Keeping our rivers navigable with flexible groynes
Expert(s) |
Robert Groenewege
There are thousands of traditional groynes in Dutch rivers. They are essential to keep waterways navigable and manage river ice. However, maintenance work on these groynes is costly and time-consuming. In a unique alliance between the government, private parties and knowledge institutes, including Deltares, an innovative alternative has been developed: the flexible groyne.
Challenge for river managers
A groyne is a structure that protrudes from the bank into the river, regulating the flow of the water and helping to keep the rivers open for shipping during dry periods. Groynes also stop river ice blocking the river. However, maintenance work on thousands of groynes is time-consuming and expensive, and some of the groynes actually need replacing. Low water levels are more frequent now, impeding river traffic, and so the need for alternatives is growing.
Long-term alliance
The idea for the flexible groyne emerged from a long-term alliance between Rijkswaterstaat, several contractors (BAM Infra, Van den Herik-Sliedrecht) and Deltares. The alliance’s shared objective is to develop innovative solutions to manage our rivers more effectively and sustainably. The flexible groyne is made up of individual concrete blocks (Xstream) that interlock firmly. A major benefit of this design is that it makes maintenance more straightforward: extra blocks can be easily added. In addition, the length, height and depth of the flexible groyne can be adjusted quickly, which comes in useful when water levels fluctuate. During dry periods, a groyne can be extended temporarily to maintain the navigable depth. In addition, the flexible groyne is more sustainable than the traditional type because less greenhouse gases are released during production and maintenance, which helps to combat climate change.
Practical trial near Kampen
During the past two years, the flexible groyne has been tested in practice in a pilot project on the IJssel River near Kampen. This trial location was chosen because there was no groyne here in the past. The flexible groyne was designed with a steeper slope (approximately 1:1), greater roughness and higher permeability than the traditional type, all of which helps to improve the flow of the water and bank protection. Deltares used a range of criteria for testing the groyne, including functionality, sustainability and ecological value, monitoring the groyne during the trial period and conducting lifecycle and multi-criteria analyses.
The results of the trial indicate that the functionality of the flexible groyne is similar to that of the traditional variety in terms of regulating the flow into the shipping channel and protecting the bank. Less scour was seen at the head of the groyne, and the scour hole was not as deep and large as with traditional groynes. A few concrete blocks rolled into the scour hole but the slope itself was not affected. After two years, however, the scour hole is still not entirely stable.
Another important result is the sustainability of the flexible groyne. The steep slopes and open structure of the groyne require less material and resources, resulting in a lower environmental impact. In addition, the Xstream blocks can be produced and reused locally, benefiting the circular economy.
Follow-up research
Although the trial is promising, some questions remain unanswered, particularly in the area of river-ice flows. No ice formation was seen during the trial period and so the effect of the flexible groyne on ice-flow management has not yet been tested. This is an important issue because traditional groynes are specifically designed to guide ice and stop it blocking the shipping channel. In addition, further research will be needed to determine how the flexible groyne behaves in a range of weather conditions and water levels, and to study the long-term effects of the flexible structure on the river bed. The application of Xstream blocks in other projects provides valuable insights that can further strengthen the introduction of the flexible groyne in a range of river conditions.
The great thing about applied research is: you have an idea, you test it in practice, and you know right away if you need to go back to the drawing board (or not)
Robert Groenewege, river expert Deltares
Prospects
The flexible groyne offers major advantages as a sustainable alternative to the traditional groyne. It provides a more efficient and flexible way to keep rivers navigable while helping to reduce the ecological impact. In the years to come, Deltares will continue to contribute to the further development of this concept with the aim of increasing the applicability and effectiveness of flexible groynes and supporting the sustainable management of our rivers.
Robert Groenewege, river expert Deltares: “I started working on this project as soon as I joined Deltares two years ago. We drove to Kampen with a watering can and a vacuum-cleaner hose in the boot of the car. We walked out carefully onto the groyne and, balancing on the Xstream blocks, we poured dye into the water on one side of the groyne. Not long after, we could see it coming out on the other side. For me, that’s the great thing about applied research: you have an idea, test it in practice, and you know right away if you need to go back to the drawing board (or not). After the success of the flexible groyne, the ‘innovation team’ is now looking at scaling up and at other types of hydraulic-engineering structures where the Xstream blocks may be useful.”
