Down towards the bottom of Francis Street, in front of Johnston Antiques, there is a loading bay that doesn’t look like much.

It’s just an expanse of black tarmac, relatively new, laid as part of the revamp of the streetscape the council has done over the past few years.

But it’s a special patch of tarmac, actually. It’s “permeable stone mastic asphalt”, a council spokesperson says.

Hard rain falling on the city’s streets and driveways and other paved surfaces can gush into the sewers and rivers at a rate that overburdens them – causing flooding.

As the climate changes, more, harder rain is expected. This problem will likely become worse and more frequent.

One solution that Dublin City Council is trying out is this spongier surfacing. They’ve some in Belmayne too, said a spokesperson, and the council is also looking at using it in future sustainable transport projects.

“These trials will help gather data about the cost, service life and maintenance requirements of permeable asphalt relative to standard bituminous surfacing materials,” they said.

How effective

Porous asphalts have been around since the 1970s, according to the book Asphalt: Materials, Science and Technology.

The asphalt on top is a bit less smooth than regular asphalt, to leave room between stones, basically, to let the water seep in, the book says.

Then below this layer of black asphalt, there’s a bed of coarse stone, often 45cm to 90cm thick, to hold some of the water as it soaks into the soil below, the book says.

“Because of the open structure of the pavement, porous asphalt offers a means to replenish water tables and aquifers rather than forcing rainfall into storm sewers,” it says.

A 15-month field test of permeable paving in front of a high school in the city of Taipei, in Taiwan, found that “Runoff peak reduction ranged from 16% for large, intense storms to 55% for small, long-duration storms,” according to a 2019 paper.

Permeable asphalt is also good at filtering out pollutants, according to the US Environmental Protection Agency (EPA) as water seeps into the ground. Instead of just flowing overland into the nearest stream, or through the storm sewers into Dublin Bay.

However, the pores in porous asphalt tend to get clogged over time – and so it becomes less effective. Over the course of the Taipei study, the paving became less effective for soaking up water and reducing runoff.

“Clogging was a problem, but infiltration rates would improve with maintenance,” the 2019 paper documenting the results of that field test says. .

Is it worth it?

Installing permeable asphalt is currently a bit more expensive. In Taipei, the paper found it was 10–15 percent more costly. In Dublin, it’s also more pricey, a council spokesperson said.

“Permeable asphalt drainage infrastructure required additional excavation over regular asphalt and is more expensive to lay than regular asphalt,” they said.

However, part of that might be because the council is doing small amounts of permeable asphalt as a test, and large swathes of regular asphalt. “Economies of scale may have played some part in the price difference,” the spokesperson said.

Even though the installation cost is a bit more than for regular asphalt, though, the full cost – all factors considered – is trickier to calculate.

“On new schemes, this [installation cost] can be offset against the reduced need for pipes and gullies,” the spokesperson said.

There’s the maintenance cost too, to consider. The porous asphalt has to be cleaned periodically or it won’t stay porous.

Power washing is the usual way to keep permeable paving clean, according to Anna Lawlor, a director at Dublin-based Driveway Paving and Construction.

Using a giant hoover on it, or grinding off the top 2cm of asphalt are other ways to restore its permeability, according to a 2016 paper on the subject.

The council spokesperson said that, “Theoretically, permeable asphalt would require more maintenance than regular asphalt, but this may be offset against costs associated with cleaning gullies.”

Reducing runoff during storms, and the flooding this can cause, also has benefits that would need to be considered when totting up the full costs versus full benefits – as does reducing the amounts of pollutants flowing into streams and other water bodies.

Yes, please

On Friday, the rain is falling kind of medium-hard when Tony Cullen answers the door to his red-brick house in Mount Brown.

The drops are big and close enough together to make you notice, and to pool in shallow puddles on the footpath and the roadway.

But they’re not enough to overload the drains and flood Mount Brown, which Cullen says happens often enough.

Cullen says he worked for years at Dublin Corporation – now Dublin City Council – in water and drainage.

Replacing his concrete footpath with permeable paving of some sort that would sop up at least some of the rain would be welcome, he says. “Yes, yes, absolutely.”

Tony Cullen in his doorway. Credit: Photo by Sam Tranum.

So far, though, the council has only used it in a few places as it calculates costs and tracks effectiveness, according to a council spokesperson. There’s the loading bay on Francis Street, for one.

“This has also been used in the Belmayne area and it is hoped to carry out trials under the Active Travel Project,” they said – so probably for cycle paths.

It’s also been used at Dublin Airport’s red car park, according to the company Roadstone, which supplies permeable asphalts.

It is likely that under standards set by the city development plan, there’s going to be a lot more porous asphalt used in the city, as well as other kinds of porous paving – and more ways of avoiding paving altogether and leaving green spaces to sop up rain.

The development plan requires the use of sustainable drainage systems (SuDS) in all new developments.

“This is required to reduce flood risk, protect water quality in our rivers and Dublin Bay and to help mitigate the impacts of Climate Change,” the council spokesperson said.

There are efforts all across the city to reduce runoff. Planned rainscapes in Stoneybatter, along the Dodder and along the Santry river, for example. Or an initiative to get people to install small “rain gardens” below their drain pipes.

The city development plan also requires that “all surface water run-off from new/ extended domestic driveways, repaired/ replacement driveways … is managed through the use of SuDS, ensuring no increase in surface water discharges to the public drainage network”.

In addition to requiring new construction to use permeable paving and other systems for managing rainfall, the development plan talks about going back and re-doing things that have already been built.

“The strategic planning of surface water management will be required in order to reduce the volume of run-off, relieve overloading pressure on the drainage network, and protect water quality in the City’s watercourses,” it says.

“This will necessitate the use and retrofitting of sustainable drainage solutions to manage surface water in place of hard grey solutions,” it says.

The council spokesperson said that “nature based solutions such as tree pits and rain gardens are preferred”, but “in many cases permeable paving will offer the best available solution”.

Beyond the council

When a heavy rain led to on-street flooding in Drimnagh and Crumlin on 20 June, the council issued a report to councillors blaming, in part, driveways.

“Crumlin and Drimnagh were developed during the 1930s/1940s to different drainage design standards and at a time when car ownership and national income levels were a lot lower than today,” it says.

Over the years, people have been paving over front gardens for off-street parking, as well as building extensions and patios. And more homes have been built, covering more ground.

“Unfortunately, this higher level of impermeable surfacing in the area – combined with more intense rainfall – has increased both,” it says.

Lawlor, of Driveway Paving and Construction, says there’s a range of permeable materials available for driveways.

There are also designs that combine an impermeable surface (concrete tracks for wheels, for example) with permeable ones (grass in between and to each side, for example).

But, Lawlor says, “It’s rare that a customer asks.”

“The main factors that we see customers consider is maintenance, durability and cost,” she says.

It isn’t a given that more permeable driveways are more expensive, says Lawlor.

“Concrete would be lower cost while a granite driveway is higher cost and neither are permeable,” she says, by way of an example.

A desire for the lowest-possible maintenance option drives people towards less permeable options, she says.

“Unfortunately most permeable paving options have the potential to be higher maintenance,” Lawlor says. “This is due to water flowing, and water can carry seeds, mud and debris into joints.”

If the council wants to people to think about permeability when they’re choosing a material for their driveway, it’ll need to tell them why, she says

“I think more knowledge and education is required as to why it’s needed to encourage people to make a conscious decision about it,” she says.

CORRECTION: This article has been updated at 12.18pm on 19 July to correct the typo “t00” to “too” in one instance.

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