How a garden on your roof could fight floods this winter

Too late now. Image: Getty.

In recent years, there seems to have been a rise in the extreme weather all over the world: from terrible flooding in Bangladesh and Pakistan, the record cold snap in North America, to one of the wettest winters on record in the UK.

Extreme events are very difficult to tackle, and in some cases there is little we can do, other than increase our preparedness and our recovery response. However there is one thing we can do in response to smaller scale, more common events such as flooding from intensive rain showers. As winter closes in on, it’s worth looking at some of the ways we can better manage excess water.

People have tackled floods for centuries, but modern urban development has thrown up a set of new challenges. The more we develop the landscape, the more rainwater stays on the surface rather than sinking into the soil. That means water gets onto the roads and into drains more quickly, bypassing some aspects of the natural hydrological cycle.

Rooftop plants (including green-roofs and roofgardens), along with rainwater collectors and "rain gardens" (small patches of greenery, which exist entirely to absorb water) can help slow things down and spread the impact of heavy rain out over a longer period. The idea is to replace some of the trees, grass, hollows and wetlands that have been lost to concrete, and so mimic a more natural flow of water.

Rooftop rain collection. Image: Ian Muttoo, CC BY-SA.

One approach that aims to manage rainwater more naturally is known as Sustainable Urban Drainage Systems (SUDS). SUDS are particularly useful in helping to manage small but frequent floods from rainfall, just as the un-urbanised landscape would.

The system has three main aims: to catch and slow down the flow of water; to improve the quality of water, by capturing and treating the pollutants it contains; and to benefit the local community by providing a green space that people can enjoy and where wildlife can flourish.

Reducing runoff

Managing water where it lands – at its source – is one of the most effective ways of reducing runoff. We can do this by creating green roofs and raingardens, for example, as well as by designing other mechanisms that slow down the flow of rainwater from our roads, roofs and driveways. These features are often linked to ponds, wetlands and temporary storage areas (called basins) which can hold and treat more water from a larger area.

Raingardens are planted in depressions and soak up rainwater that lands on surrounding roads, pavements and lawns. Image: Roger Soh, CC BY-SA.

Individual roof gardens, water collectors or raingardens may seem small-scale. But if many of these features are installed in an urban area, and linked up to a larger pond or wetland, they can have a huge impact. They can slow the flow of water, clean it of some of the pollutants it carries, provide habitat for wildlife, and help to recharge our streams and groundwater supplies in a more natural way.

Sustainable urban drainage is already being used to great effect in Portland, Oregon, where 3,500 trees have saved the city $63m in pipe replacement; it's also in use in Malmö, where 6km of water channels and 10 retention pools helped an area that previously experienced chronic flooding. In Dunfermline, Scotland, sustainable drainage is being added to a greenfield pre-development.

Dunfermline: send the rainwater towards the boggy bits. Image: University of Abertay

Too much rain to drain

Sometimes it rains so heavily that no sensible drainage system could handle the flow. Where serious and sustained flooding is caused by unprecedented rain – as happened in the UK last winter – it has to be said that these small-scale systems are less effective. Sustainable drainage measures would probably have helped initially, but no form of water management could have accommodated rainfall on that scale.

Record-breaking rainfalls combined with high tides and high water tables presented us with a perfect combination of conditions that any traditional engineering would have been hard pressed to tackle. In these cases, we need to think about how to be more resilient to floods – how we can be more prepared, reduce the impact of the flood, and recover more quickly.

Researchers in the UK have been working on this for some time now, and in some cases such approaches have become law – in Scotland, for instance, all new developments since 2006 must have sustainable drainage.

SUDS can deliver sustainable solutions to our urban water management problems. They can give us healthier urban catchments, more livable neighbourhoods, and cleaner rivers and streams. And who doesn’t love a rooftop garden? The Conversation

Rebecca Wade is a senior lecturer in environmental science at the Urban Water Technology Centre, University of Abertay Dundee.

This article was originally published on The Conversation. Read the original article.


Was the decline in Liverpool’s historic population really that unusual?

A view of Liverpool from Birkenhead. Image: Getty.

It is often reported that Liverpool’s population halved after the 1930s. But is this true? Or is it a myth?

Often, it’s simply assumed that it’s true. The end. Indeed, proud Londoner Lord Adonis – a leading proponent of the Liverpool-bypassing High Speed 2 railway, current chair of the National Infrastructure Commission, and generally a very influential person – stood on the stairs in Liverpool Town Hall in 2011 and said:

“The population of Liverpool has nearly halved in the last 50 years.”

This raises two questions. Firstly, did the population of the City of Liverpool really nearly halve in the 50 year period to 2011? That’s easy to check using this University of Portsmouth website – so I did just that (even though I knew he was wrong anyway). In 2011, the population of the City of Liverpool was 466,415. Fifty years earlier, in 1961, it was 737,637, which equates to a 37 per cent drop. Oops!

