Raising and reinforcing flood embankments
Questions and Answers on Frequently Asked Questions: Extra balancing lake Onlanden
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Why do we raise flood embankments ?
The flood embankments are too low in some places. Currently, the maximum water level is –0.20 metres. After raising the flood embankments, the water level can rise to +0.18 metres when the controlled balancing lake is used. By expanding the balancing lake, it is possible to store an additional 5.2 million cubic metres of water.
During extreme rainfall, water levels may rise emporarily. In such cases, De Onlanden can hold additional water for a few days before it is discharged into the sea. To ensure this can be done safely, the flood embankments must be made higher and stronger. This will allow us to store more water during periods of extreme rainfall.
Why are flood embankments not raised to the same height everywhere?
Raising and reinforcing the flood embankments are an important part of the Onlanden Optimisation. At various locations, the flood embankments are currently not high enough to hold a water level of NAP +0.18 metres when the entire balancing lake is in use.
The design of the flood-embankment reinforcement is based on a target year of 2050. Completion is scheduled for 2027. Stability and settlement calculations have demonstrated that after they have been raised the flood embankments will meet the required final height in 2050. For this reason, the flood embankments are built with what is known as a “surplus height.” This means that they are built higher than the height required in 2050. This approach accounts for subsidence, consolidation, and settlement, ensuring that the flood embankments will remain high enough in the long term.
What is the difference between a hard and a soft flood embankment?
A soft flood embankment is a green flood embankment without surfacing on top. A hard flood embankment does have a surfacing on the crest. A hard flood embankment does not have to be as high as a soft flood embankment, because the surfacing provides additional protection. This results in different crest heights: NAP +0.68 metres for soft flood defences and NAP +0.48 metres hard flood defences.
Where does the soil come from that we use to raise the flood embankments ?
Where possible we use soil from the area itself. This soil comes from turfing work. We also remove soil from the wide, sloping sides of the existing flood embankments. We refer to these sides as the slopes of the flood embankment.
If there is still not enough soil, we will bring in clean soil from outside the area. In the case of hard flood embankments, we remove the surfacing on top of the flood embankment temporarily. After raising the embankment, we will replace the surfacing. The soil comes from Cosun. Approximately 45,000 m3 of soil will be brought in from the region.