface penetration in sand bed channels and this influences their accuracy. Consequently, they are best suited to monitoring coarse bed streams or riprap stability (as shown in Figure 9.21)

Photograph of Brisco Monitor sounding rods attached to the upstream sloped nose of a river bridge pier. The length of the rods are protected and covered. The tips of the rods can be seen resting on riprap just in front of the nose of the pier. Photograph credit, Butch 1996
Figure 9.21. Brisco Monitor sounding rods installed at a bridge pier in New York to monitor movement of riprap (Butch 1996).

Summary. If recording a series of streambed elevations over time is of interest, sonars, magnetic sliding collars and sounding rod monitors may be used (only the sonar will record scour and fill). If a bridge owner is interested only in when a certain streambed elevation is reached, float-outs may be employed. For specific information on a pier or abutment, tilt sensors record relative rotation and movement of the structure. Additional fixed instruments may be added to the scour monitoring system to gather information on water elevations, water velocities and/or temperature readings.

Data from any of these fixed instruments may be downloaded manually at the site, or may be telemetered to another location. A scour monitoring system at a bridge may use one of these devices, or include a combination of two or more of these fixed instruments all transmitting data to a central control center. These types of scour monitors are being used in a wide variety of climates and temperatures, and in a wide range of bridge and channel types throughout the United States.

9.3.5 NCHRP Project 20-5

Most of the information in Section 9.3.4 is derived from NCHRP Project 20-5 (Topic 36-02) which was a synthesis study entitled Practices for Monitoring Scour Critical Bridges (Hunt 2008). The study assessed the state of knowledge and practice for fixed scour monitoring of scour critical bridges. It included a review of the literature and research, and a survey of transportation agencies and other bridge owners to obtain their experience with fixed scour monitoring systems.

The study found that 30 of the 50 states use, or have employed fixed scour monitoring instrumentation for their highway bridges. A total of 120 bridge sites were identified that have been instrumented with fixed monitors. The five types of fixed instruments being used in 2007 included sonars, float-outs, tilt meters, magnetic sliding collars and time domain reflectometers.

The site conditions and the types of bridges that were monitored with fixed scour instrumentation varied in many aspects. There were small to long span bridges with lengths ranging from 41 ft (12.5 m) to 12,865 ft (3,921 m). The Average Daily Traffic (ADT) ranged from 400 to 175,000 vehicles per day, and the bridges were constructed between 1920 and 1986. The site conditions included both riverine and tidal waterways, intermittent to perennial flows, and water depths ranging from less than 10 to 75 ft (3 m to 30 m). The soil conditions ranged from clay to gravel, and some had riprap protection.

The scour monitors were installed between 1992 and 2007. The earlier installations included sounding rods, magnetic sliding collars and sonars. More recent installations also include float-outs, tilt sensors and TDRs. The sonar scour monitoring system is the most commonly used device, installed at 70 of the 120 bridge sites. This was followed by the magnetic sliding collar at 21 sites. The bridge owners reported that 90% of the structures monitored with fixed instruments were piers. The remaining devices were on abutments, or in the vicinity of the bridge on bulkheads or downstream countermeasure protection.

The survey respondents indicated that high velocity flows, debris, ice forces, sediment loading and/or low water temperatures were extreme conditions that were present at the monitored bridge sites. The debris and ice forces caused the majority of damage and interference to the scour monitoring systems. They noted that the extent and frequency of the damage was often not anticipated by the bridge owner and this resulted in much higher maintenance and repair costs than anticipated.

The bridge owners provided information on their future needs for improved scour monitoring technology which include:

  • More robust devices - increased reliability and longevity
  • Decreased costs
  • Less maintenance
  • Devices more suitable for larger bridges
  • Devices that measure additional hydraulic variables and/or structural health
9.3.6 Application Guidelines

Bridge Pier and Abutment Geometry. It is clear that no single device is applicable to all bridge pier and abutment geometries. However, most bridge geometries can be accommodated with one of the scour measuring devices described in Section 9.3.4.

