Saturday, 20 December 2014

Talk : Blue-Green Cities - Managing Urban Flood Risk

Recently went to an interesting UoN public lecture talk by Dr Emily Lawson, who is a Research Fellow, UoN Faculty of Social Sciences.

The talk was entitled "Blue-Green Cities - Managing Urban Flood Risk the blue-green approach" and compared traditional "grey" flood protection methods with alternative "blue-green" approaches. The talk was based on work being done by the "Blue-Green Cities" project, whose website is This post is based on the talk, with a little extra linkage thrown in.

Dr Lawson began by providing a helpful glossary of some of the terms used:

Grey Infrastructure: Traditional flood defenses such as barriers, storm drains etc etc

Blue-Green Infrastructure: Natural, multi-functional defenses (e.g. replacing concrete, which rain runs straight off) with grass (which soaks up rain). More examples later!

SuDS:Sustainable Drainage Systems, which aim at use cost effective solutions with low environmental impact to drain away dirty and surface water run-off through collection, storage, and cleaning before allowing it to be released slowly back into water courses. Absolute SHEDLOAD of resources about SuDS available here. SuDS is now mandatory for new housing developments, although whether developers use it in a token or a wholehearted fashion is source of debate.

Infiltration:The process whereby water soaks into the ground (or doesn't if the ground is concrete)

Fluvial Flooding: Flooding from rivers, usually into their floodplain (not a problem unless the floodplain has been built over!)

Coastal Flooding: Usually caused by a combination of high tides, storm surges and wave action.

Pluvial Flooding: Surface water flooding, can happen unexpectedly, usually from intense rainfall. See "Toon Monsoon"

Dr Lawson then described what had happened in the winter of 2013/14, when many parts of the country received 2-3 times as much rainfall as usual (see image below and also this ,and this).

The averaging period used for the following assessment was 1981-2010

UK Rainfall Jan 2014
According to the MetOffice report on the events, the unusual weather was:

"...part of major perturbations to the Pacific and North Atlantic jet streams driven, in part, by persistent rainfall over Indonesia and the tropical West Pacific. The North Atlantic jet stream has also been unusually strong; this can be linked to exceptional wind patterns in the stratosphere with a very intense polar vortex."
This caused flooding in many regions, with much media coverage being given to the flooding in the (low lying) Somerset Levels. Dr Lawson pointed out that flooding in this area has happened many times before. For example, in 1919, flooding covered 30% of the farmland in the area, compared to the 10% that was flooded in 2013/4.

Grey Infrastructure
Dr Lawson commented that, whilst grey infrastructure did its job in terms of combating floods, and was sometimes the only solution (e.g. there is no "blue-green" alternative to the Thames Barrier) is also had a number of disadvantages.

In particular, it treats water as "a nuisance" that has to be got rid of as quickly as possible, via drains etc. This causes problems during intense rainfall, for example, as the drains simply can't cope with the volume of water, resulting in water backing up and flooding onto the streets.

Although modern drains and sewers separate rainwater from domestic foul water, old drains do not, so flooding can result in the escape of diluted sewage onto the streets or into local rivers. Not good.

In addition, grey infrastructure is being put under more pressure by continued building of new houses etc, which result in more land being covered in impenetrable surfaces. For example, 92% of Newcastle city centre is covered in impervious materials such as concrete.

Grey infrastructure is also relatively inflexible - expanding the size of existing sewers simply isn't going to happen - so can be vulnerable to changes in climate or population. As Prof. Antje Stokman from Liebnitz University explains in this paper:
"It is hard to believe that also Tokyo still in the 19th century used to be a very open, green and wet city. A system of segmented rice fields was planned as an integral partof the urban tissue, which did not only provide food to the citizens, but at the same time performed as a system of preventing floods and irrigation"

Incidentally, populations may go down as well as up. Hannover city centre, for example, has seen a reduction of population from 300,000 in 1970 to just 140,000 today, resulting in a decrease in wastewater from 3000 litres/sec to just 520 litres/sec. The effect of this is explained by Prof Stokman:
"...the amount of wastewater flowing through the pipe systems is much reduced due to decreasing urban density with sparser populations distributed over larger areas. As a result in many German cities the under-loaded sewers need to be flushed constantly to avoid smell and the accumulation of pathogens."

And finally, the point was made that flooding only happens for a short time of the year, the rest of the year the grey infrastructure just sits there looking ugly Maybe there is a better way?

Concrete flood barriers can be pretty ugly

Although these in Kentucky have been brightened up

Blue-Green Infrastructure.
In contrast to Grey Infrastructure, the Blue-Green approach (see also here, and here) aims to protect and restore natural water courses and infiltration processes, and also to introduce planted areas to improve soil water retention. Critically, Blue-Green measures aim to deliver benefits other than just flood control - they may increase biodiversity; improve soil quality; remove pollutants from the air; or be visually attractive. A leader in developing Blue-Green approaches to water management is the City of Portland, Oregon, US (see here). Here are a few examples of Blue-Green infrastructure :

A school in Portland, Oregon, US, installed a "rain garden" (a SuDS system) that performed the flood control function of allowing rain to infiltrate into the ground rather than running off into drains - but it also allowed the school to reduce their air-conditioning costs, as the plants reflected less of the light and heat from the sun than the concrete had. In addition, the rain garden was a valuable resource for the schools science students.

