STREAM MONITORING / FIELD MEASUREMENTS

Successful stream monitoring relies on the use of innovative methods and integrative approaches. At IWA, field measurements have a long tradition, leading to globally unique methods and insights into the processes of our flowing waters.

 A variety of measurement techniques are used in the monitoring of flowing waters to gain important knowledge about the processes relevant to many research questions in flowing waters and their surroundings. Depending on the objective, the measuring devices are used either individually or in combination. Devices used at specific points, such as the mobile bedload sampler or the so-called “ray” for measuring bed shear stress, provide sample-like results and can be flexibly used at different locations. In contrast, permanently installed geophones for capturing bedload transport intensity and turbidity probes for suspended sediment monitoring enable continuous and automated process recording. These measuring devices thus provide valuable long-term data of the relevant measurement variables.

Hydraulic Measurement Variables

Hydraulic measurement variables such as flow velocity and discharge are measured at IWA using established devices such as the ADCP (Acoustic Doppler Current Profiler), ADV (Acoustic Doppler Velocimeter), and the Flow Tracker. For measuring bed shear stress, IWA has designed an innovative measuring device, the “ray,” which directly measures the forces acting on the riverbed using load cells.

Suspended Sediment Transport – Innovation through a Combination of Direct and Indirect Measurement

A variety of methods and measuring instruments are available for recording suspended sediment transport. The appropriate methods and instruments are selected based on the specific research question and local conditions. In most cases, direct and indirect measurement methods are used in combination.

Direct methods include suspended sediment sampling, ranging from simple scoops to isokinetic sediment samplers and automated pump samplers. Additionally, indirect measuring instruments are used, which infer suspended sediment concentration by detecting a proxy parameter. Optical probes enable continuous turbidity measurements, although typically only at a single point in the water body.

One approach for determining the distribution of suspended sediment across a river cross-section is acoustic measurement using an ADCP (Acoustic Doppler Current Profiler) in combination with sample collection. When these methods are used together and the measurements are combined through conversion factors, the temporal and spatial variability of suspended sediment concentrations within a cross-section can be captured, allowing for the calculation of sediment transport and suspended sediment loads.

By adapting the monitoring methodology, additional questions can also be addressed, such as the spatiotemporal distribution of sediment plumes caused by dredging and sediment disposal, changes in suspended sediment concentration during controlled reservoir drawdowns, and the calculation of suspended sediment budgets for selected watercourse sections.

Bedload Transport – Mobile Bedload Capture Basket

Mobile bedload collectors are lowered to the riverbed and capture the transported bedload using a suitable container and a defined measurement duration. Their range of application includes steep mountain streams with high flow velocities and large grain sizes, as well as wide and deep gravel-bed rivers with finer bedload. The different hydraulic conditions and local circumstances require an adaptation of the measurement methods. At IWA, various types of bedload collectors are used, enabling reliable measurement of transport processes in turbulent mountain rivers like the Rofenache at 1980 meters above sea level, as well as during flood discharges up to HQ200 on the Danube.

Bedload Transport – Bedload Trap

The bedload trap allows for automated and long-term measurement of bedload transport across the entire range of flow. The measuring device consists of a collection container that is mounted on load cells and installed in the riverbed. The cover of the container can be hydraulically opened, allowing the bedload material to fall through a measuring slot into the collection box, where it is continuously and automatically weighed. The bedload trap must be maintained after successful measurement during low water conditions. A special case is the lifting trap on the Drau River in Dellach. It is equipped with a hydraulic mechanism that allows it to be elevated, making it accessible even at higher water levels.

Bedload Transport – Geophone System

Geophones are vibration sensors that originally come from the field of seismology. For bedload measurement, they are mounted on the underside of steel plates and installed across the entire flow cross-section at the same height as the riverbed. When bedload material is transported over the steel plates, it generates vibrations that are detected by the geophones and automatically and continuously recorded (indirect bedload measurement).

The geophone system thus provides temporally and spatially high-resolution data on the transport process. To convert the measurement data into bedload transport rates or loads, the indirect measurements must be calibrated with direct measurements (capture basket, bedload trap).

Bedload Transport – Integrative Bedload Measurement System

Bedload transport is a process that varies greatly in space and time. Existing measurement methods each have their capabilities, but when used alone, they cannot fully capture the bedload transport process. At IWA, the integrative bedload measurement system is used, in which the individual measurement methods are combined to provide qualitative measurement data.

Bedload Transport – Tracers

Radiotracers enable the measurement of transport distances, velocities, and movement paths of individual sediment particles (tracer stones). Either natural river sediments are used or artificial tracers are manufactured. At IWA, both active and passive tracers are used. Both measurement systems allow for the individual identification of the tracer stones, enabling them to be specifically located and their movements tracked. A typical tracer measurement system consists of a transmitter (“transponder” or “tag”) integrated into the stones, a reader device (“reader”) to capture the tag information, and an antenna for signal transmission.

Active transponders offer the significant advantage of a large range of about 120 meters. However, since they have their own battery, they are relatively large and their lifespan is limited to 1 to 5 years. This technology is preferably used at IWA in larger rivers such as the Danube, Drau, Gail, and Rhine.

Passive transponders are characterized mainly by their small size and longevity (less than 10 years). Since they do not have their own power source, they must first be inductively charged via the antenna for communication. This leads to a limited range of about one meter. At IWA, this technology is primarily used in smaller rivers such as the Urslau, the Strobler Weissenbach, and the Alm, as well as in side channels of larger rivers like the Inn or Rhine.

Bed Substrate – Freeze Core

In addition to common methods for grain size analysis, such as the collection of volumetric samples and the execution of Wolman counts and line count analyses, IWA also conducts freeze core sampling in collaboration with the company UWITEC.

For freeze core sampling, a metal tube is driven 1 meter or 0.5 meters deep into the riverbed. Using liquid nitrogen, the tube is cooled down to -196 °C, causing the river substrate to freeze to the tube. This process takes between 20 and 45 minutes depending on the water temperature. The tube can then be pulled out of the riverbed, and the sample can be divided into relevant layers on the shore (or on a boat).

Morphological Monitoring – GNSS Systems

At IWA, total stations and GNSS (Global Navigation Satellite System) systems are used complementarily for precise and flexible surveying, while drones equipped with photogrammetry and LiDAR provide versatile 3D models. Control points, stationary GNSS antennas, and real-time correction data enhance accuracy. Drones range from compact camera models to multirotors with LiDAR, capturing tens of thousands of points per second. These technologies are used not only for river morphology but also for monitoring check dams, where cameras and LiDAR sensors continuously record the fill level of retention basins.

Survey of Micro and Macro Plastic Transport

Plastic pollution in our environment has become ubiquitous. Research in recent years has shown that plastic is found even in the most inaccessible places on our planet. Rivers are both major pathways for the entry of plastic into the oceans and directly affected by pollution. However, there are still few standards or regulations for measuring plastic transport in flowing waters. Such standards would be of great importance for comparing measurements taken worldwide and for creating common databases.

IWA has conducted significant research in the survey of both micro and macro plastic transport. For microplastics, IWA is developing isokinetic methods to determine plastic transport in rivers. This involves combining net sampling at multiple depths with pump sampling. Nets are used to filter sufficient water for the coarser fraction of microplastics, while the finest particles are captured by pump filters down to 25µm. Regarding macroplastics, GPS tracers are used as a monitoring method, and hydrodynamic-numerical models are also employed to better understand and extrapolate transport processes.