Flood control in the Netherlands
is an important issue for the Netherlands, as due to its low elevation, approximately two thirds of its area is vulnerable to flooding, while the country is densely populated. Natural sand dunes and constructed dikes, dams, and floodgates provide defense against storm surges from the sea. River dikes prevent flooding from water flowing into the country by the major rivers Rhine and Meuse, while a complicated system of drainage ditches, canals, and pumping stations keep the low-lying parts dry for habitation and agriculture. Water control boards are the independent local government bodies responsible for maintaining this system.
In modern times, flood disasters coupled with technological developments have led to large construction works to reduce the influence of the sea and prevent future floods. These have proved essential over the course of Dutch history, both geographically and militarily, and have greatly impacted the lives of many living in the cities affected, stimulating their economies through constant infrastructural improvement.
History
The Greek geographer Pytheas noted of the Low Countries, as he passed them on his way to Heligoland BCE, that "more people died in the struggle against water than in the struggle against men". Roman author Pliny, of the 1st century, wrote something similar in his Natural History:There, twice in every twenty-four hours, the ocean's vast tide sweeps in a flood over a large stretch of land and hides Nature's everlasting controversy about whether this region belongs to the land or to the sea. There these wretched peoples occupy high ground, or manmade platforms constructed above the level of the highest tide they experience; they live in huts built on the site so chosen and are like sailors in ships when the waters cover the surrounding land, but when the tide has receded they are like shipwrecked victims. Around their huts they catch fish as they try to escape with the ebbing tide. It does not fall to their lot to keep herds and live on milk, like neighboring tribes, nor even to fight with wild animals, since all undergrowth has been pushed far back.
The flood-threatened area of the Netherlands is essentially an alluvial plain, built up from sediment left by thousands of years of flooding by rivers and the sea. About 2,000 years ago most of the Netherlands was covered by extensive peat swamps. The coast consisted of a row of coastal dunes and natural embankments which kept the swamps from draining but also from being washed away by the sea. The only areas suitable for habitation were on the higher grounds in the east and south and on the dunes and natural embankments along the coast and the rivers. In several places the sea had broken through these natural defenses and created extensive floodplains in the north. The first permanent inhabitants of this area were probably attracted by the sea-deposited clay soil which was much more fertile than the peat and sandy soil further inland. To protect themselves against floods they built their homes on artificial dwelling hills called terpen or wierden. Between 500 BC and AD 700 there were probably several periods of habitation and abandonment as the sea level periodically rose and fell.
The first dikes were low embankments of only a meter or so in height surrounding fields to protect the crops against occasional flooding. Around the 9th century the sea was on the advance again and many terps had to be raised to keep them safe. Many single terps had by this time grown together as villages. These were now connected by the first dikes.
After about AD 1000 the population grew, which meant there was a greater demand for arable land but also that there was a greater workforce available and dike construction was taken up more seriously. The major contributors in later dike building were the monasteries. As the largest landowners they had the organization, resources and manpower to undertake the large construction. By 1250 most dikes had been connected into a continuous sea defense.
The next step was to move the dikes ever-more seawards. Every cycle of high and low tide left a small layer of sediment. Over the years these layers had built up to such a height that they were rarely flooded. It was then considered safe to build a new dike around this area. The old dike was often kept as a secondary defense, called a sleeper dike.
Image:Plompetoren 3.jpeg|thumb|alt=Aerial photograph of a white stone tower near the shore|The Plompe toren, the only remainder of the village Koudekerke
A dike could not always be moved seawards. Especially in the southwest river delta it was often the case that the primary sea dike was undermined by a tidal channel. A secondary dike was then built, called an inlaagdijk. With an inland dike, when the seaward dike collapses the secondary inland dike becomes the primary. Although the redundancy provides security, the land from the first to second dike is lost; over the years the loss can become significant.
Taking land from the cycle of flooding by putting a dike around it prevents it from being raised by silt left behind after a flooding. At the same time the drained soil consolidates and peat decomposes leading to land subsidence. In this way the difference between the water level on one side and land level on the other side of the dike grew. While floods became more rare, if the dike did overflow or was breached the destruction was much larger.
