Better understanding of the canal bed
Expert(s) |
Marc Hijma
The Netherlands is a country of canals. In the years ahead, these waterways will be modified to make inland navigation future-resilient. However, this work is not straightforward. To prevent local disruption and to work faster, Rijkswaterstaat needs to know more about the interaction between canals and the subsurface. The drained bed of the Juliana Canal in Limburg was a gift: a unique opportunity for Deltares to verify previous measurements of the canal bed and improve measurement techniques.
Under a cloudy grey sky, Deltares researchers Marc Hijma and Dennis Peters wade laboriously through the heavy mud on the bed of the Juliana Canal. Five meters below ground level, they struggle around pools, shells, boulders and pits with a cart full of equipment to take soil samples and measure water permeability. In some places they sink in deep but, fortunately, the motorised cart has caterpillar tracks. “It may not be the most beautiful fieldwork location I’ve seen for Deltares but it is certainly the most unusual,” laughs Marc.
Godsend
The fact that this part of the Limburg Canal was drained for maintenance work turned out to be a godsend for Deltares. To understand the interaction between canal and subsurface better, Marc had been mapping out the canal bed for some time. Now he could take measurements on location to see if past data were correct. “With the information we collect here, we could validate our previous measurements,” he explains. “We conducted them from a ship in the summer of 2024 with technology including multibeam sonar backscatter. That involves sending sound waves down to the bed and measuring the intensity of the reflection. The results provide us with information about the material located on the bed. We supplemented this data with high-resolution photographs taken with a drone when the canal was dry. We are now comparing the results of our soil samples and tests with all that in order to establish a unique dataset for improving measurement techniques for canal beds.”
Canal maintenance
Rijkswaterstaat is also interested in this. Regular modifications such as deepening and widening are required to keep the canals navigable, and so Rijkswaterstaat asked Deltares to study the subsurface of canals including the Amsterdam-Rhine and Juliana Canals. “It must be said that the Juliana Canal is exceptional,” says Marc. “It rests on gravel, from which water drains easily. That’s why, during its construction in the 1930s, a layer of clay was laid down with a protective layer of gravel on top.”
A section of the canal was drained for a while in the autumn. It was a unique opportunity to learn more about the bed. Marc: “We could see whether that clay layer was still present everywhere and, if not, how fast water flowed away in locations where there was no more clay. We can also look at how the canal affects the groundwater system in the vicinity. We are doing this with dozens of monitoring wells that were installed in the past to monitor groundwater levels.”
Surprise
In a three-kilometre-long section, Marc and Dennis grabbed shovels themselves to take soil samples. At four locations, they used the double ring infiltrometer. That involves hammering two rings into the ground, filling the pool with water and measuring how fast the water in the inner ring sinks into the ground. “This was one of Dennis’s ideas to measure water permeability quickly,” explains Marc. “You couldn’t wish for a better colleague with fieldwork: he has everything with him in his van, from wrenches and saws to pumps.”
The pair brought in an excavator to dig trenches in the canal bottom at several locations to inspect the clay layer in more detail. In the northern part, different, much older, sediments occur under the clay layer than in the south. “For instance it was cool to see a million-year-old beach line,” says Marc. “The white sand makes a nice contrast with the dark stones. Fieldwork always throws up surprises really.”
Better measurements
With this section of the Juliana Canal now being submerged again, what was the point of the research here? Marc explains that you have to test and improve measurement techniques to understand better how canal beds work. That’s the only way to establish accurate subsurface models. “Rijkswaterstaat will use these models for designing and implementing future modifications to other canals. The knowledge we acquired in the Juliana Canal will also help my colleagues working on the Amsterdam-Rhine Canal. We expect that a clearer understanding of canal-subsurface interactions will ultimately help establish resilient infrastructure.”