School Raingarden

A related feature is a "Bioswale" (another SudS system) which aims to capture rain and run-off water, slow it down, and remove pollution from it. These can work well when used around car-parks, as the the surface run-off from these areas is often contaminated with oil, rubber and other contaminants. Another common application for these is alongside roads, where they can, again, filter and slowdown rain water runoff.

Roadside Bioswale/td>

Bioswale in construction (foreground)
Completed Bioswale in background

"Green roofs" are not a new idea, but have been getting renewed interest as a SuDS approach. Their weight, however, makes them difficult to retrofit. Nice article about their use in Portland here.

This is not a field, this is the roof of the Canadian War Museum

Permeable paving can, due to the area covered, make a huge difference to the amount of rainwater that is infiltrated back to the ground instead of running off into drains. Positive economic findings in this report from Portland.

Permeable paving

And one final example, disconnecting domestic rainwater downspouts reduced water going into drains and provides water for plants. Portland have disconnected 56,000 domestic downspouts in recent years, reducing rainwater runoff significantly.

Disconnected downspout

One in "X" years
Flood defenses are, in part, defined by the kind of flooding that they are expected to be able to deal with. This is expressed in how often the most extreme event it is designed for might happen. Typically, defenses are designed as follows:

River defences : 1 in 100 years event protection
Sea defenses : 1 in 200 years event protection
Barrier defences : 1 in 1000 years event protection
Blue-Green defences : 1 in 30 years protection

It can be seen, as mentioned earlier, that blue-green defences will often need to be used in conjunction with traditional grey infrastructure. However, the use of blue-green can significantly reduce the cost of constructing grey defenses, for example by reducing the diameter of sewer piping required.

The Environment Agency published flood maps for the UK (see here) and some examples are shown below. In each case, the colour coding is as follows:
Dark Blue : >1 in 30yr event; Medium Blue : 1 in 30 to 1 in 100 yr event; Light Blue : 1 in 100 to 1 in 1000 yr event.

Nottingham river flood risk

Nottingham surface water flood risk

Nottingham surface water flood risk - significant at bottom of valley 

Somerset Levels river/coastal flood risk

Somerset Levels surface water flood risk

Show me the money.
A recent government note on flood defense spending states that :

"Some 5.2 million properties are at risk of flooding in England. Annual flood damage costs are in the region of £1.1 billion. These costs could rise to as much as £27 billion by 2080. It has been estimated that maintaining existing levels of flood defence would require flood defence spending to increase to over £1 billion per year by 2035.

Central Government spending on flood defences will reduce in real terms over the spending review period. The Government has introduced a new flood defence funding system, which it believes will help to meet the shortfall. The new funding arrangements seek to encourage more local investment in flood defences, so that schemes that might not be funded nationally may still go ahead.

There are concerns about the extent to which local communities are able to contribute to flood defence funding. While the number of properties at risk of flooding may not rise in the short term, there could be a significant increase in the longer term if current spending levels do not increase."
And also comments that :

"There was a significant increase in flood defence spending from 1997 to 2010—spending increased by three-quarters in real terms. Central Government spending on flood defence in 2010-11 was cut soon after the Coalition Government was formed. Spending was reduced in year by £30 million or 5%. In the 2010 Comprehensive Spending Review (2011-12 to 2014-15), a total of £2.17 billion in central government funding was provided for flood and coastal defence. This represented “a six per cent fall in central government funding”. The Committee on Climate Change calculated that this represented a real term cut of around 20% compared to the previous spending period."
See also this very recent BBC article.

Regarding the question of whether blue-green approaches are of economic benefit, a recent DEFRA report which looked at the application of blue-green approaches instead of a proposed large "grey" project in London, basically says "We don't know". This was due to a variety of factors, including that many of the benefits of blue-green approaches are qualitiative.

On the other hand, another DEFRA report, looking at the use of SuDS in new housing developments comments, amongst much else, that:

The differences in the costs between each drainage Standard is scale related, with SuDS costs showing greater benefit against the other Standards at large sites, with reduced cost benefit for small sites.

"SuDS have very great advantage for developments on permeable catchments where rainfall runoff can be infiltrated compared to the drainage designs based on the other Standards.

...In these [impearmeable site] situations SuDS, particularly with the use of permeable pavements, are generally advantageous over the other Standards...

SuDS generally have little cost advantage for medium and large sites where ground conditions require lining.

The issue of whether SuDS can or cannot be considered as contributing to public open space is very important to clarify, as costs associated with loss of land for dwellings is significant where vegetated systems are used.

There is a significant cost advantage in using SuDS for flat sites, but in contrast their use on steep sites tends to constrain site layout and may cost more than drainage schemes designed using the other Standards.

The study found that the approach of the design team in developing a development layout and the SuDS strategy will have a large impact on the capital costs and only small changes in both the approach will have a large impact on the cost and affordability of the SuDS scheme."

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Dr Sara Goodacre on Spiders
Green Fuel from Seaweed
Living on Frack-tured Earth

Image Sources
2014 Jan Rainfall, Flood Wall, Painted Flood Wall, RainGarden, Bioswale, StreetsideBioSwale, Canadian War Museum, Permeable paving, Disconnected Downspout

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