How Do You Build a City Where the Sea Was? Push the Water Back
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Large areas of several countries stand on ground that was recently seabed, lake bed or marsh, created deliberately by draining or filling. The engineering is well understood and the ground never quite stops moving.
The two basic methods
Land is made from water in one of two ways. Draining encloses an area with an embankment, pumps the water out and keeps pumping, leaving ground that sits below the surrounding water level permanently and depends on continuous operation to stay dry. Filling raises the ground instead, dumping sand, rock or excavated material into shallow water until it stands above the tide, which requires enormous quantities of material and produces land that does not need pumping. The choice depends on the depth, on what material is available and on what the ground is for, with draining suiting broad shallow areas and filling suiting harbours, airports and city extensions.
Where it has been done at scale
A few places have done this on a landscape scale:
- •The Netherlands, where a substantial share of the country is drained land
- •Singapore, which has grown its area by around a quarter by filling
- •Hong Kong and Macau, where much of the built centre stands on fill
- •Tokyo Bay, Osaka and several Japanese airports built offshore
- •Boston, Chicago and much of lower Manhattan
- •Dubai and other Gulf cities, including artificial islands
Why the ground keeps sinking
Made land subsides, and the reasons are unavoidable rather than the result of poor work. Draining removes water from soil, and soil that was held apart by water compacts once it is gone, which lowers the surface immediately and continues for decades. Peat soils shrink further because exposure to air lets them decompose, so the ground literally disappears. Filled land compacts under its own weight as the fill settles and as the soft seabed beneath it is squeezed, which continues for years and is why major airports built this way have required continuous jacking of structures. The result is that reclaimed ground generally sits lower than when it was made and requires ever higher defences.
How the Dutch did it
The country most associated with this has a sequence worth following, because the techniques developed over eight centuries. Early work built dykes around naturally drained marsh and used ditches and sluices to let water out at low tide, which works only where the land is above the low water mark. Windmills driving scoop wheels from the fifteenth century allowed water to be lifted rather than merely released, which made deeper areas workable, and mills were arranged in series to raise water in stages. Steam pumping from the nineteenth century enabled drainage of large lakes that mills could not manage. The largest scheme of all closed off an inland sea in 1932 and drained several enormous areas from it over subsequent decades, creating an entire province.
What it costs elsewhere
Making land destroys the place it was made from and moves the effects around. Intertidal mudflats and salt marshes are among the most productive habitats there are and are exactly what is easiest to reclaim, so the losses have been enormous and are the main reason those habitats are scarce. Filling requires vast quantities of sand, which is dredged from the seabed or bought from other countries, and Singapore's demand has been substantial enough that several neighbouring states banned exports, citing damage to their own coastlines. Dredging destroys the seabed communities where the material is taken. Changed water flow around new coastlines causes erosion and deposition elsewhere, which frequently requires further engineering.
The takeaway
Land is made either by enclosing an area and pumping it dry, which leaves ground below water level needing permanent pumping, or by filling shallow water with sand and rock. The Netherlands, Singapore, Hong Kong and Tokyo Bay have done it at landscape scale. Made ground subsides as soil compacts and peat decomposes, and reclamation destroys mudflats and requires dredged sand that damages other coastlines.