The dune landscape
The formation of the dune landscape
The dunes in the Netherlands are on the upper side of the banks. They consist of sand from the bottom of the sea or rivers. The question that rises is: how is it possible that sand from the bottom of the sea ends up on top of the dunes. The answer is that this primarily happens because of the tidal flows and the wave movements.
The tidal flows
There are three different types of tidal flow in the sea:
- The flood current,
- The ebb current, and
- The rest current.
The flood current transports water along the coast from south to north. The ebb current moves the opposite way and transports water from north to south. The water of the sea is in fact being moved up and down. Because of the strong power of the western wind however, the flood current is stronger than the ebb current. In the long term, more water is moved from south to north than the other way around. This is called the rest current. The rest current determines the direction of the sand movement along the coast.
The wave movements
The direction of the sand movement is determined by the rest current. Consequently, the wave movements actually move the sand inland. Close to the coast, the water gets shallower. The waves from the sea are curbed by the bottom and as a result, the waves ‘break’ and sand is detached from the sea bottom. Because of the breaking of the waves, a branding is created. The surf waves throw the sand up onto the beach during flood tide.
When the sand dries up during ebb tide, the wind can move further inland and banks can develop (the sand moves when the wind blows over the bold and dry loose sand with a speed of greater than five metres per second). Salt loving plants hold the sand together, resulting in dunes developing on top of the banks.
The salt-loving plants hold together the small dunes and stop the new sand grains blowing away. This means the dunes get higher and higher. After the rise, marrams settle, which is a type of grass highly suitable for holding sand together and which is resistant to dryness.
All small dunes eventually form a closed dune row. This dune row can develop into a sea strip: a solid seawall which is high enough to dam water during a storm flood. A sea strip can grow to more than 20 metres high and can either settle towards an existent sea strip or a little further away from it. The latter can for instance happen when a surf ridge gets so high that it remains uncovered during ebb tide. The sea water trapped between both sea strips may become fresh, and eventually a layer of peat or clay will originate. This new surface between the dunes is called a primary dune valley.
The origin of old dunes
The oldest surviving bank with dunes of the Netherlands was created about 5,000 years ago. Until the beginning of the era many new banks with dunes originated. These dunes gradually raised because the sea level kept on rising. The dunes that originated in this period, are called old dunes. The old dunes have a height of 10 metres. Primary dune valleys are situated between the rows of old dunes.
The origin of young dunes
About 500 years before Christ, the sea started to rise again. A very strong western wind caused storm floods, which wiped away large pieces of the coast. The old dunes were partly ruined at many places, and completely destroyed at others. For this reason, there are no old dunes along the south-west and northern coasts of the Netherlands.
A lot of sand was made free through of coastal erosion. This sand formed new young dunes in the period from 1200 until 1600 years after Christ. In North and South Holland, these young dunes are situated on top of the old dunes. The older the dunes, the further inland they are situated. Moreover, young dunes are much higher and have more relief than old dunes: the young dunes can grow to over 50 metres high.
As described earlier, the dune scenery is a dynamic scenery, always developing. Coastal erosion and high winds force the sand of the dunes to move again. This happens mostly with the young dunes, since these dunes have the least plants growing on them (as a result of which sand is not held together properly) and suffer most from the power of the sea.
In areas where the sand of the dunes is not held together properly (because there are too few plants or because plants have disappeared), wind erosion can occur. The wind blows holes in the dunes, resulting in so-called 'wind pits'. These wind pits can become so deep that they get close to the level of the groundwater.
The surfaces which are created by the wind, are called secondary dune valleys. Because secondary dune valleys lie so close to the ground water, they are often wet underneath. Sometimes, they even have a small lake inside them.
The closer to the sea, the more dynamic the dune scenery. The scenery keeps on changing because of the influence of the sea and the wind. Many plants cannot live in a salty environment with such dynamic circumstances. For this reason, the number of different plants close to the sea is limited. We could say that the more dynamic the scenery, the lower the diversity of the plants (also, the fewer types of plant). Plants which can stand these dynamic circumstances, are for instance sand oats and blue sea thistle. These plants only occur in the dunes.
Further inland, the number of different plants increases. The plants which grow in the dunes also occur in other places in the Netherlands. The combination of plants is unique, however, and typical for the dune area. Moreover, the further inland you get, the further the succession is. If you wish to learn more about what exactly succession is, you can read it here.
The soil usage
The soil usage in the old dunes is different to the soil usage in the young dunes.
The soil usage in the old dunes
The old dune scenery has little relief and was, partly because of its firm underground of banks, a good place for habitation. The wet underground however meant it was unsuitable for agriculture. The boggy beach surfaces were used as grass land and as an extension of the habitation area. First however, the beach surfaces had to be drained. On top of the banks, there is mostly forest.
Around the 17th century, the bulb culture was increasing. To be able to cultivate bulbs, you need a limy soil and a constant ground water level of 55 centimetres below the surface. To make this possible, the upper layer, which was deficient in lime, was removed, setting free the limy soil below. The soil was dug to 55 centimetres above the ground water level and the bulb culture was made possible. The soil surfaces created in this way are called ‘geests’.
The soil usage in the young dune scenery
The young dune scenery, which was full of relief, was not suitable for agriculture either. The young dunes were used as sea walls and for the storage of drinking water. The fact is, that under the dunes, is a fresh water bubble. This freshwater bubble is created because rain water drains into the dune sand. Because freshwater has a lower density than salt water, the rain water floats on top of the salt water. This creates a fresh water bubble.
To be able to extract the drinking water, water basins are placed in the dunes. In 1853, the first drinking water companies were founded, who extracted this drinking water from the dunes.
Problems in the dune scenery
There are a number of problems in the dune scenery. These problems are both caused by both nature and humans. One of these problems is coastal erosion.
The dynamics along the coast line of the Netherlands can cause problems. While the coast is broadened by the supply of new sand at some areas, coastal erosion occurs at others, because of the influence of water and wind. To fight coastal erosion, the threatened beaches are raised by the Ministry of Waterways and Public Works. Nevertheless, the Dutch coastline is still moving further inland.
Another problem of the dune scenery is withering. The withering of the dune scenery has a number of different causes.
One is the extraction of drinking water in the dunes. This may be advantageous for human beings, but it certainly is not for nature, because the dunes gradually dry up
Another cause is the withering of the polders by farmers. The farmers drain polders near the dune scenery to make them suitable for agriculture. Because of this drainage, the ground water level in the dunes decreases, which leads to withering.
Finally, withering can be caused by reforestation of the dunes. At many places, broad-leaved trees are replaced by coniferous trees. Coniferous trees do not lose their leaves and they evaporate all year long. As a result, they evaporate much more water than broad-leaved trees. This may also lead to withering.
Pollution and recreation
To fight withering, water has been pumped into the dunes since the fifties. This is called infiltration. However, this water is from rivers and therefore differs from the water that is originally in the dunes: water from rivers is nutritious, whereas water in dunes is not. As a result of this nutritious water, the dune flora was gradually replaced by new plants which did actually not belong in the dunes.
Pollution of the dunes is also caused by recreation. People who go to the beach may leave their waste on the beaches and in the dunes. They also walk on the dune plants responsible for binding the sand of the dunes. This makes the dunes more vulnerable to wind erosion.
It is very important that we take good care of the dune scenery!