The construction method of dikes has changed over the centuries. Popular in the Middle Ages were wierdijken, earth dikes with a protective layer of seaweed. An earth embankment was cut vertically on the sea-facing side. Seaweed was then stacked against this edge, held into place with poles. Compression and rotting processes resulted in a solid residue that proved very effective against wave action and they needed very little maintenance. In places where seaweed was unavailable, other materials, such as reeds or wicker mats, were used. Another system used much and for a long time was that of a vertical screen of timbers backed by an earth bank. Technically these vertical constructions were less successful as vibration from crashing waves and washing out of the dike foundations weakened the dike.
Much damage was done to these wood constructions with the arrival of the shipworm, a bivalve thought to have been brought to the Netherlands by VOC trading ships, that ate its way through Dutch sea defenses around 1730. The change was made from wood to using stone for reinforcement. This was a great financial setback as there is no naturally occurring rock in the Netherlands and it all had to be imported from abroad.
Current dikes are made with a core of sand, covered by a thick layer of clay to provide waterproofing and resistance against erosion. Dikes without a foreland have a layer of crushed rock below the waterline to slow wave action. Up to the high waterline the dike is often covered with carefully laid basalt stones or a layer of tarmac. The remainder is covered by grass and maintained by grazing sheep. Sheep keep the grass dense and compact the soil, in contrast to cattle.
Developing the peat swamps
At about the same time as the building of dikes the first swamps were made suitable for agriculture by colonists. By digging a system of parallel drainage ditches water was drained from the land to be able to grow grain. However, the peat settled much more than other soil types when drained and land subsidence resulted in developed areas becoming wet again. Cultivated lands which were at first primarily used for growing grain thus became too wet and the switch was made to dairy farming. A new area behind the existing field was then cultivated, heading deeper into the wild. This cycle repeated itself several times until the different developments met each other and no further undeveloped land was available. All land was then used for grazing cattle.Image:KinderdijkMolens klein.jpg|thumb|right|alt=Typical Dutch scene with a series of windmills along the waters edge|The windmills of Kinderdijk, the Netherlands
Because of the continuous land subsidence it became ever more difficult to remove excess water. The mouths of streams and rivers were dammed to prevent high water levels flowing back upstream and overflowing cultivated lands. These dams had a wooden culvert equipped with a valve, allowing drainage but preventing water from flowing upstream. These dams, however, blocked shipping and the economic activity caused by the need to transship goods caused villages to grow up near the dam, some famous examples are Amsterdam and Rotterdam. Only in later centuries were locks developed to allow ships to pass.
Further drainage could only be accomplished after the development of the polder windmill in the 15th century. The wind-driven water pump has become one of the trademark tourist attractions of the Netherlands. The first drainage mills using a scoop wheel could raise water at most 1.5 m. By combining mills the pumping height could be increased. Later mills were equipped with an Archimedes' screw which could raise water much higher. The polders, now often below sea level, were kept dry with mills pumping water from the polder ditches and canals to the boezem, a system of canals and lakes connecting the different polders and acting as a storage basin until the water could be let out to river or sea, either by a sluice gate at low tide or using further pumps. This system is still in use today, though drainage mills have been replaced by first steam and later diesel and electric pumping stations.
Image:Kruik 1.JPG|thumb|left|alt=Round brick building of gothic architecture with steel beams protruding from the windows|De Cruquius is one of the three pumping stations that drained the Haarlemmermeer
The growth of towns and industry in the Middle Ages resulted in an increased demand for dried peat as fuel. First all the peat down to the groundwater table was dug away. In the 16th century a method was developed to dig peat below water, using a dredging net on a long pole. Large scale peat dredging was taken up by companies, supported by investors from the cities.
These undertakings often devastated the landscape as agricultural land was dug away and the leftover ridges, used for drying the peat, collapsed under the action of waves. Small lakes were created which quickly grew in area, every increase in surface water leading to more leverage of the wind on the water to attack more land. It even led to villages being lost to the waves of human-made lakes.
The development of the polder mill gave the option of draining the lakes. In the 16th century this work was started on small, shallow lakes, continuing with ever-larger and deeper lakes, though it was not until in the 19th century that the most dangerous of lakes, the Haarlemmermeer near Amsterdam, was drained using steam power. Drained lakes and new polders can often be easily distinguished on topographic maps by their different regular division pattern as compared to their older surroundings. Millwright and hydraulic engineer Jan Leeghwater has become famous for his involvement in these works.