Most instruments are adaptable in some degree to vertical piers and abutments. Sloping piers and spill-through abutments present difficulties for most instrument configurations; however, driven rod instruments, such as the automated sliding collar, that are not fastened to the substructure can be used on sloping piers and abutments. Adapting scour instrumentation to a large spread footing or pile cap configuration also presents challenges.

Flow and Geomorphic Conditions. Each class of scour measuring instrument will not be applicable to all flow and geomorphic conditions. While some limitations stem from the capabilities of the device itself, some pertain to whether the device is installable given the geomorphic and flow conditions. For example, sounding rods have not performed well in sand-bed streams, although the addition of a large base plate to the sounding rod could help correct the problem, and sonar devices may be best suited for tidal waterways where problems with debris are not as common as they are for riverine bridges.

All devices using a driven rod configuration (including TDR) will have limitations imposed by bed and substrate characteristics. Pre-drilling, jetting, or augering may permit installation under a wide range of conditions, but these techniques may be expensive and could be difficult over water. The connecting conduit required by the manual-readout sliding collar device is vulnerable to ice and debris impact, but the instrument proved surprisingly durable at field test sites with significant debris.

Low-cost fathometers are applicable to a wide range of streambed characteristics and flow and geomorphic conditions, but ice and debris in the stream can quickly render a fathometer inoperable. Strategies such as placing the transducer close to the streambed may reduce, but won't eliminate, the vulnerability of this instrument to ice and debris.

Float out devices are simple to fabricate and relatively inexpensive. Installation in the dry on an ephemeral stream or where a coffer dam can be installed can be accomplished with drilling equipment available to most DOTs. Installation under water, however, would be difficult.

Tilt Meters. Tilt meters are placed on the bridge superstructure and above-water substructure components making installation easier and less expensive than other fixed instruments. Tilt meters measure the movement of the bridge itself, therefore the bridge must be redundant enough to withstand some movement without failure (Avila et al. 1999). This will allow maintenance forces sufficient time to remotely observe the movement and send crews to inspect the bridge and close it, if necessary. However, it is difficult to set the magnitude of the angle at which the bridge is in danger. Bridges are not rigid structures, and movement can be induced by traffic, temperature, wind, hydraulic and earthquake loads. It is necessary to observe the "normal" movement of the bridge and then determine the "alarm" angle that would provide sufficient time for crews to travel to and close the bridge to traffic. The California Department of Transportation (Caltrans) has accomplished this by installing the tilt meters, monitoring normal pier movement for several months (ideally, they recommend one year), and setting the "alarm" angles based on the unique "signature" of each monitored pier on any given bridge.

9.4 SELECTING INSTRUMENTATION

Developing the monitoring program in a Plan of Action requires identifying the specific instruments, portable and/or fixed, and how they will be used to monitor scour. Selection of the appropriate instrumentation will depend on site conditions (streambed composition, bridge height off water surface, flow depth and velocity, etc.) and operational limitations of specific instrumentation (e.g., as related to high sediment transport, debris, ice, specialized training necessary to operate a piece of equipment, etc.).

Engineering judgment will always be required in designing instrument specifications to maximize the scour information collected within the given resources. Specific issues related to the use of either fixed or portable instruments include:

  1. For fixed instrumentation, the number and location of instruments will have to be defined, as it may not be practical or cost effective to instrument every pier and abutment.
  2. For portable instrumentation, the frequency of data collection and the detail and accuracy required will have to be defined, as it may not be possible to complete detailed bathymetric surveys at every pier or abutment during every inspection.

Some monitoring programs will involve a mix of fixed, portable and geophysical instruments to collect data in the most efficient manner possible. Furthermore, portable instrumentation should be used to ground-truth fixed instrumentation to insure accurate results and to evaluate potential shifting of the location of maximum scour.

