How Do You Farm the Bottom of a Lake? Wall It Off and Pump

The term covers a tract of low-lying ground that has been surrounded by an embankment so that it is cut off hydrologically from the water outside it, and then drained. The essential point is the separation. Inside the bank the water level is controlled entirely by human decision, set by how much is pumped out and when, and it bears no relation to the level of the sea or river on the other side. That level is usually held well below the outside level, which means water flows in continuously through the ground and must be removed continuously. The arrangement is permanent maintenance rather than a single act of construction.

The sequence has been essentially unchanged for centuries:

• •Build a ring embankment around the area to be enclosed
• •Cut a ring canal outside it to carry away the water pumped out
• •Pump the enclosed water out, originally with windmills in stages
• •Cut a network of ditches inside to collect groundwater
• •Lead those ditches to a pumping station that lifts water to the ring canal
• •Keep pumping indefinitely, since seepage never stops

Draining low ground sets off a process that makes the problem worse over time, which is the central difficulty of the whole enterprise. Waterlogged peat and clay are held up partly by the water in them, so removing that water compacts the soil and the surface drops immediately. Peat exposed to air then oxidises and disappears altogether, lowering the surface further, year after year. That means the drained land sinks relative to the sea outside, so the difference in level grows, so more pumping is needed and the embankments must be raised. Large parts of the Netherlands now sit several metres below sea level as a direct result, and the deepest are close to seven metres below.

The pumping technology is the constraint that determined what could be attempted and when. Windmills fitted with a scoop wheel could lift water about a metre and a half, so draining anything deeper required a staircase of several mills each lifting to the next, which is why surviving groups of mills stand in lines. That limited practical depth and made large projects slow and unreliable, since a windless week stopped everything. Steam engines from the early nineteenth century removed both limits at once, lifting far higher and running regardless of weather, which is what made draining a substantial lake feasible. Diesel and then electric pumps followed, and modern stations move enormous volumes under automatic control.

The scale achieved in the Netherlands is difficult to appreciate from a map. Roughly a sixth of the country's land area has been made this way, including whole provinces created within living memory. A lake north of Amsterdam was drained in the 1850s to create farmland and now holds the national airport, sitting several metres below sea level on the old lake bed. An inland sea was closed off by a barrier dam completed in 1932 and then drained in sections over decades, creating an entire province with new towns, farms and roads on ground that had been under water. One further section was abandoned unfinished in the 1990s, when opinion shifted towards keeping water rather than removing it.

Enclosing low ground with a bank separates its water level from the sea outside, letting people set that level by pumping, which never stops because seepage never stops. Draining compacts the soil and oxidises peat, so the surface sinks further and the task grows. About a sixth of the Netherlands was made this way, including an airport on a drained lake bed and a whole province on a former inland sea.