Greening cities

The MAX:R+I team have examined the opportunities for the expansion and exploitation of green infrastructure in London. The topic is huge and extremely complex, so our analysis has in many places been necessarily unsophisticated. However, using our engineering expertise and analytical experience, we have taken care not to over simplify. Our work provides a meaningful overview of the usefulness of different cited benefits, alongside a description (be it quantitative or qualitative) of what designs would achieve a significant city scale effect.

In our view, some of the mechanisms we examined look promising. Our work suggests that a greener city could be less prone to overheating, with a lower flood risk, slightly less polluted air, a lower food-related carbon footprint, and improved citizen mental and physical health. On the other hand, we were unconvinced by the evidence for value added in terms of building insulation, attenuation of traffic noise, and biofuel production: these may just be more trouble and expense than they are worth. The table below summarises our findings. The interventions we describe are not necessarily the only way of achieving the desired result, but one suggestion.

One of our findings was that, while the predicted percentage reduction in PM10 concentration brought about by increasing urban tree cover by 10% is small, the total amount of PM10 removal is predicted to be 1500 tonnes per year. This may not seem significant, but it could be that using trees for removing this pollution from the city is a cheaper and simpler exercise than attempting to reduce traffic source emissions by the same amount. This example highlights the importance of understanding the goal of a green intervention: it may not always be the most effective method, but it may be the cheapest/simplest/nicest means to a desired end.

Our research focussed on finding the realisable, city scale benefits of green infrastructure, and in some cases these were positive. However, some of the areas that appeared less promising may still be beneficial - either when considered at a smaller scale or in conjunction with other benefits. For example, food growing space can be useful at an individual level; greening the walls of a single street canyon can benefit that individual street’s air quality; green roof storm water attenuation can relieve pressure on an individual branch of the sewer; green roofs designed to attenuate storm water could also help mitigate the urban heat island if deployed across an entire borough.

What we have presented does give an insight into which of the benefits are generally the most promising, but we have also raised some new questions regarding the importance of consideration of scale, intent, and multiplicity of the various benefits in any given situation.

The modern city, with domestic and industrial heat use, can lead to an increase in urban temperatures of several degrees when compared to the surrounding countryside. This effect is known as the Urban Heat Island (UHI). As well as an increased cooling building load requirement in summer and higher energy-related emissions, UHI can have direct impacts upon the health of city inhabitants. It can potentially lead to respiratory difficulties, exhaustion and general discomfort.

Plants can help alleviate the effect of UHI in two ways; shading surfaces, and evapotranspiration (the combined effect of evaporation and transpiration) from the plants themselves and the soil they grow in. Both of these phenomena are well understood and supported in literature. Given the right conditions evapotranspiration can reduce immediate air temperatures to the plant by 2-8°C, air that will then extend out beyond the vicinity of the planted areas to provide a wider-reaching cooling effect [citation Taha]. The shade provided by a plant’s canopy can reduce the ground surface temperature below their canopy by 3-8°C depending upon the foliage of the plant used [citation Lin].

At Max Fordham, we examined the increase in green area that would be required in order to maintain the current maximum surface temperature, under the UK Climate Projections 2080s high emissions scenario. Surface temperature is one of the contributing factors of air temperature, which is the main driver behind natural ventilation. An increase in air temperature would decrease the annual range of hours in which natural ventilation become feasible.

To do this we used the STAR tool, a free to use online assessment tool developed by The Mersey Forest and The University of Manchester. The tool enables users to estimate surface temperature based upon percentage land coverage (i.e. what percentage of land is buildings, green and blue surfaces, etc.) across a range of temperature scenarios as a potential result of climate change.

Within a typical London borough approximately 30% of the area is ‘green cover’. For this scenario the STAR tool model predicts that the maximum surface temperature in these areas is 27-28°C under current climate conditions. Using the UK Climate Projections future climate data for 2080 under a ‘high’ emissions scenario, this maximum surface temperature could be maintained by increasing the green cover to 55%. With no increase in green area the STAR tool predicts that the surface temperature would rise to 32-33°C.

Within London’s central boroughs away from London’s parks the percentage of green cover is closer to 20%. For this scenario the STAR tool predicts a maximum surface temperature of around 30°C under current climate conditions. To maintain this temperature the percentage of green cover would need to increase to just under 40%.

The tool does not take into account warm or cool winds that may be blowing, increase in shading due to the vegetation, rainfall or variable wind speeds. Although the mitigating effect of green areas based upon evapotranspiration alone shows promise, this is by no means the full picture.