Table 9.1 summarizes the advantages and limitations of the various instrumentation categories. In general, fixed instrumentation is best used when ongoing monitoring is required, recognizing that the location of maximum scour may not always be where the instrument was originally installed. This could be the result of geomorphic conditions and changes in the river over time, or an initial miscalculation when the instrument was installed. Portable instruments are best used where more areal coverage is required, either at a given bridge or at multiple bridges. Portable instruments provide flexibility and the capability to respond quickly to flood conditions; however, if a portable monitoring program becomes large, collecting data may become very labor intensive and costly. Additionally, deployment of portable instruments may require specialized platforms, such as trucks with cranes or booms, or the use of an under bridge inspection truck. Geophysical instrumentation is best used as a forensic tool, to evaluate scour conditions that existed during a previous flood. The primary limitation of geophysical equipment is the specialized training and cost involved in deploying this type of measurement.

Table 9.1. Instrumentation Summary by Category.
Instrument Category Advantages Limitations
Fixed Continuous monitoring, low operational cost, easy to use Maximum scour not at instrument location, maintenance/loss of equipment
Portable Point measurement or complete mapping, use at many bridges Labor intensive, special platforms often required
Geophysical Forensic investigations Specialized training required, labor intensive
Positioning Necessary for portable and geophysical instruments
9.4.1 Portable Instruments

Within the portable instrument category, the use of physical probes is generally limited to smaller bridges and channels (Table 9.2). It is a simple technology that can be effectively used by personnel with limited training, but may be of limited use as the flow depth or velocity increase, such as during flood conditions. Portable sonar instruments may be better suited for large bridges and channels, but they too can be limited by flow conditions based on the deployment options available. Sonar may also be limited in high sediment or air entrainment conditions, or when debris or ice accumulation are present.

Table 9.2. Portable Instrumentation Summary.
Best Application Advantages Limitations
Physical Probes Small bridges and channels Simple technology Accuracy, high flow application
Sonar Larger bridges and channels Point data or complete mapping, accurate High flow application

Positioning equipment is required to provide location information with any portable or geophysical measurement (Table 9.3). The approximate methods are useful for any type of reconnaissance or inspection level monitoring, but are obviously limited by accuracy. The use of standard land survey techniques, using a total station type instrument or in the case of hydrographic surveying, an automated range-azimuth type device, can provide very accurate positional data. However, these instruments require a setup location on the shoreline that may be difficult to find during flooding, when overbank water and/or riparian vegetation limit access and line-of-sight. These approaches can also be somewhat slow and labor intensive. In contrast, the use of GPS provides a fast, accurate measurement, but will not work under the bridge.

Table 9.3. Positioning System Summary.
Best Application Advantages Limitations
Approximate methodsRecon or inspectionNo special training or equipmentAccuracy
Traditional land survey methodsSmall channels or areal surveysCommon technique using established equipmentShore station locations, labor intensive
GPSMeasurement up to bridge faceFast, accurateCannot work under bridge

Another important factor in designing a monitoring program is the cost of the instrumentation and data collection program. Portable instrument costs can be readily identified, but the cost of installation and operation are more difficult to quantify, since this will depend on site specific conditions and the amount of data needed. Based on field experience, Table 9.4 provides general guidelines on cost information. These costs should be used cautiously in an absolute sense, given unique site-specific conditions and/or the changes in cost that can occur with time and new research and development. This information may be most useful as a relative comparison between different approaches.

Table 9.4. Estimated Cost Information for Portable Instruments.
Instrument Cost Cost for Installation or Use Operation Cost
Physical Probes< $500varies by use varies, minimum 2-person crew for safety
Portable Sonarfish-finder - $500;survey grade - $15,000 +/-varies by use varies, minimum 2-person crew for safety
Traditional land survey$10,000 +/-varies2-3 person crew
GPS$5,000 for submeter accuracy, $20,000 + for centimetervaries1-2 person crew
9.4.2 Fixed Instruments

Fixed instrument devices include sonar, sounding rods (automated physical probe), magnetic sliding collar, float out devices, tilt sensors, and time domain reflectometers (Table 9.5). Based on field experience, the sonar type devices work best in coastal regions and can be built using readily available components. They provide a time history of scour, yet have difficulty in conditions with high debris, ice, and air entrainment (Zabilansky 1996). Therefore, if a sonar device is selected for a riverine environment, these conditions may limit when data is collected and the quality of the data record.

