Dynamic coastal management
When the sea is calm and the waves lapping calmly on the tide line, the water brings sand to the beach. And the wind blows the sand into the dunes. When the wind blows very hard and the waves pound on the beach, sometimes whole chunks of the first row of dunes are knocked off and the wind leaves battered dunes behind. The sand ends up back in the sea. Some of that sand washes away, some is brought back to the beach by the wind and water.
Retention of sand
The play of water and wind determines where the coastline ends up; without human intervention, the coast appears to be eroding. Therefore, for years all kinds of interventions, such as rows of poles, reed fences and especially marram grass have been used to retain sand in the dunes. In this way, attempts were made, with varying degrees of success, to prevent erosion and maintain the coastline.
Maintain basic shoreline
Since 1990, Rijkswaterstaat has had the task of maintaining the basic coastline. This prevents the Netherlands from shrinking due to erosion. To maintain that basic coastline - which has been updated several times since 1990 - Rijkswaterstaat provides sand replenishment. To this end, sand is extracted from the deep North Sea with large suction dredgers from a ship to be sprayed just off or on the coast. The applied sand strengthens the coastal foundation, which consists of foreshore, beach and dunes. Some of the applied sand blows into the dunes. This form of coastal maintenance has caused solid accretion of dunes in many places.
Acidified dunes
By securing the dune with marram and poles, a static first row of dunes was created over the years: the sea verge. Then, since the 1990s, beach sand from the sand replenishments has been blowing against it. Thus, the first row of dunes continued to grow in the direction of the sea, and the sea bar became higher and higher. As a result, even more sand was held back. This ensures that hardly any sand gets through to the dunes behind it. These are also secured with marram grass, overgrown and acidified by nitrogen. Unfortunately, the calcareous beach sand - which counteracts acidification - does not really get there. This is bad for biodiversity and creates one-sided vegetation. Meanwhile, all kinds of specific plants have disappeared from the dunes.
Dune turnoff
If the fresh sand remains only in the sea wall, the dune becomes higher but hardly more solid. During a heavy storm, the sand from that high, narrow row of dunes easily beats off, and then the sand disappears back into the sea. So the dune does not fundamentally grow. And given the expected rise in sea level, this is necessary. It doesn't have to be fast, but with a little extra drifting sand, the dune continues to grow. So to get the sand further into the dune for a really sturdier dune volume, with a healthy soil, we have to take action. Because the sea level is rising, and for that we also need stronger coastal defenses.
Dynamic coastal management
To get the sand past the first row of dunes, the sea wall, into the dunes, various measures have been devised. A lot of experimentation has been done and it seems to work well. We can cap the dunes, we can create notches and we can dig drift pits. Relatively speaking, these are small interventions, which give the wind more freedom. Next, the accretion of the dunes is a natural process, in cooperation with the Department of Public Works, which takes care of the sand replenishment. What are these measures and how does it work?
Dunes capping
If the sea verge has become too high, preventing sand from sliding through and also making it vulnerable to erosion, we can excavate the upper part of the dune. Then space can be created again, so that the sand ends up in the dune behind it. Of course, the first row of dunes will remain more than high enough to withstand a storm. The excavated sand will be worked into the dune on the inside, thus making the dune stronger again.
Digging Kerfs
A notch is a V-shaped lowering in the first row of dunes. Wide on the beach side, the point in the dune. This acts like a funnel bringing the sand further behind the first row of dunes. How big the notch becomes depends on wind direction, sand availability and the height of the sea water during a storm. A well-functioning notch will drift out a little deeper by itself until it reaches equilibrium, and with enough wind and enough supply of sand, the notch will maintain itself. If the notch becomes too deep, we take management measures: we plant marram grass or close it again. If the notch threatens to grow thick, we dig it out again. The dune behind the crevice can rise up to one meter per year, increasing the volume of the dune and the strength of the flood barrier. This management is carried out by the permit holder on site.
Dust pits
Sometimes the excavation of dune tops and the digging of pits is combined with the construction of drift pits. A large part of the sand that has been deposited ends up in these pits and is then carried further into the dune. This way, the calcareous sand reaches further - which is good for biodiversity - and the dune grows further on.
Natural reinforcement
So with relatively small interventions, in addition to RWS's sand replenishment, we can reinforce the process of sand drift to the underlying dunes, thus allowing the dune to "grow" with the expected sea level rise. This is how we strengthen water safety. We do this mainly by making space after which nature takes over our work. This only happens in places along our coast where the dune as a flood barrier is much wider than just the first row of dunes.
Restoring biodiversity
At the same time, we are restoring biodiversity; calcareous sea sand gives rare species a chance to return to the dunes. That is why we are also digging holes in places where the dune is now just about firm enough to cope with a considerable rise in the sea level. This also gives lichens, dune violets and wheatears, for example, a chance.
