Researchers at the University of Bonn in Germany have developed a low-cost, reliable method to monitor the water levels of rivers continuously. The sensor, which can be attached to a bridge, building or tree near the river, is less likely to be damaged during extreme weather and can transmit the water level to an evaluation centre via mobile communication. It is based on a low-cost global navigation satellite system (GNSS) receiver and antenna that uses satellite signals to measure the height of the antenna above the river surface.

The cost-effective sensor is suitable for area-wide flood warning systems and allows the water level of rivers to be monitored around the clock.

There are many ways to measure the level of a watercourse, but they all have limitations. Some methods, like using a yardstick or staff gauge, are simple but not always effective. More advanced radar solutions may be more reliable but can be damaged by high water levels and may not allow for continuous monitoring. Additionally, remote reading can be difficult with these devices, and they can be expensive.

A measuring device that does not have these disadvantages has already been in service for two years on the Lower Rhine River in Wesel, Germany. According to the University of Bonn, it is cost-effective, reliable and capable of continuously transmitting the water level to an evaluation centre via mobile communication. In principle, this means that such a sensor is suitable for providing a densely-distributed network for flood and drought warning systems.

“The core of our device is a low-cost GNSS receiver and antenna,” explains Dr Makan Karegar of the Institute of Geodesy and Geoinformation at the University of Bonn. This is a sensor that can conventionally determine the position of its location with several metres of accuracy. It does this using the US GPS satellites and their Russian counterparts, GLONASS. “However, satellite signals can also be used to measure the height of the GNSS antenna above the river surface,” Karegar says.

This is because the waves transmitted by the satellites are partially picked up directly by the antenna. The rest is reflected from the nearby environment (in this case the water surface) and reaches the receiver via a detour. This reflected part, therefore, travels longer. When superimposed on the directly received signal, it forms certain patterns called interference. These can be used to calculate the distance between the antenna and the water level.

“We can attach the GNSS antenna to any structure, whether it’s a bridge, a building, or a tree or fence next to the river,” Karegar explains. “From there, it can measure the river level around the clock without contact – to within around 1.5 cm on average. And yet it is less likely to be damaged during extreme flooding events.” The accuracy of the method does not match that of a radar-based sensor. However, it is completely sufficient for the intended use. At just under 150 euros, the device is also considerably cheaper than its advanced counterpart.

The GNSS antenna is connected to a microcomputer called a Raspberry Pi. “The device is about the size of a small smartphone, yet it has enough power to calculate water levels from raw data,” reports Prof. Dr Kristine Larson of the Institute of Geodesy and Geoinformation. Thanks to its flexibility and low power consumption, the microcomputer is very popular among hobbyists, who use it to realize a wide variety of projects. It can be powered by solar cells and then works completely stand-alone. It can also transmit its data via mobile networks.

“The software we wrote is open source,” Larson explains. “So it can be used by anyone for free.” The researchers also make all the information about their project available on the Internet. Interested parties can therefore easily reproduce the measuring device. (see the GitHub page here)

However, the process has one disadvantage: It is only suitable for rivers with widths of at least 40 meters. “This is the smallest radius from which the antenna can receive the reflected satellite signal,” Karegar says. “If the watercourse is too narrow, most of the reflected signals come from the land.” But those involved plan to further optimize their evaluation code. They hope that this will enable them to obtain reliable results for smaller rivers such as the Ahr in Germany, which experienced severe flooding in 2021.

Read more about the research here: Publication: Makan A. Karegar et al.: Raspberry Pi Reflector (RPR): A Low-cost Water-level Monitoring System based on GNSS Interferometric Reflectometry; Water Resources Research; DOI: 10.1029/2021WR031713

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