Table 9.5. Fixed Instrumentation Summary.
Type of Fixed Instrumentation Best Application Advantages Limitations
SonarCoastal regionsRecords infilling; time history; can be built with off the shelf componentsDebris, high sediment loading, ice, and air entrainment can interfere with readings
Magnetic Sliding CollarFine bed channelsSimple, mechanical deviceVulnerable to ice and debris impact; only measures maximum scour; unsupported length, binding
Tilt SensorsAllMay be installed on the bridge structure and not in the stream-bed and/or underwaterProvides bridge movement data which may or may not be related to scour
Float-Out DeviceEphemeral channelsLower cost; ease of installation; buried portions are low main-tenance and not affected by debris, ice or vandalismDoes not provide continuous monitoring of scour; battery life
Sounding RodsCoarse bed channelsSimple, mechanical deviceUnsupported length, binding, augering
Time Domain ReflectometersRiverine ice channelsRobust; resistance to ice,debris, and high flowsLimit on maximum lengths for signal reliability of both cable and scour probe

Sounding rods, typically a dropping rod with a method to measure the displacement occurring, have been found to work best in coarse bed channels, and are a simple mechanical type of device. They have had difficultly in channels with fine sediments where sediment accumulation around the sliding components has led to binding. Additionally, they are limited by the maximum amount of travel that the sounding rod can realistically achieve, given problems with unsupported length vibration and augering. In contrast, the magnetic sliding collar device works best in fine bed channels, where it is possible to drive the supporting rod into the streambed. It, too, is a simple mechanical type device, but it is also limited by concerns with unsupported length, binding and debris.

The float-out type sensors have worked well in ephemeral channels, and are a low-cost addition to any other type of fixed instrument installation. They have been successfully used when buried either in the channel bed, or in riprap, and can be placed at locations away from structural members of the bridge, which may not be possible with the other types of fixed instruments.

Tilt sensors are installed above water on the bridge superstructure or substructure and may be used on a wide variety of bridges and sites. They measure overall structure movement and, therefore, do not have to be located at the specific location of the scour, as is the case with the other fixed instruments discussed in this chapter. Tilt sensors do not provide information on scour depths. The bridge must be redundant enough to ensure that when movement is detected there is enough time to close or repair the bridge. In addition, bridge movements and rotations may be caused by a variety of load factors, and it may take some time to establish the "signature" movements particular to a bridge which are not due to scour.

The fixed instrumentation selection matrix, Table 9.6 was developed during the synthesis study (Hunt 2008) to complement the countermeasure selection matrix (Table 2.1). See Chapter 2, Section 2.3 for a discussion of the various symbols used in both tables. If fixed instrumentation is to be used to monitor a bridge, this table provides additional items to be considered in deciding between the various fixed instrument options. It was developed based on the results of the synthesis study state survey and literature search (Section 9.3.5). Table 9.6 includes additional categories for suitable river environment for the various fixed instruments:

Type of waterway - riverine / tidal Bed material
Flow habitExtreme conditions
Water depth

The bridge geometry includes information on the characteristics of the bridges for the different types of instruments:

  • Foundation type

The table includes additional items regarding the monitoring system capabilities which may be mandatory or desirable criteria for a particular bridge site:

  • Continuous data monitoring
  • Remote technology

Additional river environment factors listed in Table 2.1 (river type, stream size, bend radius, bank slope, and floodplain) are not listed in Table 9.6 as they do not directly influence the selection of fixed instrumentation.

Table 9.6

The installation experience by state for each type of fixed monitor for those that responded to the synthesis survey (Section 9.3.5) and also from the literature search are included in the last two columns of Table 9.6.

The cost of fixed instrumentation and the data collection program is an important factor in the selection process. Table 9.7 provides general guidelines on cost information. This table may be used with Table 9.4 for portable instruments to compare relative cost information between the different monitoring approaches. As with portable instruments, the most quantifiable cost for fixed instrumentation is the equipment cost. The installation, operation, maintenance and repair costs are more difficult to ascertain.

