Nature-based prevention of urban flooding

A new nature-based strategy aiming to prevent flooding emanating from rainwater in urban areas has been introduced by the Department of Housing, Local Government and Heritage (DHLGH).

Published in May 2024, Nature Based Management of Urban Rainwater and Urban Surface Water Discharges: A National Strategy, rationalises how nature-based rainwater solutions can be implemented in urban areas.

The strategy states that the best approach is to seek to replicate the capacity of greenfield areas to absorb, store, and treat rainwater, insofar as possible, drawing on numerous examples of use that has been adopted across the world. This is an approach whereby urban areas that would, traditionally, have been dominated by paved areas incorporate specially designed landscaped areas into which urban rainwater runoff is directed.

Various terms are used to describe this approach, including water sensitive urban design, ‘sponge city’, nature-based sustainable urban drainage, or urban nature-based solutions.

These landscaped areas are primarily designed to take in, store, and treat urban runoff and to then discharge the treated runoff at a slower rate back into the existing urban drainage network. While the design will allow excess flows to bypass the landscaped areas and flow directly into the existing drainage network, the landscaped areas will have removed most of the pollutants which are contained within the “first flush” of runoff from the paved landscape.

Reducing the percentage of impermeable surfacing and increasing the area of urban landscaping result in multiple benefits in addition to the reduction in rainfall induced flood risk and runoff pollution. These benefits include reduced “heat island” effect and urban noise, a more pleasant urban environment that supports active and sustainable transport modes and increased biodiversity.

Ideally the integration of nature-based rainwater management will form part of the broader approach to urban development across the settlement.

An increasing risk

Traditionally, urban areas have been designed so that rainwater is directed from buildings and impermeable surfaces into collections points (gullies) and from there into an underground network of pipes, ultimately discharging into local rivers and streams.

Analysis of European flooding records between 1960 and 2010 suggests that there has been increased flooding in the autumn and winter periods from rainfall throughout the northwest of Europe, including Ireland.

The strategy outlines how in urban areas, a large proportion of the surface area is made up of hard surfaces which are impermeable, meaning that all rainwater falling on that area remains on the surface and must be managed by an urban drainage or rainwater management system.

With the resulting polluted rainwater, which comprises flooding – water which cannot be contained by existing drain infrastructure – leads to increased pollution in two ways:

1. rainwater falling on many urban surfaces such as roads, streets, and roofs will become contaminated by a wide range of pollutants before entering the drainage system and being discharged into receiving waters; and
2. in many older urban areas, the drainage system is combined and, therefore, rainwater runoff is mixed with sewage and wastewater. As rainfall increases beyond the sewer capacity, these systems overflow into local waterbodies, with resultant pollution.

Although existing infrastructure is arguably incapable of meeting demand, the strategy concludes that it is “neither practical nor sustainable” to simply replace all existing urban drainage networks with larger pipes. Simultaneously, it is not possible to stop rainwater falling onto urban areas; indeed increasing intensities are likely.

At the same time, there are two established trends that, taken together, mean that the traditional approach to urban design is not sustainable. These are:

1. the continuing growth in urban population; and
2. the changes in climate that have already happened and will continue to happen, with predicted increases in rainfall intensity and longer periods of summer drought with high temperatures.

As such, nature-based solutions, the DHLGH asserts, offer a sustainable alternative. Writing for eolas Magazine, Minister of State at the Department of Housing, Local Government and Heritage with responsibility for Nature, Heritage and Electoral Reform, Malcolm Noonan TD, observes: “Not only can nature-based solutions provide attractive features and focal points within the landscape, they can also provide significant ecological value.

“Designed to manage rainwater, they slow and store runoff and allow it to soak into the ground. Their benefits are wide-ranging, filtering out pollutants that would otherwise end up in our waters, reducing flooding and easing the burden on combined sewer systems.”