In fact, the City of Liverpool’s peak population was recorded in the 1931 Census as 846,302. Its lowest subsequent figure was recorded in the 2001 Census as 439,428 – which represents a 48 per cent decline from the peak population, over a 70 year period.

Compare this to the population figures for the similarly sized City of Manchester. Its peak population also recorded in the 1931 Census as 748,729, and its lowest subsequent figure was also recorded in the 2001 Census, as 392,830. This also represents a 48 per cent decline from the peak population, over the same 70 year period.

So, as can be seen here, Liverpool is not a special case at all. Which makes me wonder why it is often singled out or portrayed as exceptional in this regard, in the media and, indeed, by some badly briefed politicians. Even London has a similar story to tell, and it is told rather well in this recent article by a Londoner, for the Museum of London. (Editor’s note: It’s one of mine.)

This leads me onto the second question: where have all those people gone: London? The Moon? Mars?

Well, it turns out that the answer is bit boring and obvious actually: after World War 2, lots of people moved to the suburbs. You know: cars, commuter trains, slum clearance, the Blitz, all that stuff. In other words, Liverpool is just like many other places: after the war, this country experienced a depopulation bonanza.

So what form did this movement to the suburbs take, as far as Liverpool was concerned? Well, people moved and were moved to the suburbs of Greater Liverpool, in what are now the outer boroughs of the city region: Halton, Knowsley, St Helens, Sefton, Wirral. Others moved further, to Cheshire West & Chester, West Lancashire, Warrington, even nearby North Wales, as previously discussed here.

In common with many cities, indeed, Liverpool City Council actually built and owned large several ‘New Town’ council estates, to which they moved tens of thousands of people to from Liverpool’s inner districts: Winsford in Cheshire West (where comedian John Bishop grew up), Runcorn in Halton (where comedian John Bishop also grew up), Skelmersdale in West Lancashire, Kirkby in Knowsley. There is nothing unique or sinister here about Liverpool (apart from comedian John Bishop). This was common practice across the country – Indeed, it was central government policy – and resulted in about 160,000 people being ‘removed’ from the Liverpool local authority area.

Many other people also moved to the nearby suburbs of Greater Liverpool to private housing – another trend reflected across the country. It’s worth acknowledging, however, that cities across the world are subject to a level of ‘churn’ in population, whereby many people move out and many people move in, over time, too.

So how did those prominent images of derelict streets in the inner-city part of the City of Liverpool local authority area come about? For that, you have to blame the last Labour government’s over-zealous ‘Housing Market Renewal Initiative’ (HMRI) disaster – and the over enthusiastic participation of the then-Lib Dem controlled city council. On the promise of ‘free’ money from central government, the latter removed hundreds of people from their homes with a view to demolishing the Victorian terraces, and building new replacements. Many of these houses, in truth, were already fully modernised, owner-occupied houses within viable and longstanding communities, as can be seen here in Voelas Street, one of the famous Welsh Streets of Liverpool:

Voelas Street before HMRI implementation. Image:

The same picture after HMRI implementation Image: 

Nonetheless: the council bought the houses and ‘tinned them up’ ready for demolition. Then the coalition Conservative/Lib Dem government, elected in 2010, pulled the plug on the scheme. 

Fast forward to 2017 and many of the condemned houses have been renovated, in a process which is still ongoing. These are over-subscribed when they come to market, suggesting that the idea was never appropriate for Liverpool on that scale. 

At any rate, it turns out that the Liverpool metropolitan population is pretty much the same as it was at its peak in 1931 (depending where the local borough boundaries are arbitrarily drawn). It just begs the question: why are well educated and supposedly clever people misrepresenting the Liverpool metropolis, in particular, in this way so often? Surely they aren’t stupid are they?

And why are some people so determined to always isolate the City of Liverpool from its hinterland, while London is always described in terms of its whole urban area? It just confuses and undermines what would otherwise often be worthwhile comparisons and discussions. Or, to put it another way: “never, ever, compare apples with larger urban zones”.

In a recent Channel 4 documentary, for example, the well-known and respected journalist Michael Burke directly compared the forecast population growths, by 2039, of the City of Liverpool single local authority area against that of the combined 33 local authority areas of Greater London: 42,722 versus 2.187,708. I mean, what bizarre point is such an inappropriate comparison even trying to make? It is like comparing the projected growth of a normal sized-person’s head with the projected growth of the whole of an obese person, over a protracted period.

Having said all that, there is an important sensible conversation to be had as to why the populations of the Greater Liverpool metropolis and others haven’t grown as fast as maybe should have been the case, whilst, in recent times, the Greater London population has been burgeoning. But constantly pitching it as some sort of rare local apocalypse helps no one.

Dave Mail has declared himself CityMetric’s Liverpool City Region correspondent. He will be updating us on the brave new world of Liverpool City Region, mostly monthly, in ‘E-mail from Liverpool City Region’ and he is on twitter @davemail2017.