Our analysis found that increasing green cover in London boroughs away from large parks from 20% to 40% could mitigate the warming effects of climate change so that 2080 surface temperatures are similar to those experienced today.

The urban landscape is a jigsaw of impermeable spaces, increasing the surface run-off that flows directly into surrounding waterways. This becomes particularly apparent during a period of heavy rain where water that in a rural environment would be attenuated by vegetation and soil. Also infiltration through other permeable surfaces can instead result in flooding, erosion and sewer surcharging both in the cities themselves and in areas downstream.

An increased green infrastructure can help mitigate surface water run-off by replicating natural systems within the urban environment. These techniques are known as Sustainable Urban Drainage Systems (SUDS). The benefits of SUDS are widely publicised and have become established targets within a number of sustainable development frameworks. The London Plan even goes so far as recommending that all proposals should include green roofs and walls where feasible to aid sustainable urban drainage.

Focusing on green roofs, we examined the coverage necessary to limit the run-off from non-permeable surfaces within London during a severe storm event to within the current sewer capacity. This was defined as the difference between the 1 in 30 year rainfall rate and the 1 in 100 year rainfall event with an additional 30% allowance for climate change.

The surface area in London is approximately split into 30% green space, 30% buildings, and 40% other non-permeable surfaces. The increased rainfall rate, calculated at 50mm/hr, could be absorbed by installing green roofs with a soil depth of 350mm on 50% of London’s rooftops. This assumed that no additional attenuation is required for the green areas themselves and for simplicity ignores such inconveniences as pitched roofs.

While the practicality of installing green roofs in existing rooftops in cities needs to be examined further, for stormwater attenuation alone the inclusion of green roofs on new-building and refurbishment projects can be beneficial to not only that building but the city as a whole.

Installing green roofs evenly across 50% of London’s rooftops would provide sufficient attenuation so as not to overload the current sewer network in a 1 in 100 year, 1 hour storm event (including an additional 30% allowance for climate change).

Noise pollution is inevitably a cause for concern in any bustling urban metropolis. The presence of traffic, machinery, sirens and the general background noise will create an unwanted disturbance for those living and working in the city. This will only be exacerbated by sound bouncing off the hard, vertical surfaces that make up a typical street canyon. Noise levels within the city can be as high as 75-85dBA (A-weighted decibels), comparable to a Boeing-747 taking off (as measured along the take-off path, 2 miles from the end of the runway).

Occupants are unlikely to open street-facing windows in favour of quieter internal conditions. Therefore a building’s cooling and ventilation systems will have to work harder to maintain internal comfort resulting in higher energy-related emissions.

Vegetation is both softer and less reflective than the majority of surfaces within a street canyon. This means that green facades have the potential to absorb a greater percentage of sound wave energy hitting them. Typically a brick surface will only absorb 2% of the sound wave energy, whereas a green wall can absorb around 70%.

Using a ray-tracing computer simulation tool, CATT-acoustic, we simulated the potential attenuation offered by varying green wall scenarios on a typical urban street canyon.

The analysis showed that even if 100% of the street canyon was covered with green walls, the noise at street level would only reduce by 2dBA. This is a value barely perceptible to the human ear. The World Health organisation suggests that hearing impairment becomes an issue for outdoor industrial, commercial, shopping and traffic areas at 70dBA. Providing green walls on 100% of street canyon areas would not reduce current noise levels to this level.

The effect does become more apparent at higher levels, but at 5^th floor level the reduction would still only be around 4dBA. There are more effective, and less costly, acoustic façade treatments to reduce the effect external noise has on building occupants.

The closer you can place an acoustic barrier to the source of unwanted noise the more effective that barrier will be. A green wall, needing to be fixed to a building façade set back from the road itself; can never be as effective as measures that control the noise at source.

Acoustic barriers are most effective when placed close to the source of unwanted noise. Since the green facades are fixed to buildings and some distance from the source, they will only ever be of very limited effect.

The effect of applying green walls to 100% of the surfaces within a trafficked street canyon would only reduce the sound level by 2bB, barely perceptible to the human ear.

Apples from South Africa, tomatoes from Spain and onions from the Netherlands. The source of the UK’s food supply is very much a global network. The UK imports 50% of its own food with 75% of fruit and vegetables originating overseas (source). Even during the peak growing season, more than 30,000 tons of apples, tomatoes and onions are imported into the UK every month. This is despite being species which the UK has a long history of cultivating (citation).

The transport of food, both imported and home grown, accounts for 1.9% of the UK’s total CO₂ emissions (citation). A third of this comes from internal HGVs (heavy goods vehicles) as food processing companies look to concentrate their production capacity and manufacturing processes. By doing this, they increase the average distance between production and consumer. Moving from localised production to centralised production, we are increasing the carbon footprint of our home grown produce.

