How does the Waterline work?
The Waterline was best known for its flooding defences. Until the 19th century, areas were flooded by breaking dams and dikes. By 1815, the ability to create floods was technically perfected using an ingenious system of intake and drainage to control the water level. Both the Old Dutch Waterline in 1672 and, more recently, the IJzer Line (Belgium) during the First World War proved that inundation was effective at stopping the enemy.
Basins and dikes
The power of flooding lies in the fact that while the water surface is the same everywhere, the ground underneath is not. A water depth of 40 to 50 cm was enough to render an area impassable and unnavigable. A military engineer divided the flooded areas into a number of basins of different water levels. Ordinary polder dikes were sometimes used for flood control. In other cases, flood dikes were built with a stop-log sluice to partially retain the water where the water flow crossed the basin’s edge.
Sluices and culverts
Sluices and culverts were used to fill and maintain the water level in the basins. Flood sluices within the Line were controlled with stop logs and gates. New techniques were also developed to control water. Former Inspector General of Public Works, Jan Blanken, for example, designed the waaiersluis, an flood sluice gate, that could be opened against high water.
Filling the basins
Initially, floodwater came from the river Lek (near Vreeswijk) and the river Vecht (near Muiden). The three basins north of the city of Utrecht were filled with salt water from the Zuiderzee (later the IJsselmeer). The three basins between Utrecht and the Lek were to be flooded with water from the Lek via the Vaartse Rijn canal, flood canals and the inlet sluice near Vreeswijk. The area between the Lek and Waal rivers was divided into to two basins. The basin north of the river Linge was filled with water from the Lek via an flood sluice gate near Werk aan het Spoel. The southern basin received water from the Waal via a sluice near Dalem. Both basins were also filled with water from the Linge via two flood sluice gates near Asperen.
A new basin was added in 1870. Thanks to the new flooding works north of the Lek and between the Lek and the Waal, it was possible to flood the area in only 3-4 days (formally 1-4 weeks).
In 1930, the flooding system was upgraded for a third time as a result of the Zuiderzee works and the digging of the Amsterdam–Rhine Canal. The canal divided one of the basins in two. This had two consequences. First, an attacker could raise the water and wash away the defence forces. And second, the canal drained the floodwaters away. To offset this danger, the Plofsluis was built on the Amsterdam–Rhine Canal. A barrier fort was added near Pannerden to prevent the enemy from blocking the water tower of the Nederrijn and Lek rivers.
The Defensive Flooding Operations Act
In addition to being a threat to the enemy, the Line also created problems for the residents of the flooding regions. Floodwaters drowned crops. Salts and minerals left behind by the floods rendered the soil useless for long periods. In the time of the Old Dutch Waterline, farms also broke dikes to let water drain from other areas already under water.
The Defensive Flooding Operations Act of 1896 made flooding safer and regulated damage compensation. Compensation was so favourable that farmers were happy when an exercise or mobilisation happened on their land. The Flooding Operations Act is still in force and was last changed in 1989 (for the New Civil Code) and in 1996 (for the Act establishing the Coordination Act for National Emergencies).
In war, an open line of fire was considered very important. So the region surrounding the defensive works was divided into concentric zones of 300, 600 and 1000 metres – the Prohibited Areas. Within these zones, all sorts of construction and agricultural regulations applied. The military authorities wanted to view each building plan. For example, the Prohibited Areas Act of 1853 specified that only wood-frame houses could be built within 300 metres of a defensive work, except with permission from the Minister of War. In the middle zone (300 to 600 metres) houses could be built with stone foundations (up to 50 cm above ground), and the chimney could be built in stone, but the rest of the structure had to be built from combustible materials. In the outermost zone, all building materials were allowed in theory, but in case of either an official state of war declaration or a mobilisation ordered by a military commander, all buildings, trees and other obstacles were to be cleared without any legal proceedings. Those who sustained damage received compensation. The clearing of buildings, houses and crops was necessary for an open line of fire. The Prohibited Areas protected Amsterdam, The Hague, Rotterdam, Utrecht and other towns and cities behind the Waterline. In addition, the capital was protected by the Stelling van Amsterdam.
The Prohibited Areas Act of 1853 had never been modified while in force. In 1951 the law was suspended, and then finally officially repealed in 1963. The 110-year old law had an enormous effect on spatial planning of the landscape within the Waterline. One of the most striking examples is the city of Utrecht. Until 1951, the city was forbidden from developing east due to the double lines of forts. And around the fortresses of Naarden and Weesp, there remain countless wood-frame houses along the Line as a reminder of the Prohibited Areas.
Access points were areas of higher ground that could not be flooded. Forts and other defensive works defended the country from these areas.
The best-known access point was the Houtense Vlakte, a higher-lying former riverbed east of the city of Utrecht.
Several forts were built here along the Kromme Rijn.The number of access points grew as more dikes were built as foundations for railways and roads. New defensive works were built to protect this infrastructure. In addition, new canals like the Amsterdam–Rhine Canal had consequences for how high the water level could rise during a flood. To make the canal unnavigable and maintain a high enough water level during a flood, a Plofsluis was added. This type of floodgate was filled with sand and gravel. During an attack, the underside could be exploded, thereby blocking the canal.
The integration of defensive works in the landscape is a unique feature of the New Dutch Waterline. Defence forces took advantage of a good view from the forts and a clear shooting range. But camouflage was important as well. Forts were camouflaged through a planned landscaping following the rural English landscape style. To care for the plants, the army even had its own nursery.
The plants had different functions depending on the section of the fort. Poplars, willows and elms along the moat helped the fort blend into the surroundings. Hawthorn, blackthorn, elderberry and rose bushes acted like barbed wire, making the fort difficult for the enemy to storm. Near the sentry boxes limewood and horse chestnut trees provided shade and beauty. Grass was planted in open fields, along slopes, and on rooftops to protect the soil. Grass planted on the rooftops of bombproof buildings ensured that rainwater infiltrated quickly into drinking water reservoirs. Water lilies and other plants were grown in the moat as camouflage from the enemy in the air. A fringe of reeds was maintained along the fort’s moat and other waterways.