Anthroposaurus
The Decay of a City
Abstract
Contents Updated: Wednesday, December 15, 1999
The Decay of a City
Just what would happen if a modern city were abandoned to nature? How long would it be before it began to crumble and how long would it take for obvious signs of it to disappear? This whole scenario has been described in a comprehensive article in the New Scientist (20 July 1996), written by Laura Spinney, a freelance science writer, of which this is a synopsis.
In geological terms civilization is merely skin deep. If a modern city-like London-were suddenly abandoned by its population, its inevitable decay would begin immediately. Any suburban gardener knows the perpetual battle against weeds and anyone living in a city can see the hardy weeds growing in neglected corners and pavement cracks, even part way up walls and in rooftop gutters. Hardy weeds like dandelions would be flowering in odd places within weeks of the disappearance of public services. Their long tap roots would make their way down into the narrow cracks into which the seed has nestled and begin to force them wider for more aggressive weeds, like elder, to exploit.
One of the commonest destructive weeds in Britain is not a native of the island. It is buddleia, brought from the Himalayas in Victorian times to offer a long flowering season and attract butterflies, it can now be seen growing in the most unlikely places. Buddleia grows fast and its many seeds are easily dispersed by the wind. It has powerful roots used to thin soil on rocky substrata, ideally suited to penetrating the bricks and mortar of modern buildings. In London and other urban centres it can be seen growing out of walls and eves. A buddleia can be a substantial bush in only a few years, prizing out bricks and stones from walls and leaving a thick organic litter of its dropped leaves and branches-its wood is soft and its branches easily break. The damp leaf litter traps dust falling from the walls by the action of bacteria, lichens and mosses, and the wind blown dust from elsewhere. Laura Spinney tells us that within five years, roads, pavements and city plazas will be covered with weeds and a rich turf of clover.
The reason why clover thrives is that it is a nitrogen fixing plant and the stone, brick and mortar dust at first is low in nitrogenous matter. Not for long though. Besides clover, more substantial plants can fix nitrogen-alder trees will be one of the first trees to occupy the virgin space of the abandoned city. The nitrogen such plants provide in the poor quality soil enriches it for other invaders. As the dust and plant litter thickens into a soil, deeper rooting plants can find a home. Seeds blown in or dropped by birds, which would shrivel in the sun on a hot tarmac road, find sufficient soil to take a hold and their bigger stronger roots break apart what is left of pavements and tarmacadam.
This is not conjecture. The town of Pripyat, built for the workers of Chernobyl and abandoned when the power plant went critical, now looks as if it had been hit by an earthquake, with conrete paving stones smashed and pushed up almost a metre by tree roots. And that in only a decade.
After as little as five years the streets will have built up a litter of grasses, fallen leaves and branches. At any time thereon only a long hot spell is needed to cause a fire hazard. Sunlight focused through a partly filled bottle or a lightning strike could ignite the litter and the city would be engulfed. The fire releases the nutrients from the organic litter and adds to it that from the newly gutted buildings. The remains of furnishings and roof timbers are added to the deepening and enriching layer of soil. Before the fire the town merely looked derelict but now it looks destroyed. Rose bay willow herb, the characteristic leggy purple weed of wartime bombed sites will be rampant.
In low lying places like London there is another destructive force waiting to assert itself when the human population had departed. Flood. The Thames flood barrier was built because of the growing danger of London flooding as it slowly sinks into the North Sea. In January 1996 it had to be shut for three consecutive tides to prevent a combination of storm and spring tides from endangering the city. Once the control of the barrier had gone a surge tide would pour into the central area and damage the embankments. Thereafter lesser surges would cause flooding through the damaged parts of the embankment and further decay would be rapid. Soon flooding at the spring tides would be commonplace. The defences against river and tide would erode and subside until flooding would occur several times a year and the low lying areas would revert to the marshes and water meadows that they were originally.
About ten years only have passed but already the willow herb will be yielding to marsh and meadow plants. Animal wildlife will have found its way along the long corridors of the abandoned road and motor ways, tube trains and railway lines into the central area. Certain protected areas in central London already prove that species of plants unusual in urban sprawls, like the common orchid, can strike root given freedom from molestation by mankind. Within ten years the centre will be full of kestrels, foxes, hedgehogs and bats, not to mention frogs, newts and toads in the newly established marshes. Birch saplings will be thriving.
