Lesson 7 of 6
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Module 11: Monitoring Equipment

May 31, 2021

[vc_row][vc_column][vc_single_image image=”918″ img_size=”full” alignment=”center”][vc_column_text]An essential part of any environmental monitoring or measurement project is the equipment used. Some equipment, like meters and field kits, are intended for spot sampling. Others, like sondes, data loggers and data buoys, are designed for long term monitoring applications. While buoys, sondes and automated samplers tend to be specific to water quality projects, sensors and data loggers can be used to monitor nearly any environmental parameter. Online datacenters, when used in conjunction with telemetry and a data logger offer real-time data and project access from anywhere, at any time. Project equipment should be chosen based on accuracy requirements, budget and whether or not remote monitoring is required.

Buoy-based systems are typically moored as either a single-point or two-point mooring, based on environmental and application-specific factors.

Single-Point Mooring

Single-point moorings are not common, but they require the least amount of mooring equipment. This setup can be deployed in very calm waters with minimal instruments. A single-point mooring should only be used when all sensors and equipment are housed within an instrument cage or deployment pipe. Hanging sensors risk getting damaged or entangled with the anchor line. A cage or pipe protects the instruments from entanglement, subsurface debris, and currents without affecting sensor readings.

In a single-point configuration, a mooring line connects the buoy directly to a bottom chain and anchor. The sensors are typically housed within a central deployment pipe or attached to a rigid instrument cage. The anchor, bottom chain, and mooring line are assembled and attached to the buoy prior to deploying the system.

Two-Point Mooring

Two-point moorings are the most common deployment configuration. This is the recommended setup if sensors will be hanging at multiple depths in the water column. In a two-point setup, the mooring lines are pulled away from the data buoy by two smaller marker buoys. This configuration leaves the water column below the buoy available for sensors, without risk of entanglement with anchor lines. It also offers greater stability if there are currents or wave action at the location.

A two-point mooring requires a larger deployment area than a single-point mooring, as the marker buoys are typically set about ten feet away from the data buoy. Additional mooring lines run from the marker buoys to bottom chains and anchors at the seafloor. The increased system stability from the two-anchor setup is well worth the extra equipment, as is the expanded area for hanging sensors. If there is significant subsurface debris or other risks present, deployment pipes or instrument cages can still be used.

Real-Time Scour Monitoring

Real time scour monitoring include sonar, magnetic sliding collars, and float out devices. In conjunction with a data logger, any of the just named devices can be used.  All 3 of these devices measure elevation changes in the streambed.  When excessive sediment occurs, these instruments will indicate the elevation change and transport that information to a datalogger.

While other instruments, such as tilt sensors, sounding rods and piezoelectric sensors can also be used for monitoring, they are not as well known or preferred. Tilt sensors measure bridge movement, not sediment erosion, which requires partial structure failure before action is taken. Sounding rods and piezoelectric sensors can provide false or inaccurate measurements. Sonar, magnetic sliding collars, and float-out devices, on the other hand, all measure streambed erosion in areas of potential scour.

Of all the scour monitoring methods available, sonar is the most widely used due to the ease of installation and the wider range of applicable site conditions. In addition, sonar instruments provide continuous bed level data, while magnetic sliding collars and float out devices only indicate when scour has reached a certain depth.

Sonar Instruments

Sonar instruments, also known as acoustic transducers, use sound waves to “ping” the bottom of a waterway. The corresponding echo will indicate depth to the riverbed. When the transducer is angled toward an area of potential scour, the sensor will measure the level of erosion occurring. Most sonar instruments are mounted directly to a pier or substructure of a bridge. This indirect measurement method limits interference from debris in the water and makes the instruments easier to maintain.

Sonar is a popular scour monitoring solution as it can continuously measure and return data. They are also easily connected to a telemetry station for real-time data transmission. These systems are also easy to implement, as they do not need to be installed or buried in the riverbed at all. However, depending on the location and environmental conditions, some above surface debris (e.g. ice flows) can interfere with the sonar. As with any monitoring solution, regular maintenance is recommended.

These instruments can be equipped with one sonar transducer to monitor a specific area, or multiple to measure elevation changes in a wider range around the pier. If a stream migrates, shifting the potential scour area, it is fairly simple to move the system to the new at-risk pier. In addition to their appeal as a continuously monitoring device, sonar-based scour monitoring systems are recommended in deep channels, waterways with loose sediment beds, and biologically active areas, as they are not as susceptible to biofouling, in-water debris or shifting sediment.

Magnetic Sliding Collar

Sliding collar solutions are comprised of a rod and a ring driven into the riverbed. As the sediment erodes, lowering the bed elevation, the collar slides down the rod. Magnetic triggers in the rod will determine the collar’s depth based on its location on the rod.

While magnetic sliding collars can be automated or manually read, they only measure maximum scour depth. Manual magnetic sliding collar installations are inexpensive and can be easy to install during low flow events. However, they are very susceptible to debris in the water. Automatic magnetic sliding collar systems are more expensive, but also more robust than their manual counterparts. However, in biologically active environments, such as estuaries and tidal rivers, barnacle growth and other biofouling can occur. This will can interfere with scour readings, as the biofouling can prevent the ring from moving on down the rod.

Both versions of the sliding collar method should be firmly driven into the streambed. Installations are easier in shallow rivers and during low flow events. In addition to debris and biofouling, sliding collars are susceptible to subsoil shifts. Any lateral movement of a waterway can make the deployment ineffective.

Float-Out Device

Float-out devices are buried at varying depths in a potential scour area. When scour occurs, removing the sediment covering the device, the instrument will float to the surface. A wireless signal is transmitted to a nearby data logger to indicate its release.

These instruments have an extended lifespan (5-10 years for battery operated float-outs) to maintain operability for several seasons if they are not triggered in the first year. As they are buried beneath the bed, they are not susceptible to damage from debris at all. They are also very easy to install in dry beds (intermittent streams) and riprap, though they can also be installed in wet channels with a trained diver.

While relatively inexpensive and maintenance free, these devices do have several drawbacks. One such drawback of the float-out device is that it will only provide data once the scour has progressed beyond a certain depth and the instrument becomes uncovered. However, several devices can be buried at multiple depths below the bed to signal scour progress.

It is recommended to have a level of redundancy when using float-outs, as once the instrument is buried, its functionality cannot be verified. These instruments only produce a signal once uncovered, and their status cannot be checked in the interim. Any instrument failure may result in unsafe conditions, as there will be no warning of critical scour levels.[/vc_column_text][/vc_column][/vc_row]