Circular cities..? Real time material flows for London

Since it looks as if we are going to introduce the climate measures for buildings mainly via municipalities or cities, I have recovered some of my older research material about urban sustainability, developed when I worked at Wageningen University. The urban approach makes sense, buildings are not isolated functions, they are connected via infrastructure and grouped in cities. And since the city also seems to be an international focal point for environmental impact reduction, (majors initiatives) it is interesting to do an exercise at city level.

Its not only interesting as an exercise. As earlier argued, smart cities (and circular cities) are cities with a disaster plan. Since might distribution systems fail, in a city you are trapped like rats. Which is not unthinkable, just look at Belgium that has already warned for electricity blackouts. And last summer several energy plants in Western Europe have been shut down because the cooling water was too hot. Similar problems can occur, in water, food and material distribution systems. And how is a city going to ensure that its citizens retain a basic level of facilities? Not with smart meters, smart parking, and smart whatever, probably smart things don’t work anymore. Or, maybe perhaps, because residents like in Belgium have massively purchased diesel generators….

But then, what are we talking about when it comes to material flows for building? Cities are of course ‘consumptive systems’ par excellence: there is a lot going ‘in’, and hardly anything coming out, except waste. That kind of metabolism has already been tried to document for some cities. An example that appeals to the imagination is London, which has been keeping track of what goes in and out of the city. Which offers a very interesting insight into the “consumption” of an urban system, the quantitative picture, as a reference for further research. The data are somewhat outdated (2000) , but that does not matter for the exercise. [1]


The table below shows the material flows through the city of London as part of a large inventory survey, in the framework of the ecological footprint of London at the time. The table is very special, because it shows more than seems at first glance.

If we look closely, we see that the apparent raw material consumption for construction in that year is 27 million  tons. This is detailed in the other columns: 14 million tonnes is waste and 13 million tonnes is net added. In other words, more waste has been created than net addition to the stock, the material efficiency is less than 50%! And theoretically, London could have created its entire addition from residual flows! Wood already shows a much better score: net addition is 3 times the waste flow. It also shows that if the primary flows are better utilized, the actual consumption drops significantly, just like the new inflow.

This is a first indicator of how an urban environment can better manage its resources by looking at the entire stock and flows, rather than at individual buildings.

It is also interesting that we normally normalize buildings over their lifespan, because that is the only way to gain some insight into the ‘current’. But when we look at stock levels, the picture changes: Then materials become a real time stream, just like water and energy, which can be managed per year! The flows to maintain the ‘orbanism’, the urban organsim.

It is not entirely clear from the report, but suppose that Construction and Wood are the two construction-related flows. Together, 14,862,000 tonnes are net added to the stock. In that case you could say that the fraction of renewable materials is roughly 12%. Only that is the net addition. If we look at the total flow through the city, apparent consumption, that is only 6%. Either, assuming that the wood would be FSC and renewed, and the others not, the city has a ‘cycle efficiency’ of 6%. (closed cycle efficiency). Compare that with energy, we can also show the amount of renewable energy in real time / annually. This way also for materials in real time. (The amount of actually recycled material is not entirely clear, which could increase this a bit)

Total M2 floor in city

A step further is to relate these data to the amount of m2 buildings in the city. Table 2 contains some data that I could find for that period, mainly houses and offices, which will be by far the largest part.


If we now divide the amount of materials by the available m2 we see that:

Of the apparent consumption, 0.1 tons per m2 of floor has been invested. Or, for the net added quantity, it is 0.05 ton / m2 of floor. You could see this as the amount of material needed to maintain the building stock, and to renew a part of that annually. If I assume an average new construction weight of 1 ton / m2 (about NL average for traditional houses), then about 10% of the original weight is needed per m2 to maintain it! So every 10 years the materials replace themselves. That is not entirely true of course, in fact it is 5% and 20 years, but because of the inefficiency of the material use that is higher.

This is, of course, somewhat speculative, more study and detailed figures are needed, but it gives an idea of ​​how it can be evaluated at the urban scale level.

The question is whether infrastructure is included in these figures. Then it would look better for buildings, of course. But the question is whether you should separate that: buildings can hardly function without infrastructure, or roads, and roads do not make sense if they do not go somewhere. So for every m2 of building a piece of infra is responsible, or included …. But that is another discussion.

Newly added m2

For 2003 I was able to find data on new built homes and offices in London. Sufficient for the exercise. Again assuming that new construction requires 1 ton / m2 (also in UK), this leads to the following table:


Net added is 2.6 million tons, which is approximately 8.5% of the apparent total material flow. And so is 91.5% for maintenance and waste.

The previous figures can therefore be corrected for maintenance only: and the 0.1 tonnes will then be 0.09 tonnes, or 90 kg per m2 of floor.

The waste stream provides us with another interesting indicator when we relate it to the built-up area of London as a whole. “Greater London” has a surface of 1584 km2, or 158400 hectares. And so the construction waste stream is about 100 tons per hectare. Either about 1 house of 100 m2 per hectare.

Land need

Suppose we would make all of renewable material, then that would give us a reference for how much land is needed to generate those sources, and a measure of the vitality of the city. Assuming that timber construction can be 50% lighter than traditional construction (as we know from earlier calculations) and that we work with waste, the net stock addition can halve, or 7.5 million tonnes approximately. 1 hectare of forest yields about 5 tons of wood per year, which then requires 1500000 hectares , continuously to keep the construction and maintenance going. That is 15000 km2 and about 10 times the area of ​​the city itself. In other words, a city needs about 10 times its surface to be insured of materials when it is self-reliant.

(Of course: many optimizations are possible, and also variations between cities. But then, this is also only building material, although that is already 40 to 50% roughly of all materials)

Empty space

There is another interesting comparison to be made when we look for instance at the empty indoor space in the city: At that time about 25 million square feet of offices were empty in London, or 2.3 million m2. Which is therefore more than the need for the next 6 years, based on the new constructed figures. New construction could thus be avoided, which could further reduce the flow of materials through the city. It is also almost the same as the housing demand for 1 year.


The example shows that interesting comparisons can be made for a city at this scale level, which can help creating a more sustained use of materials. Of course, that requires high-level management and organization, in order to bring together and utilize the various streams. (But that’s no different from the organisation of producing an importing new materials and their extraction, even though that has already been organised at the moment. Nothing happens by itself.) London by the way was already investigating how they could manage this at the time.

All this combined with reuse and recycling possibilities, a more closed cycle approach for cities with regard to materials comes into view. This way of analysing gives cities more control over what exactly is happening in their community, and could therefore also be, or should be, steering for new-building permits. At least, if we want to tackle and organize the environmental impact and climate change mainly through cities.



[1] City Limits. A resource flow and ecological footprint analysis of Greater London

Author(s) :Best Foot , Year 2002 . However, the report seems not available at the time. I put a copy for download on this website: london-citylimits-complete-report

Author: ronald rovers