Pet dogs will have gone wild and the larger breeds possibly hybridized with wolves from abandoned zoos. Cats would certainly be thriving. Feral cats thrive now but largely on the kindness of old ladies. Nevertheless they kill a lot of prey, even if they do not necessarily eat it because they have access to nourishing KittyBits or even waste bins. Once the old ladies had departed the feral cats would soon learn to eat their kills. Pigeons, house mice and sewer rats however will find the going too tough without their providers, the humans. Voles, field mice and shrews would return.
Many of the buildings are still standing proud. Though blackened with fire, overgrown with creepers and with buddleia growing from their ledges, the more solid buildings are surviving. In the city, the modern steel glass and concrete buildings just look neglected among the foliage at ground level. Windows are broken and the concrete is stained with soot but the absence of pollution in the form of acid rain actually suits the alkaline nature of concrete. They are a haven for cliff nesting birds like kestrels and sparrowhawks. Still, even these buildings are strong only in the short term. In a few decades they will be capped with ivy growing down from the rooftops, dropped by birds. Ivy will also be creeping up the walls, eventually reaching a height of over 100 feet.
The basis of their structure is steel and steel is not long lived. The metal bridges over the Thames like Blackfriars Bridge, unserviced by maintenance gangs and painters would be falling into rust after only 30 years. After fifty years the bridges are collapsing eventually to leave only their masonry piers poking out of the river. The debris from the collapsed bridges would partly obstruct the river forming a series of weirs which salmon would be, by now, using on their way up the river to spawn.
After about thirty years the central areas will be impenetrable because of the growth of a thicket of elder plants 15 feet in height. In more open spaces the birch saplings are looking like mature trees, forming a birch wood. Birch trees are happy growing on rubble. Sycamores and maple are also establishing themselves on the thickening layer of soil. Wooden buildings will be the first to have disappeared, lost by fire and rot. At the same time all the trappings of the interiors will have decayed away, prey to funguses or foraging insects.
Even after fifty years the solidly built buildings will still be recognizable, rising out of the forest, but more and more walls crash down as the years pass, undermined by flood or roots. The fallen rubble is rapidly covered in leaf litter and mud in the floodable areas, and so pass out of sight. After three or four centuries nothing will remain but hummocky piles under the canopy, and even the city centre skyscrapers, their steel cores rusted into weakness have collapsed into piles of broken glass, concrete and rust. Carbon dioxide dissolved in the rainwater has gradually eaten away at the alkaline cement until the enclosed steel structure becomes accessible.
Those subject to flooding go first, the decaying vegetation in its flooded cellars, being acid, rotting the concrete faster than elsewhere. The rusted material is bulkier than the steel. So once oxygenated water gets access, decay is swift, the rusting steel actually cracking open the sound concrete which remains. Flooding at the base also allows the building to tilt, accelerating a collapse. After a few centuries, just before they collapse some of the tower blocks will look like drunken men, leaning here and there. Unlike New York the buildings of London are not built upon natural solid rock but upon huge rafts of concrete suspended in the clay. When these rot the towers tilt and collapse. The biggest and strongest of them will not last longer than 500 years, by which time London will have reverted to nature. The river would now be much wider, the artificial embankments which channeled it unnaturally having long gone.
Beyond this time, signs of human occupation still exist but they are more subtle. The forest is not quite natural because of the exotic species which still survive having been brought here by mankind. Sycamores abound and sweet chestnuts are doing well. Some bird species like certain parakeets can survive. Wolves, or hybrid dog-wolves, feed on feral deer from fancy estates and feral pigs.
After a thousand years, the forests and marshes will now be mature and nothing will remain visible of the Great Wem. No building is likely to be standing. Any that are, will be old fashioned stone built types built of large cut slabs proving a natural stability independently of mortar.
Thus it is that older structures of a simpler technology can last longer than modern structures. Almost everything that we make in the modern world will disappear into dust or rust in a thousand years. In 65,000 times that period, what can we expect to see of such a civilization.