Table 9.7. Estimated Cost Information for Fixed Instruments.
Typed of Fixed Instrumentation Instrument Cost with Remote Technology ($)(1) Instrument Cost for Each Additional Location ($) Installation Cost Maintenance/Operation Costs
Sonar12,000 - 18,00010,000 - 15,500Medium to high; 5 to 10-person days to installMedium to High
Magnetic Sliding Collar13,000 - 15,50010,500 - 12,500Medium, minimum 5-person days to installMedium
Tilt Sensors10,000 - 11,0008,000 - 9,000LowLow
Float-Out Device10,100 - 10,6001,100 - 1,600Medium; varies with number installedLow
Sounding Rods7,500 - 10,0007,500 - 10,000Medium; minimum 5-person days to installHigh
Time Domain Reflectometers5,500 - 21,700500LowMedium
(1) Cost per device will decrease when multiple devices share remote stations and/or the master station.

Instrument costs generally include the basic scour monitoring instrument and mounting hardware, as well as power supply, data logger and instrument shelter/enclosure, where applicable. This cost may not include miscellaneous items to install the equipment such as electrical conduit, brackets and anchor bolts which may be included as part of the contractor installation cost.

The cost of the scour monitoring installations can vary dramatically due to different factors such as site conditions, the group and/or the experience of the personnel installing the equipment, the type of contract, and the installation requirements. Larger bridges and deeper waterways are more expensive to instrument than smaller bridges in ephemeral or low water crossings. Scour monitors may be installed at certain sites by the state maintenance group, or another agency with equipment they own, or by students. More complicated installations and sites may require specialized contractors and construction equipment to install the scour monitoring devices.

Most recent installations of fixed instrumentation have used remote technology to download data to avoid repeated visits to the bridge site. Although this increases the initial equipment cost, it can substantially reduce the long-term operational costs of data retrieval. Site data retrieval involves sending crews to the bridge and access may include security clearance, lane or bridge closures, and equipment such as snooper trucks or boats. Remote technology can also increase safety to the traveling public because it permits real-time monitoring during the storm events which may result in earlier detection of scour.

Factors that contribute to increased scour monitoring installation, inspection, maintenance and repair costs include: larger bridges; complex pier geometries; bridges with large deck heights off the water; deeper waterways; long distance electrical conduit runs; more durable materials required for underwater tidal installations; the type of data retrieval required (i.e., Internet, satellite); lane or bridge closures and maintenance-of-traffic; and installation and access equipment such as boats, barges, snooper trucks, drills and diving teams.

9.5 FIXED INSTRUMENT CASE HISTORIES

9.5.1 Introduction

The following case histories were selected for this section because they cover a range of geographical locations and types of fixed scour monitoring instrumentation. This section provides descriptions of the systems as well as details on the installations and implementation of the scour monitoring programs.

9.5.2 Typical Field Installations

Alaska Installations. To better understand the scour process and to monitor bed elevation at bridge piers, the U.S. Geological Survey (USGS) and the Alaska Department of Transportation and Public Facilities operate a network of streambed scour-monitoring stations in Alaska (Conaway 2006a). To date they have instrumented 20 bridges with sonar and river stage instrumentation (Figure 9.22). In 2007, 16 bridges remained in the scour monitoring program. These stations provide state engineers with near real-time bed elevation data to remotely assess scour at bridge piers during high flows. The data also provide a nearly continuous record of bed elevation in response to changes in discharge and sediment supply. Seasonal changes as well as shorter duration scour and fill have been recorded. In addition to the near real-time data, channel bathymetry and velocity profiles are collected at each site several times per year.