We have compared the World Health Organisation (WHO) recommended fruit and vegetable intact (citation) with average UK crop yield figures published by the Department for Environment, Food and Rural Affairs (DEFRA) (citation). This was to examine whether London could produce its own food.

We calculated that if 25% of London’s green areas were turned over to food production we could supplement 20% of London’s fresh fruit and vegetable requirements. This was on the assumption that the crop yield for ‘urban farming’ would match those achieved on agricultural land. If this additional fruit and vegetable supply were to offset imported food stock this would reduce London’s food related CO₂ emissions by 5% (assuming field grown produce).

There is a view that the benefits of food production in the urban environment should not be judged on yield alone. During our research we came across a number of studies, reports, opinions and anecdotal evidence that local food production can have a positive impact upon the community. This could particularly benefit the participant’s mental and physical health; whilst also playing a part in a healthy eating education (citation) at a time where obesity is an ever increasing area of concern.

Conversely growing tomatoes out of season under glass is less environmentally friendly than importing them from Spain. The UK tomato season runs from June to October without the need for artificial heating and lighting. In Spain the tomato season runs from March to November, again without the need for artificial heating and lighting. The carbon footprint associated with transporting in season Spanish tomatoes can be up to ten times less than that of the heating and lighting requirements for growing tomatoes in the UK out of season (citiation).

To provide 20% of London’s fruit and vegetable needs, 25% of London’s existing green space would need to be turned to food production.

This would have the added benefit of reducing transport related carbon dioxide emissions by 5% and could help to improve the physical and mental health of the city’s occupants.

Hot water heating and space usage accounts for roughly a third of the UK’s total energy use. The vast majority of this heating demand is met through the combustion of fossil fuels and is the source of 40% of the UK’s CO₂ emissions.

Biofuel could provide a means of reducing the CO₂ emissions associated with space and water heating through a means that does not deplete fossil fuels. The carbon released in the combustion of biofuels can be recaptured as more biofuel stock is grown. Biofuels are by no means a new development. Much of Rudolf Diesel’s early work, inventor of the engine that bears his name, centred on the use of biofuels. At the 1900 World Exhibition in Paris, he produced an engine that ran on peanut oil.

Thankfully our understanding of biofuels has moved on since Diesel’s work and biofuel power stations need not smell of peanuts. One of the most common energy crops grown today is Miscanthus, a fast growing grass with relatively high energy contents. We used benchmark data for average annual heating requirements for a number of generic building types. Following that we examined the growing area of Miscanthus required for the crop to replace gas combustion as the dwelling’s heat source.

A sole heat source an area in the region of 10-30 times the building’s footprint would need to be set aside for biocrop production for a typical UK house. For a two storey PassivHaus dwelling this would be reduced to 2.5-5 times the building footprint. For a typical five storey office block the space required would be an unrealistic 60-100 times the building’s footprint.

The Forestry Commission have also looked into using biofuel as a source of heating. The 300,000 tons of timber produced by London’s existing woodlands could provide heat for approximately 30,000 homes, only 0.5% of London’s existing housing stock. However biofuels as a heating source seem to have potential in more rural areas where space is not at a premium.

With the land requirement for biofuel production ranging from 2.5-100 times the footprint of the building in question, it is unrealistic for biofuels to replace fossil fuels as the primary means of providing space and hot water heating.

It has been widely reports in the media how parts of London managed to breach the EU’s annual pollution limits within the first week of 2017. Far from being a London-centric problem, high pollutant concentrations have also been reported in Aberdeen, Exeter, Glasgow and Stoke-on-Trent., amongst other areas. The health effects of excessively high urban air pollution are alarming; in the UK the death of 50,000 people can be attributed to the chronic and acute effects of air pollution annually. 

Plants can aid in alleviating urban air pollution through four main mechanisms:

• Dry deposition - pollutants, suspended in air, intercepted by the plant’s foliage;

• Wet deposition - pollutants, dissolved in moisture, intercepted by the plant’s foliage;

• Gaseous uptake - gaseous pollution absorbed by the plant through their stomata; and

• Rhizosphere bioremediation - organisms near the roots of the plant absorbing and metabolising pollutants into substances of use to the plants and the organisms themselves.

