Milner on Biodiversity #12: Future Proofing Cities

March 9, 2023

As I indicated last month (Blog #11), authorities around the world are becoming increasingly concerned about the likely impact of climate change on cities and city life, especially the effects of extreme weather events. A month’s rain fell in a single hour recently in Auckland, New Zealand; few cities could cope with this kind of assault, but when even moderate cloudbursts have recently resulted in deaths, damage and disruption, perhaps some new thinking and action is urgently needed. In London where relatively minor downpours often cause local flooding even now, I fear for the functioning of the city as a whole in a more turbulent future. At least London has the good fortune to be built on fairly level ground while many other cities are not so lucky. Flooding is often exacerbated by inadequate drainage and poor-quality building construction, a combination which frequently turns an inconvenient problem into a lethal one, with spiralling overall costs.

Paradoxically, the current approach to groundwater management has also resulted in making drought conditions worse. They paved paradise, put up a parking lot as Joni Mitchell sang. The predominant strategy of most urban authorities seems to be to cover as much of the land surface with impermeable concrete, for example in London by allowing front gardens to be turned into parking lots, covering open spaces and reducing greenery. ‘Our cities are designed to get rid of the most precious resource we have, as quickly as possible’ says Bruce Reznik, director of a sustainable water NGO in Los Angeles, where recent storms have failed to ease the area’s water shortages. In London drought conditions are common in summer months, a trend that is likely to intensify, putting all greenery under stress, while vast amounts of rainwater and greywater is wasted by sending it down drains. In my own street in north London saplings are constantly having to be renewed when they fail to get established due to a lack of accessible groundwater; evidence that the obsession with tree-planting is not supported with appropriate aftercare.  

Under ‘normal’ circumstances, rainwater’s destination depends largely on the intensity of the precipitation. Drizzle mostly evaporates again or ends up in groundwater.  Heavier rain rapidly fills watercourses which may easily overflow; in cities it is carried away in street drains, most of it ending up in urban waterways like rivers, canals and lakes, with only a small proportion of the total actually sinking into the ground.  Cloudbursts or other violent storms can rapidly overwhelm ordinary drainage systems. In nature major downpours neither evaporate nor are carried away; the water accumulates on site filling up low-lying areas – ponds, lakes and marshes, and then gradually percolates down to replenish the groundwater. River meanders, flood meadows and marshland effectively smooth the effect of violent weather events creating conditions for both vegetation and wildlife to thrive. Cities by their predominant design do the opposite, but as climate change effects intensify could nature’s way offer a way forward? Could cities be adapted to contain more excess rainwater rather than trying to funnel it out as fast as possible?

In the countryside a natural approach is increasingly being used to regulate the flow of rivers; levees along both the Rhine and the Danube having been deliberately breached in recent years restoring large areas of natural flood meadow, and this approach is now being actively pursued on the Somerset Levels and in the East Anglian Fenland in the U.K. There are several advantages of this sustainable approach; groundwater is replenished, soil fertility benefits from annual inundation, and nature benefits enormously from wildflowers and birds to fish and frogs.

Some cities are experimenting with a completely novel approach to water management. In part of East Los Angeles the innovative Sustainable Stormwater Capture Project is already starting to blossom. New parks, sinkaways and ‘drywells’ have been constructed to retain stormwater rather than waste it, partly financed by a special ‘polluter pays’ tax on every square metre of impermeable surface. The funds raised have been used to develop new parks and enable a spectacular regreening of the area that is popular with residents. A similar approach to sustainable water management is visible in other cities around the world where water conservation measures are a condition of the design of all new developments, so that gardens and other green areas can ensure the capture of rainfall on a regular basis.  

In China, where flooding due to violent rainfall is a particular concern, this idea is being taken further. The idea of whole ‘sponge cities’ has been developed with wetlands and woodlands being promoted within an entire urban area so that excess rainfall can be absorbed rather than overwhelming local drainage systems ‘aligning the urban environment more closely with natural forest systems’ according to official publicity. Taking matters even further Liuzhou Forest City claims it is ‘set to challenge perceptions about urban living’ by effectively integrating forest with buildings in a comprehensive design for a whole new city. These ideas are effectively sustainable and multi-purpose, countering pollution and improving the environment for the benefit of people and nature while maximising retained water and minimising runoff and potential disaster.

Of course, not all the flooding problems in urban areas are the result of excess precipitation on the city itself but are caused by excess runoff from deforested areas upstream, especially mountain areas. Loss of upland forest and woodland usually results in restricted vegetation and relatively unstable soils that are incapable of absorbing heavy rainfall. Excessive erosion then results in the siltation of watercourses leading to downstream flooding or worse. In the English Lake District a few years ago a major road was washed away during heavy rain causing major disruptions for months. It was no coincidence that the place where the problem occurred was immediately below completely bare slopes denuded of their former natural woodland cover.

In considering future-proofing cities another issue that arises from predicted climate change is that the ‘heat island effect’ is set to get worse. Currently the energy use in cities is rising inexorably with increased need for air-conditioning to keep buildings cool. But pumping hot air out into the atmosphere at increasing cost is obviously unsustainable and new ways of keeping both buildings and the outside air cool are urgently needed. Trees and other areas of greenery such as green roofs and green walls all contribute, while the installation of solar panels above green roofs can multiply the benefit by maintaining the humidity of the roofs for the benefit of diverse urban nature. An aerial view of most big cities, including London, reveals that very few public or large commercial buildings have either solar panels or green roofs while the few solar panels are mostly to be seen on private houses. London may be lucky in having many parks, gardens and squares, but serious future-proofing is an urgent issue that clearly needs a sophisticated approach combining nature with technology in ways that are still at the experimental stage.

Edward Milner, London

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