Each bridge is instrumented with a retractable, pier-mounted sonar device. At locations with multiple scour critical piers, sonar transducers were mounted at each pier. The sonar transducers were mounted either at an angle on the side of the piers near the nose or on the pier nose in order to collect data just upstream of the pier footing. Many of Alaska's bridges are situated in locations too remote for landline or cellular telephone coverage. The scour monitoring instrumentation on the remote bridges has incorporated Orbcomm, a constellation of low-earth-orbiting satellites. Data is sent from the bridge to a passing satellite, which then relays it on to an earth station which then forwards the data to specified email addresses. The network of scour monitoring sites is dynamic, with locations being added and removed annually based on monitoring priority and the installation of scour countermeasures. Instrumentation is subject to damage by high flows, debris and ice, and repairs at some sites can only be made during low-flow conditions.

In 2002 one sonar scour monitor was installed at the Old Glenn Highway Bridge over the Knik River near Palmer (Figure 9.23) (Conaway 2006b). There are two bridges that cross the Knik River at this location. The active bridge was built in 1975, is 505 ft (154 m) in length, and is supported by two piers. The roadway approaches to the active bridge significantly contract the channel. Approximately 98 ft (30 m) upstream is the original bridge which is no longer open to vehicular traffic. Two guide banks extend upstream of both bridges and route flow through a riprap lined bridge reach. All piers are approximately aligned with the flow. The Knik River is a braided sand and gravel channel that transports large quantities of sediment from the Knik Glacier. The braided channel narrows from approximately 3 mi (4.8 km) wide at the glacier mouth to 0.07 mi (0.12 km) at the Old Glenn Highway Bridge where the channel is subject to a 4:1 contraction during summer high flows.

Map of Alaska in outline indicating south central detail area. The topographic map of the detail area is centered approximately at Anchorage Alaska and shows 16 active stream monitoring sites spread from almost Fairbanks to Kodiak Island.  Sketch credit, Conaway 2006a
Figure 9.22. Active streambed scour monitoring locations in Alaska (Conaway 2006a).

Ariel photograph of the Alaskan Knik River Old Glenn Highway bridges during a summer high flow. On the labeled central active bridge the pier instrumented is also labeled. Sketch courtesy of U.S. Geological Survey
Figure 9.23. Oblique aerial photograph of the Knik River Old Glenn Highway bridges during a summer high flow (Courtesy of U.S. Geological Survey).

The right-bank pier of the new bridge was instrumented with a retractable, pier-mounted sonar monitor. This retractable arm was designed to prevent ice and debris flows from damaging the sonar bracket, as had occurred in other scour monitoring installations in Alaska. Stage data were measured by a nearby USGS stream gage. The sonar was mounted at an angle on the side of pier near the nose in order to collect data just upstream of the pier footing. Data are collected every 30 minutes and transmitted every 6 hours via satellite. When bed elevation or stage thresholds are exceeded, data transmissions increase in frequency. The Knik River was the only site within the monitoring network that had large changes in bed elevation each year. Annual scour ranged from 17.2 to 20 ft (5.2 m to 6.0 m). These near real-time data for the Knik River and other sites in Alaska are available on the USGS website.

New York Installations. NYSDOT has installed twenty-seven sonar scour monitors at three bridges on the South Shore of Long Island in Nassau and Suffolk Counties in New York (Hunt 2003). Wantagh Parkway over Goose Creek is a 93 ft (28.3 m) bascule bridge (Figure 9.24), Wantagh Parkway over Sloop Channel was a 576 ft (175.6 m) long bridge, and Robert Moses Causeway over Fire Island Inlet, a 1,068 ft (326 m) bridge (Figure 9.25). These have served as both short and long-term solutions to the scour problems at these bridges. In 1998, following a partial pier collapse at Wantagh Parkway over Goose Creek, it was found that the streambed at one pier had experienced approximately 29 ft (8.8m) of localized scour since it was built in 1929. In order to ensure that these bascule piers were safe, several options were investigated and a scour monitoring system and program was designed for the bridge.

A nearby bridge, Wantagh Parkway over Sloop Channel was also examined. It was found to have similar problems with respect to scour of the piers. As a result, four scour monitors were installed at the bascule piers of Goose Creek, and ten monitors were installed at Sloop Channel. In addition, a water stage sensor was installed at each bridge. The scour monitors were approved by NYSDOT within one week of the 1998 failure, and they were designed, custom-built and arrived at the site ten weeks later. The sonar mounting brackets were made of stainless steel due to the harsh tidal environment. For data retrieval the system employed remote telemetry via a modem and telephone landline. The power was supplied using solar panels for the fixed bridge at Sloop Channel and using the electrical system on the bascule bridge at Goose Creek.

