Low-cost sensor developed for river level monitoring

Researchers at the University of Bonn have developed a cost-effective method that allows the water level of rivers to be monitored around the clock, which will be useful for area-wide flood warning systems. Their work has been published in the journal Water Resources Research.

There is a wide range of methods to determine the level of a watercourse — from very simple ones (by yardstick or staff gauge) to advanced radar solutions. But they all have a catch: most measuring devices can be damaged due to direct exposure to the high water level, many do not allow continuous monitoring, remote reading is difficult, or they are simply too expensive.

In Wesel on the Lower Rhine, however, a measuring device that does not have these disadvantages has already been in service for two years: 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,” said Dr Makan Karegar of the Institute of Geodesy and Geoinformation (IGG) at the University of Bonn. This is a sensor that can conventionally determine the position of its location with several-metre accuracy, which it does using US GPS satellites and their Russian counterparts, GLONASS.

“Satellite signals can also be used to measure the height of the GNSS antenna above the river surface,” Karegar added. 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 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 said. “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, the device is also considerably cheaper than its advanced counterpart.

The GNSS antenna is connected to a Raspberry Pi microcomputer, whose flexibility and low-power consumption makes it popular among hobbyists for a wide variety of projects. As noted by IGG’s Prof Dr Kristine Larson, “The device is about the size of a small smartphone, yet it has enough power to calculate water levels from raw data.” It can be powered by solar cells and then works completely standalone. It can also transmit its data via mobile network.

Larson said the researchers’ software is open source, so it can be used by anyone for free, and that all the information about the project is available on the internet. Interested parties can therefore easily reproduce the measuring device.

The process does have one disadvantage: it is only suitable for rivers with a width of at least 40 m, as this is the smallest radius from which the antenna can receive the reflected satellite signal. “If the watercourse is too narrow, most of the reflected signals come from the land,” Karegar said. The researchers plan to further optimise their evaluation code so that they may obtain reliable results for smaller rivers.

Image caption: The water level sensor can be operated with solar cells and then works completely standalone with no maintenance. Image ©Makan Karegar/University of Bonn.