Polder

• 1. Introduction

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  A polder is reclaimed land, drained and diked off from open water, and during the First World War Allied engineers used one as a weapon: opening the sluices along Belgium's Yser River at high tide and shutting them at low tide until the surrounding polders became an inaccessible swamp, helping stop the advancing Imperial German army. So a polder can be farmland or a tool of war, depending on who controls the water. What exactly is this strip of engineered land, and why does keeping it dry take constant, permanent effort?

• 2. What a Polder Actually Is

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  A polder is a low-lying tract of land enclosed by dikes, and it comes in three forms: land reclaimed from a lake or the seabed, floodplains cut off from a river or the sea by a dike, and drained marshes, sometimes called koogs in Germany. Ground inside a drained marsh subsides over time, and because peat decomposes once it is exposed to air, a polder built on former peatland sinks lower than its original level.

  Water works its way into a polder constantly, through groundwater infiltration, rainfall, and river or canal flow, so the excess has to be pumped out or drained through sluices opened at low tide. Dikes built from sand risk collapsing once saturated with water, while dry peat, being lighter than water, can fail to hold it back in dry seasons; muskrats compound the problem by tunneling into the barrier itself, and several European countries hunt the animal specifically because of it.

• 3. A Dutch Specialty

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  The Netherlands earned its association with polders because its engineers became internationally known for developing the techniques needed to drain wetlands and turn them into usable farmland, giving rise to the saying that God created the world, but the Dutch created the Netherlands. The country has reclaimed as much as 20 percent of its own land area from the sea at one point or another, holds roughly 3,000 polders, accounting for about half of the total polder area found across northwest Europe, and by 1961 about half of its own territory, some 6,800 square miles, had been reclaimed. The first embankments in Europe date to Roman times, the earliest polders to the 11th century, and the oldest polder still standing, Achtermeer, to 1533.

  The IJsselmeer stands as the country's most famous polder project, but it followed a long line of major reclamations: Beemster, drained between 1609 and 1612, Schermer between 1633 and 1635, and Haarlemmermeer in 1852, a polder that today contains Schiphol Airport. The 20th-century Zuiderzee Works added Wieringermeerpolder in 1930, Noordoostpolder in 1942, and Flevopolder, completed in stages between 1956 and 1968, which now ranks as the largest artificial island on Earth.

• 4. Named Polders Worldwide

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  The windmills of Kinderdijk, a UNESCO World Heritage Site, stand inside the Alblasserwaard polder, while nearby Beemster carries the same World Heritage status; the Mastenbroek polder, drained around 1363 to 1364, ranks among the oldest medieval polders still in use, and Zuidplaspolder, along with Denmark's Lammefjord, marks the lowest point in the European Union. Bangladesh's polders are cultivated for agriculture as well as flood defense, reducing long-term waterlogging after the storm surges that follow tropical cyclones.

  In Germany, land reclaimed by diking is called a koog rather than a polder, managed historically under a Deichgraf system similar to the Dutch model and made famous by Theodor Storm's novella The Rider on the White Horse; the Nordfriesland district alone contains 36 koogs, and neighboring Dithmarschen another 12. Southern Germany flips the term on its head: there, a polder means a retention basin recreated by deliberately opening a dike during river floodplain restoration, the near opposite of holding water back along a coast. In Japan, engineers reclaimed Lake Hachirōgata in Akita in 1964, creating an entirely new settlement, the town of Ōgata, on the drained lake bed. Polders also underlie parts of two American cities, New Orleans and the Sacramento-San Joaquin River Delta, showing the same Dutch-born technique put to work an ocean away from where it began.

  Several cities in Brazil's Paraíba Valley, in the state of São Paulo, sit on polders claimed from the floodplains around the Paraíba do Sul river, while the Chinese city of Kunshan alone contains more than 100 separate polders, distinct from the far older reclamation projects further north in Jiangnan. In the United Kingdom, part of the village of Meathop, near Grange-over-Sands, was reclaimed only as a side effect of building a railway embankment, and further examples stretch from Kerala's Kuttanad region in India to the Black Bush Polder along Guyana's Corentyne river in Berbice.

  Belgium concentrates a dense cluster of its own polders along the coast and the Scheldt estuary, among them De Moeren near Veurne, the polders of Muisbroek and Ettenhoven around Ekeren, and Prosperpolder near Doel and Antwerp. Canada's Bay of Fundy region holds a comparable cluster, including the Tantramar Marshes, the Holland Marsh, and the Minas Basin near Grand Pré, Nova Scotia, all reclaimed using the same diking principles first perfected an ocean away in the Netherlands.

  France's Polders de Couesnon sit near the tidal abbey of Mont-Saint-Michel in Normandy, while Denmark clusters its own polders around Filsø, Kolindsund, and Lammefjorden. Romania's stretch of the Danube Delta forms one of the largest wetland reclamation zones in Eastern Europe, and in Italy, polders line the delta of the river Po, including the Bonifica Valle del Mezzano.

• 5. Governing the Water

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  Repeated flooding disasters led the Dutch to establish water boards, called waterschap when situated more inland and hoogheemraadschap in the Holland region nearer the sea, tasked with maintaining the dikes protecting each polder, the waterways running through it, and the various water levels inside and outside its borders. These boards hold their own elections and levy their own taxes, independent of other government bodies, making them among the oldest continuously functioning democratic institutions in the country; the cooperation they demanded across every rank of society gave its name to the Dutch tradition of consensus politics known as the Polder Model.

  The catastrophic 1953 flood forced a rethink of how dikes get designed, replacing older assumptions with a calculation of risk as probability multiplied by consequences. That approach set an acceptable flood risk from the sea at once in every 4,000 to 10,000 years, and once in every 100 to 2,500 years for river flooding, a policy the Dutch government still updates as new threat data comes in.

• 6. Exported Around the World

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  The word polder itself traces back through Middle Dutch polre and Old Dutch polra to pol-, meaning a piece of land raised above its surroundings; the word has since been borrowed into 36 languages. Bangladesh alone has 139 polders, 49 of them facing the sea and the rest strung along the branching channels of the Ganges-Brahmaputra-Meghna delta, built in the 1960s to hold back tidal flooding and salt water.

  China's Jiangnan region, on the Yangtze River delta, built polders as early as the 10th through 13th centuries, administered through a self-monitoring system called the Lijia, which grouped 110 households under a headman known as a lizhang. A grain chief, or liangzhang, managed each polder's water system, while a dike chief, or tangzhang, kept the embankments themselves in repair, a division of labor that let a technique born in the Netherlands take root independently, thousands of miles away, under a completely different bureaucracy.