Photograph of the control tower of the Wantagh Parkway bascule bridge over Goose Creek. Conduit to the scour monitor can be seen running down the side of the pier.
Figure 9.24. Conduit to sonar scour monitor at Wantagh Parkway over Goose Creek.

A scour monitoring program and manual were developed for the Wantagh Parkway Bridges. This was the first procedural manual to be developed for scour monitors. The manual provided the opportunity to work through various scenarios should these bridges continue to experience scour. The program included round-the-clock monitoring even during storms. It included critical streambed elevations for each pier, procedures for normal and emergency situations, a Plan of Action should certain scour elevations be reached and troubleshooting, maintenance and servicing instructions. An effective communication system for all responsible parties was established.

The installation of sonar scour monitors at Robert Moses Causeway over Fire Island Inlet is a long-term solution to the scour issues at that bridge. The flow rate was estimated to be over 492,000 cfs (13,932 m3/sec) for the 100-year storm. Riprap scour protection had been placed at some piers over the years, and according to the FHWA guidance, riprap should be monitored when used as a countermeasure at piers. In 2001, sonar scour monitors were placed at 13 piers, a water stage sensor was installed, and the Long Island scour monitoring manual was revised to include the new system. This was a complex design and installation due to the proximity of the bridge to the Atlantic Ocean, the deep-water conditions, the pier configurations and the high flow rates. In order to ensure that the underwater sonar brackets could clear the pier footings to measure the streambed elevations, this design incorporated a new type of adjustable tripod stainless steel bracket (Figure 9.26).

Photograph from the water looking up along the large Robert Moses Causeway. Two remote stations can be seen fixed to the piers.

Photograph of a sonar installation at the Robert Moses Causeway. Taken at water level the sonar monitor and the conduit are seen affixed to the pier.
Figure 9.25. Solar panel, remote station and (inset) conduit to sonar monitor at Robert Moses Causeway over Fire Island Inlet.

Photograph of a work crew in a boat between piers under a bridge. An adjustable stainless steel sonar mounting bracket is lying across the gunnels prior to underwater installation.
Figure 9.26. Adjustable stainless steel sonar mounting bracket prior to underwater installation.

Summary - New York Installations . The scour monitoring systems at Goose Creek and Fire Island have been in operation for since 1998 and 2001, respectively (Hunt and Price 2004). When the Sloop Channel Bridge was replaced in 1999, the monitoring system was salvaged and has been used for spare parts for the other bridges. The scour monitoring program includes the routine monitoring of these bridges, including data acquisition and analysis; round-the-clock monitoring during scour critical events; the preparation of bi-weekly graphs of the streambed elevations and tide gage data; periodic data reduction analyses and graphs; and routine maintenance, inspection, and repairs. In 2004, a total refurbishment of the Goose Creek system was completed. This included the installation of the latest operating system software and a new bracket for the sonar transducer at one monitor location. An underwater contractor installed the new bracket and also strengthened the scour monitor mountings at the other three pier locations. The condition of the scour monitors and the accuracy of their streambed elevation readings are checked during the regularly scheduled diving inspections at each bridge. Also, substantial marine growth and/or debris on the underwater components is cleared away during these inspections.

California, Arizona and Nevada Installations. In preparation for El Niño driven storm events, a variety of instruments were installed at bridges in the southwest in late 1997 and early 1998. Five bridges were instrumented in California, five in Arizona and four in Nevada. The equipment included automated sliding collar devices, low-cost sonar, multi-channel sonar, float-out transmitters and sliding rod devices (Figures 9.27 and 9.28). These installations provided an opportunity to test a number of new concepts, including 2- and 4-channel sonar devices, application of early warning concepts (by defining threshold scour levels and automated calls to pagers when that threshold was exceeded), and development and refinement of the float-out instrument concept.