A vast body of work is available calculating the effects that vegetation could have on urban air pollution levels. A typical study focusing upon the Greater London Authority[i] examined the removal of PM₁₀ (particulate matter less than or equal to 10 micrometres in diameter) by dry deposition. Essentially this is wind-blown particles collecting on the trees. They found that the existing tree stock removed approximately 1% of the total PM₁₀ concentration from the air, or 0.3µg.m^-3. They predicted that by increasing the percentage green coverage away from major parks from 20% to 30% a further 1% reduction in total PM₁₀ concentrations could be achieved. Although 1% seems small, put in absolute terms around 1000 tons/year is arguably a worthwhile contribution. This is valued at over £200 million per year using the UK social damage costs[ii].

The location of this new planting is very important as poorly conceived arrangements of street trees can increase local pollution disrupting airflow that would otherwise dilute the pollutants[iii]. Green street canyons can minimise the disruption to wind flow in the street, while keeping vegetation close to the source of the pollution. One key study examined the effect of green street canyons on localised Nitrogen Dioxide (NO₂) and PM₁₀ concentrations. It predicted that by installing green walls to cover 50% of the façade surface area along the street canyon NO2 concentrations could be reduced by approximately 20% and total PM₁₀ concentrations by 35%[iv].

This work highlights the crucial role that wind speed plays in pollutant removal, calculating absolute PM₁₀ concentrations based upon above-roof wind speed values. Max Fordham have used this information to calculate the absolute annual average concentrations of PM₁₀ for a narrow street canyon with 50% green wall coverage. Under this scenario, PM10 concentrations ranged from 31 to 35µg.m^-3, averaging at approximately 34µg.m^-3. This 3% reduction compared to the current concentrations along London’s road network would have a noticeable positive health impact. But at the scale of undertaking to cover half of inner London’s streetscape with ivy the task should not be underestimated.

By increasing urban tree coverage from 20% to 30% total PM10 concentrations could be reduced by 1%.

Alternatively installing green walls on 50% of the street facing façade area along narrow street canyons could reduce roadside PM10 concentrations by 3%.

[i] Tallis, M., Taylor, G., Sinnett, D., Freer-Smith, P. (2011). ‘Estimating the removal of atmospheric particulate pollution by the urban tree canopy of London under current and Future Environments’. Landscape and Urban Planning 103 (2), pp. 129-138.

[ii] DEFRA (2015). ‘Air Quality: ‘Economic Analysis’ (online) Available at: https://www.gov.uk/guidance/air-quality-economic-analysis [13/02/2017 Date Accessed].

[iii] Vos, P. E., Maiheu, B., Vankerkon, J., Janssen, S. (2013) ‘Improving Local Air Quality in Cities: To Tree or Not to Tree?’ Environmental Pollution 183, pp. 113-122.

[iv] Pugh, T. A. M., MacKenzie, A., R., Whyatt, J. D., Hewitt, C. N. (2012). ‘Effectiveness of Green Infrastructure for Improvement of Air Quality in Urban Street Canyons’, Environment Science Technology 46 (14), pp. 7692-7699.

Much of the marketing material for green roofs boasts of their insulating effect, with water retained by the green roof acting as a thermal barrier reducing heat loss. However many green roof experts are quick to add a note of caution when using the insulating effect of green roofs as a main selling point.

The effectiveness of green roofs as a means of insulating buildings is dependent on a wide range of factors not linked to the design of the green roofs themselves. The design and location of the building, roof construction type and daily variations in the weather all play a part. Together with green roofs themselves, they are an example of an active energy system, collecting, processing and releasing energy. Unlike dry building materials the thermal properties of green walls are constantly changing making it difficult to accurately calculate the thermal resistance of the system, the ‘U-value’. Without this key attribute, including the green roof in the thermal calculations for a building is next to impossible.

However the impossible has never deterred us at Max Fordham. By ignoring the energy transfers inherit in green roofs, we can make a first order estimate of the performance by looking at the ‘simple’ U-values for a typical soil build up and associated moisture content. Through this method, we found that a green roof with a soil substrate depth of 100mm provides the same insulation as just 10mm of mineral wool.

Studies have been undertaken that suggest we are being a little harsh. One by Columbia University found that average winter heat losses through the roof of a low rise academic building were reduced by 20% and summer heat gains reduced by 60% when a green roof was installed.

While the rooms below the green roofs may benefit from reduced heat losses in winter and heat gains in summer, the benefit to the building as whole is likely to be small. As storey height is increased the relative contribution of heat losses and heat gains through the roof to the performance of the whole building diminishes reducing the insulating impact of the green roof.

We are by no means against green roofs and they certainly offer many benefits to the building on which they are specified and collectively to the urban environment as a whole. However in terms of insulation they only offer a minor, tertiary benefit.