Drowning Dublin: can the capital keep its head above water?

With 52km of coastline, three major rivers, and a city centre largely reclaimed from the sea, flooding is the most significant climate change risk facing Dublin.

Sea levels are projected to rise by 3-4mm a year internationally, in Dublin Bay they have already risen by 6-7mm annually in the last 20 years. Wave heights, the frequency of coastal storms and extreme rainfall events are also increasing, leaving many Dublin households in a vulnerable position.

“Flooding is normally the third-biggest issue facing Dublin after housing and homelessness, and it’s a situation that’s getting much more difficult,” says Gerry O’Connell, senior engineer with Dublin City Council’s flood projects and flood warning division.

When Hurricane Charley hit in 1986, with 200mm of rainfall in a single day coupled with extreme tides, it was seen by many as a once-in-a-lifetime event. At the time, there had not been flooding as bad in the capital for 100 years.

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From 1986 to 2000, there were very few recorded incidents of significant flooding in Dublin, and Charley began to look like something of a freak. However, as the century turned, what had appeared to be aberrant weather began occurring with increasing regularity.

“In 2002, there was a tidal event with more than 1,100 buildings flooded. Before 2002 we didn’t really know we had a problem with tidal flooding because the previous big event had been in 1922,” O’Connell says.

In the autumn of 2008 came another a once-in-a-century rainfall event, this time concentrated on the north of the city, with 19 areas experiencing severe flooding, many of which had no previous history of floods.

Almost a year later there was more extreme rainfall. This was a once-in-50-years event, again largely affecting the north city, both in coastal area such as Clontarf and also inland in Cabra and Finglas. Sandymount, on the southside, was also hit by this flood.

Worst flooding

Then in 2011 came the worst flooding since 2002. There was more extreme rainfall, rivers broke their banks and 1,250 buildings were flooded in the city. In the decade since, severe storms, record tides and intense rainfall episodes or “extreme weather events” have all become regular occurrences.

“Between 2000 and 2018 there was 150mm of sea level rise recorded – that’s as much as in the last century in less than 20 years. And it’s not just static sea-level rise – as the water gets deeper waves get higher for the same strong winds,” O’Connell says.

“In a comparison between 1961-1990 and 1981-2010 there was a 5 per cent increase in rainfall nationally and forecasts are for a 20-30 per cent increase in rainfall intensity by the end of the century.”

About 20,000 buildings are at “significant flood risk” in the city, he said.

“Some are at risk of a 10-year flood event, for others it’s 30, 50 or 100 years.”

However, most of these at-risk areas are protected by flood defences, he says.

There has been significant investment in flood defences and flood management schemes in recent years. Protections have been implemented and are continuing on Dublin’s three main rivers –the Liffey, the Tolka and the Dodder. Construction of a major flood defence scheme for Sandymount will begin next year, and a flood defence wall is also planned for Clontarf.

However, it hasn’t all been plain sailing, particularly in relation to this last scheme. “We get a lot of resistance in some places. The opposition to the flood wall in Clontarf has been fierce,” O’Connell adds.

Flood defences

The council in 2008 secured permission from An Bord Pleanála for flood defences in Clontarf, one of the main victims of the 2002 extreme tidal flood. However, they were rejected by locals largely on the grounds they would spoil the view of the bay.

The council offered to reduce the height of the protective embankment, but this also met resistance. Eighteen months ago it came forward with a new scheme, which it hopes to submit to An Bord Pleanála in 2023 following more public consultation. In the meantime, large yellow sandbags remain stationed along the coast.

In addition to Dublin’s inherent disadvantages – its extensive coastline and reclaimed lands – cities by their nature have higher flood risks due to lower levels of vegetation and the extent of hard surfaces, O’Connell says.

“In some places in the inner city 90 per cent of the area is impermeable – paved over or roofed. In new estates developed with gardens and green areas, 40 per cent of the area is impermeable and 60 per cent is permeable, but over time the green areas start to disappear, with pressure to build and with people concreting their driveways.”

The construction of extensions and paving of front gardens was thought to be a contributing factor in the floods of a decade ago, when areas previously flood-free were inundated.

Water butts

In addition to its own flood protections, the council is encouraging people to take steps including installing water butts and permeable paving instead of concrete in driveways. In relation to new developments, there has to be enough water storage capacity to cater for 100-year flood events on site, and water can only discharge into the drainage system at a rate of 2 litres per second per hectare.

The ability to manage flood risks in new developments is crucial, given projected population increases and the demand for housing in the city. Not only will the rise in the number of Dubliners increase pressure on the drainage system, there is the question of whether those new development can keep their heads above water, particularly somewhere like the Poolbeg peninsula, which is expected to provide enough homes for 8,000 people at the very end of the city’s docklands.

“In relation to Poolbeg, yes, ultimately it will be submerged,” says O’Connell, but it might be a while off yet. “It is very hard to predict when; it could be 200 to 300 years, and these things can change if we stem the increase in climate change.”

In reality, he says, new developments on Poolbeg may be easier to protect than some existing areas.

“Sandymount is at a lower level than Poolbeg, parts of it used to be a lake. Some parts of the north inner city such as East Wall would also be lower relative to the mean sea level.”

In all events, he says, all the flood defences and management measures in the world can only go so far to protect the city. “They are a sticking plaster unless something radical is done to combat climate change, in my opinion.”