By Richard Marijnissen
We Dutch take great pride in our massive dikes and delta works which have kept the sea at bay for centuries. As a hydraulic engineer myself, I naturally considered flood defences to remain at the forefront of future climate adaptation. However, in my PhD research at the University of Wageningen, I realized that strategically using the areas next to the flood defences may help to adapt more effectively to sea-level rise. Such is the case of the “Wide Green dike”, a pilot to strengthen the dikes with the clay trapped by the salt marsh plants growing along the north Dutch coast. This storyline is about my research into the safety of a Wide Green Dike, and the lessons learned that might help more regions adapt to sea-level rise.
The challenge
“How could flood protection and nature be combined into one integrated dike design?”
Dike reinforcement is of utmost importance in the Netherlands, a low-lying country whose primary flood defences should now comply with more strict safety standards to better prepare for a changing climate. However, the coastal areas next to these dikes also need to be preserved to maintain their ecological value. Two different uses, flood protection on the one hand, nature values, on the other hand, both competing for space, and both negatively affected by sea-level rise.
The Wide Green dike pilot aims to combine the two, and hence I investigated it as a case-study in my PhD.
The dikes at the north-eastern Dutch coast
The Ems-Dollard bay
In the north-eastern corner of the Netherlands, draining into the Wadden Sea is the Ems river estuary. Located within is the 100 square kilometres Dollard bay, which harbours typical salt marsh or salt-tolerant plant species and is a bird refuge of global significance. Nevertheless, the ecosystem is threatened by increasingly high sediment concentrations blocking sunlight from penetrating into the water. The centuries of intertidal area being converted to polders, deepening of the estuary for navigation and ongoing natural sedimentary processes in the bay, have all been linked to increasingly high turbidity in the estuary today.
Combining two worlds
Inspired by the wide grass dikes in Germany, the Regional Water Authority Hunze en Aa's in the Netherlands, started a pilot to combine the needs for nature and flood protection. The so-called "Wide Green dike" pilot is a wide and gradually sloping dike that merges smoothly into the salt marsh without additional asphalt protection at the seaward side of the dike. Most importantly, the dike will be reinforced over time with the excess sediment from the Dollard. Clay is extracted from the three locations shown in the map: 1) at the harbour of Delfzijl, 2) within polder Breebaart, and 3) from a borrow pit at the Wide Green Dike pilot site itself.
The Wide Green Dike pilot
The Wide Green Dike pilot started in 2018 by digging a clay pit. The borrow pit is approximately four hectares with an average depth of over one and a half meters and a breeding island for birds in the middle. The extracted marine clay was deposited more than a year ago on dewatering sections next to the pilot site to get the necessary clay.
The research question
For my research, I estimated how much clay is captured by the pits and the salt marsh versus how much clay is needed to reinforce the dike under various sea-level rise scenarios. The main question:
"Is reinforcing a dike with clay captured in the pits and salt marshes actually feasible to combat sea-level rise?"
The salt marsh, the pit and the dike
Wrapping all these complicated processes into manageable model calculations was perhaps the most challenging step of my research. I focused on the nine (9) most essential ones, rather than aiming to fully represent all processes.
What do the models predict?
The pit
The pit is already filling up with clay nicely and is projected to be refilled within 22 years (around the year 2040). As the sea-level rise accelerates in the future, the pit is flooded more frequently and clay is trapped in the pit more often. As a result, the pit will keep filling up faster in the high sea-level rise scenarios. In any case, my modelling results also show that 75% of the infilling takes place during the first ten years after the excavation of the borrow pit. Given that a deeper pit is more efficient in settling sediment, the remaining infilling will take longer with the increasing elevation of the bottom of the pit.
The dike by 2100
As long as the sea-level rise doesn’t accelerate too much the pit and salt marsh can accumulate enough sediment not to drown. However, in the highest sea-level rise scenario (H) the sea-level rises too fast for the salt marsh to keep up and the marsh drowns eventually. Waves break in shallow water, thus as the salt marsh lowers relative to the sea level, the dike has to repel stronger waves. Preventing the salt marsh from drowning in high sea-level rise scenarios is necessary to limit the reinforcement needed of the Wide Green Dike.
Balancing the ambitions
While the pit in the salt marsh will refill with clay, the clay in it will not have compacted as much as the clay deposited over decades inside the marsh. As a result, the amount of clay gathered from re-excavating a pit will always be lower.
"Should new clay be retrieved somewhere else in the salt marsh, or could the remaining salt marsh stay undisturbed by re-excavating the same pit over and over again?"
Under a low sea-level rise scenario, re-excavating the pit is not effective. The infilling is not fast enough to re-excavate the pit over three times this century. For a high sea-level rise scenario, (re-)excavating one pit is much more efficient considering that the clay infilling rate will accelerate with the sea-level rise. This allows for many more re-excavations, but also more clay for dike reinforcement will be needed.
There should be a balance with the intended natural values. For optimal sediment capture deep pits that are extracted often would be needed, but to allow new natural marshes to emerge shallow pits that are extracted infrequently might be more preferable.
Lessons learned
Reinforcing flood protection against sea-level rise to maintain safety against flooding remains one of the major challenges this century. Dikes are and will always be an integral part, but a wider green perspective will help cope with the challenge more effectively. In essence, we need to recognize and employ nature’s engineers, the salt marshes, to reinforce coastal protection.
The Wide Green Dike pilot is a good example of an integrated solution. The process of clay accumulation in the pits both aids in reducing turbidity in the estuary, and provides sufficient material for dike reinforcements in the future. Nevertheless, challenges remain to balance nature and flood protection values.
Moving forward
In my PhD research, I only scratched the surface of all the details needed to make this pilot a success. There are still many more questions those involved in the Wide Green Dike pilot are working on. For example:
- How much clay is needed on the Wide Green Dike exactly, taking into account everything from multiple storms to cows walking on the dike's slope?
- How much clay is actually accumulating inside the pit of the Wide Green Dike pilot?
- What does it take to turn the sediment from Dollard into clay suitable for a dike?
- and so much more
While many questions remain, the general scientific insights my studies found about the Wide Green Dike pilot will help the real implementation in the Dollard, and possibly elsewhere.