"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Summary of Fixed Instrumentation for Field Measurement of Scour and Deposition

By David S. Mueller

Abstract

Scour and fill in streams can be measured in real time by using one of two basic types of equipment: fixed instrumentation installed at the site or portable instrumentation used by field personnel. This summary will address only fixed instrumentation. Each type of equipment has advantages and limitations; thus, equipment selection is dependent on the site configurations, stream conditions, and desired data.

A significant amount of research has been devoted to the development of instrumentation that can be installed on bridge piers and abutments to measure streambed elevation. Some of these instruments provide a continuous measurement of the streambed location while others measure only the minimum streambed elevation (maximum scour) since the time of installation. These instruments commonly have been grouped into four broad categories: sounding rods, driven/buried rods, sonar instruments, and other buried devices. Table 1 provides a list of the various devices.

Table 1. Types of fixed scour-monitoring instruments

Sounding Rods Driven or Buried Rods Sonar Devices Other Buried Devices
BRISCO1 Monitor Horseshoe Collar
Magnetic Collar
Piezo-Electric Probes
Heat Dissipation Gage
Photo-Electric Cells
Trip Switch Probes
Conductance Probes
Single Transducer Systems
Multiple Transducer Systems
Scanning Sonar Systems
Radio Transmitters
Buried Chains
Pressure Transducers

Research sponsored by the Federal Highway Administration and the National Cooperative Highway Research Program has led to the development of several viable technologies; however, improvements to these technologies and investigation of additional technologies are necessary to provide adequate fixed scour-monitoring equipment for all conditions.

The BRISCO Monitor is a sounding-rod or falling-rod instrument. 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 of the streambed caused by the weight of the rod and the vibration of the rod from flowing water.

Driven/buried-rod scour monitors derive their name from the fact that a rod must be driven or buried into the streambed. Driven/buried rods can be divided into two general classes-those that have a sensor mounted in a collar that slides down the rod and those that have sensors mounted directly on the side of the rod. Research has been directed toward both types of rods but those that have a sensor mounted in a collar may be more economical and easier to install than those with sensors mounted on the sides of the rod. The instruments utilizing a sliding collar or trip switches can measure only maximum scour and not subsequent deposition. The piezo-electric probes, heat-dissipation gage, photo-electric cells, and conductance probes can measure both scour and deposition. Installation of all driven/buried rod systems can be difficult and this makes the technology undesirable in streams with coarse or hard bottoms. Damage by ice and debris is also a potential problem.

Acoustic or sonar technology has been used for many years to measure the depth of water. Although sonar is a viable technology for measuring both scour and deposition in a variety of conditions, it has limitations. Use of this technology is limited in shallow streams with very high velocities where minimum depth requirements and air entrainment are potential problems. Very high concentrations of fine sediment in suspension can scatter and absorb the sonic pulse, preventing reflection of a detectable echo. The transducers are subject to damage and (or) blockage by debris and ice. However, acoustics are generally the most cost-effective alternative for many situations and provide a continuous measurement of both scour and deposition.

Low-cost scanning sonar could be developed into a viable technology. The low-cost scanning sonar hardware is currently available. Scanning sonar could be mounted on the side of a bridge pier, reducing the potential damage or blockage by debris. The scanning feature would allow an entire area around the pier to be monitored rather than a single point. Software could be developed to identify and transmit warnings of scour or debris accumulations at the piers.

Buried chains are one of the most common and oldest methods for measuring scour. The depth of scour is determined after a flood has receded by carefully excavating the area around the chain and locating the depth to which the chain is disturbed. Some recent attempts have used radio transmitters buried in the streambed that float out and are detected when scour has reached a particular elevation.

Pressure sensors that measure the pressure of the bed material have been used successfully along the Colorado River in the Grand Canyon for more than 3 years. One pressure sensor measures the stress of the overlying sediment, water, and atmosphere; a second pressure sensor measures the stress of the overlying water and air. The advantage of the buried pressure sensors is that, unlike many of the previously discussed technologies, they would not be affected significantly by debris and would allow both scour and fill to be monitored.


1 Any use of trade, product, or firm names is for description purposes only and does not imply endorsement by the U.S. Government.

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