Photograph showing a sonar scour monitor being bolted to the nose of a pier. The installation is made in dry conditions with the monitoring device placed just above streambed level
Figure 9.27. Installation of a sonar scour monitor on Salinas River Bridge near Soledad, California (Highway 101) by CALTRANS.

Photograph showing a sonar scour monitor bolted to the nose of a pier. The heavy steel casing holding the monitoring device is bolted at approximately streambed level and steel conduit runs up the nose of the pier.
Figure 9.28. Close up of sonar scour monitor on Salinas River Bridge near Soledad, CA.

To support the California, Arizona, and Nevada installations, a buried transmitter float-out device was developed for application on bridge piers over ephemeral stream systems. As summarized in Section 9.3.4, this device consists of a radio transmitter buried in the channel bed at a pre-determined depth. When the scour reaches that depth, the float out device rises to the surface and begins transmitting a radio signal that is detected by a receiver in an instrument shelter on the bridge. Installation requires using a conventional drill rig with a hollow stem auger (Figure 9.29). After the auger reaches the desired depth, the float out transmitter is dropped down the center of the auger (Figure 9.30). Substrate material refills the hole as the auger is withdrawn.

The float out device can be monitored by the same type of instrument shelter/data logger currently being used to telemeter low-cost fathometer or automated sliding collar data. The instrument shelter contains the data logger, cell-phone telemetry, and a solar panel/gell-cell battery for power (Figure 9.31). The data logger monitors the sliding collar and sonar scour instruments, taking readings every hour and transmitting the data once per day to a computer at a central location (e.g., DOT District). A threshold elevation is defined that, when reached, initiates a phone call to a pager network. The bridge number is transmitted as a numeric page, allowing identification of the bridge where scour has occurred. The float out devices are monitored continuously, and if one of these devices floats to the surface, a similar call is automatically made to the pager network.

Although the float out devices had not been tested extensively in the field, in late 1997 and early 1998 more than 40 float-out devices were installed at bridges in Arizona (4 bridges), California (1 bridge), and Nevada (4 bridges). Most devices were installed at various levels below the streambed as described above; however, several devices at bridges in Nevada were buried in riprap at the base of bridge piers to monitor riprap stability (Figure 9.32).

Photograph of truck mounted drilling rig and operators. Drilling is with hollow stem auger next to a bridge pier in dry conditions.
Figure 9.29. CALTRANS drilling with hollow stem auger for installation of float out devices at Salinas River Bridge (Highway 101) near Soledad, CA.

Photograph showing a truck mounted drilling rig with a hollow stem auger set up. Location is next to a bridge pier in dry conditions. Two workers are about to place the red float out device down to the drilled depth.
Figure 9.30. Installation of float out device on Salinas River Bridge near Soledad, CA.

Photograph of solar panel and instrument box mounted on a bridge outside the bridge rail
Figure 9.31. Typical instrument shelter with data logger, cell-phone telemetry, and a solar panel/gel-cell for power.

Photograph of placement of a float out device within the riprap matrix next to a bridge foundation. Placement is in dry conditions and by hand.
Figure 9.32. Installation of a float out device by Nevada DOT to monitor riprap stability.

One of the bridges instrumented experienced several scour events that triggered threshold warnings during February 1998. In one case the automated sliding collar dropped 5 ft (1.5 m) causing a pager call-out. Portable sonar measurements confirmed the scour recorded by the sliding collar. Several days later, another pager call-out occurred from a float-out device buried about 13 ft (4 m) below the streambed.

In both cases, the critical scour depth was about 20 ft (6 m) below the streambed. However; pager call-out was ineffective in alerting maintenance personnel during non-office hours and no emergency action was called for to insure public and/or bridge safety. Consequently, a programmed voice synthesizer call-out to human-operated 24-hour communications centers was implemented at other bridges. This illustrates the importance of effective and well-defined communication procedures, and the on-going need for comprehensive scour training at all levels of responsibility.

Updated: 09/26/2011

FHWA
United States Department of Transportation - Federal Highway Administration