As Climate Changes, Engineers Work on Ways to Manage Santry River Flooding

Tom Mullins remembers the last time the Santry River flooded.

“You should have seen there, about two years ago. Acres were flooded,” says Mullins, as he heaves himself up from the weeding that he’s doing, around the stems of young apple trees at the Santry Community Garden.

He points beyond the walls of the garden to the banks of the river near the lake. “They had to cordon the whole place off it was so bad,” says Mullins.

The river swelled the artificial lake, which had been formed from a widening of the river, flooding not only the Santry Demesne but the adjacent Swords Road, slowing the busy traffic.

That could happen again, and more. As the climate – including precipitation patterns – changes, the Environmental Protection Agency (EPA) predicts increased flooding along rivers like the Santry.

So engineers in Dublin City Council are working on ways to manage future overflows – from bending the Santry River back towards its original route, to greening the wider neighbourhood, to cleaning the river of pollution.

The Santry

“This is the area predicted for flooding and climate change and there’s quite a lot of it in this area,” said John Stack, a senior executive engineer with Dublin City Council, at last week’s meeting of the council’s central area meeting.

On slides, he showed a flood map of the Santry River area, with different scenarios that the future may bring.

The Santry River rises in Harristown, near Dublin Airport. It flows south-east through Sillogue in Ballymun, Santry, Coolock, and Raheny, eventually splashing into the Irish Sea by Bull Island.

An increase of rainfall of 20 percent would likely see significant flooding along the river’s catchment, the map showed.

That would mean washes of water on the M50 in Ballymun, into housing estates off the Swords Road in Santry, and large parts of Raheny during periods of heavy rain. This is predicted as a mid-level scenario by the Office of Public Works.

Built Up

The river’s hinterlands have changed dramatically in the last two centuries, said Stack, the council engineer.

In the 1830s, the area was agricultural. It was full of fields and forestry, and only sparsely populated. These days, the river flows through a heavily urbanised area with a catchment of 9.9 square kilometres, according to the EPA.

The Santry River is categorised by the EPA as a heavily modified waterbody. As the area was built up, its course was significantly altered to make it a receptacle for stormwater.

“It was straightened and facilitated the rapid removal of water from the area,” says Stack.

These changes have not been without adverse effects. “Urbanisation fundamentally alters how water moves through a catchment,” says Stack.

In an agricultural area, rain falls, soaks into the ground, and makes its way into a river. The ground holds the water, and also filters out pollutants, says Stack.

In an urban area, however, the rain picks up pollutants from the ground and brings them to the river.

This has contributed to the Santry River being designated, in the past, as “at risk” by the EPA – based on the likelihood that it wouldn’t be made pollution-free in the near future.

“There’s been a considerable number of pollution incidents over the past few years,” says Stack.

Some were due to illegal dumping, according to EPA research. Others were down to the pressure on the river to do so much: provide drainage, handle sewage overflow, and carry heavy rainfall and stormwater.

Pollution of the river is having an adverse impact on the UNESCO biosphere of North Bull Island, the EPA has noted too.

What to Do

“The big pressure is urbanisation,” says Stack, the engineer.

There’s a tension between demands under Ireland 2040, the government’s long term development plan for the country, to build denser, and the need to keep the water clean, says Stack.

Stack said the council has a few solutions in play. He didn’t have exact figures as of yet but said that it was going to be expensive.

The river restoration plan would see the Santry River returned to its natural route. The stormwater action plan sets out how Santry can prepare to cope with excess water from heavy rains.

For that, the council plans to trial green infrastructure, says Stack, using “nature and plants and vegetation to soak up water into the ground, to purify it and allow it to reach back into the river as it would have done naturally”.

Back at the Santry Community Garden, O’Donoghue says he agrees that green infrastructure solutions are a good starting point. But councillors also need to push for more green solutions within the city development plan, he said.

Rooftop gardens that hold heavy rainfall so it doesn’t run straight off buildings and into an already overburdened drainage system, for example.

“This should be in place now while building is happening rather than having to invest in it down the line,” he says.