Overflow Generator

In November ’20 and again in February - March ‘21 the overflow generator was installed on several strips of the levee in the Living Lab Hedwige-Prosperpolder. The overflow generator is a set-up that can simulate overflow of the levee which occurs when the water level of the river exceeds the height of the levee. The water is supplied to the generator via pumps. Water continuously flows down the slope of the levee via a demarcated test strip. The intensity of the overflowing water can be changed and cameras and other flow measurement devices are used to monitor the tests and state of the levee. 

Number of tests

A number of overflow tests have been done to determine the erosion resistance of the vegetation and cover layer during various levels of water flow intensity and in different circumstances. The effect of varying overflow heights and discharges was studied, but also of other factors such as animal activity or the type of covering. Two examples of tests will be discussed in more detail; the tests with the presence of a tree and the tests on a repair measure with geotextile (EPDM) anchored with sandbags. 

"My take home message from this series of tests: once it goes wrong, it goes very quickly."

André Koelewijn

Presence of a tree

The first example is the overflow test with a tree. In November 2020, a test was performed on a tree to evaluate the scour around the tree stem and its influence on levee stability. Due to the fast collapse of a cliff along the slope, the purpose of that test was not achieved and therefore repeated in February 2021. At the end of day one, there was some erosion at the toe, including large parts of the vegetation that were washed out, but the tree was still standing. On the levee slope, removal of vegetation at discrete places was observed, but not to the extent that it compromised the levee slope stability. 

During the second day, damage was created after a couple of small holes were drilled on the slope of the levee to enhance the inflow of water into the sand core, simulating the presence of small animal burrows. After some additional runs, sand accumulation was present at the toe, and a depression was visible on the levee slope. After another two minutes this depression quickly manifested into a collapse of the cover layer. 

Why did this happen? The hypothesis is that the entry of water into the levee due to a small natural burrow and the added drilling holes, has created an elevated water level and pressure inside the levee core. This led to water flow inside the sand core and sand being transported outwards at a lower point near the toe. In this process, the artificial holes played a role, only speeding up the inflow of water in the levee core and it is thought that the natural (reactivated) holes mainly caused the failure. In the end, sand entrainment and outflow led to mass deficit under the levee cover and subsequently to levee slope collapse. There was no specific relation with the tree. 

The main learnings here are:

• Burrows or holes that create contact between the surface water and the levee core are a major risk.
• Collapse evolves very quickly and leaves no or little time for emergency response in a real overflow situation. 

"Two things stay with me after this test. The speed of the collapse evolution was impressive. And the realisation that even small burrows may be a risk for levee safety."

Davy Depreiter

Testing repair measure

The second test is the overflow test on a repair measure. In December ‘20, the damage at this section was repaired with geotextile (EPDM) and sandbags. The upper part of the foil was placed under the cover layer, through an incision, to prevent water from flowing underneath the foil. The sides of the foil were attached with anchored sandbags. The focus of the overflow tests was on the upstream transition between grass cover and geotextile. In addition, during the second day it was tested how well the sides of the repaired sections would resist overflowing water. 

As the water near the toe contained sand on the second test day, it was obvious that the water could easily flow underneath the EPDM sheet from the sides, and reach the damaged part of the levee, resulting in more erosion. The damage under the sheet was not visible, however, the sides of the damage became unstable, and whole lumps of soil fell into the hole. Also stress to the side wings exerted by the foil withstanding overflow reinforced this process. For safety reasons, this test was stopped.

"We saw very exciting aspects. Whole lumps of soil collapsed into the hole, which made us stop the test, due to safety reasons."

Stephan Rikkert

The main lesson is that the attachment method of the foil to the undamaged part of the levee is very important. There are many different types of anchors to attach the sheet, but once water can flow underneath the sheet it is not very effective anymore. 

Further research

The overflow test results ask for further research, for instance, a follow-up test looking into the attachment of the foil at the sides and the effectiveness of different kinds of anchors. To get a better understanding of levee strength, further experiments, such as wave overtopping tests and a breach experiment, are planned at a later stage of the project. These experiments will provide further insight into the behaviour of the levee as well as emergency measures during extreme conditions. Knowing what can go wrong, will make our levees stronger in the end!