Anja Murray: We cannot continue to ignore Natural Flood Management in Ireland

In terms of flooding, the impact of more frequent storms is exacerbated by the reduced capacity of our landscapes to attenuate floodwaters. Since the 1960s, profound land-use changes across the country have left landscapes far less able to absorb and process water. For example, the vast majority of wetlands across Ireland have been drained away. Hard paving in urban areas and soil compaction from intensive farming both lessen the permeability of land. Replacing woodlands, scrub and wetlands with roads, buildings, and intensively managed agricultural land reduces the space available for floodwaters to rest. Instead of allowing rivers to spill over onto floodplains during times of heavy rainfall, floodplains across the country have been either consumed by industrial and urban development or drained and ‘reclaimed’ to increase farm productivity, removing what capacity they had to hold water in times of heavy rainfall. When water has no place to go but straight to the river channel by the quickest route, we end up having to compensate by widening and deepening channels by dredging.

Dredging clears and straightens rivers to speed up the flow of water off adjacent lands. This helps to reduce waterlogging on agricultural fields, although can exacerbate flooding in the wider catchment. When record-breaking volumes of rainfall run down through a catchment so quickly, there is a much greater chance that rivers will burst their banks and flood towns and cities. Increased flows necessitate ever bigger structural defences to protect urban areas from flood waters.

There is an effective and holistic approach to flood management known as ‘Natural Flood Management’. This is a set of solutions which essentially work to reduce the intensity of flood peaks by slowing the flow of water through a river catchment, so that after a storm event, not all the water that falls on the catchment enters river channels in such a short frame of time. Approaching flood management with this wider, landscape-scale perspective involves broadening the scope of flood management to consider how water moves from mountains and farmland down into river channels and how different land use and management affects the rate of flow.

Natural Flood Management (‘NFM’) generally involves measures such as restoring wetlands; establishing woodlands in parts of a catchment where they are most effective at holding back heavy surface flows of water during a storm event; reconnecting rivers to their floodplains and re-profiling rivers. Across Europe and further afield, NFM has gained recognition as a viable and cost-effective approach to reduce peak flows during flood events and thus help protect settlements from flood damage.

The extent to which any of the actions contributes to flood attenuation depends on a detailed understanding of the hydrology of the catchment and how, exactly, water flows through it. It is not an approach that is as easy as planting up an occasional woodland or restoring a bog here and there — instead there are many caveats and complexities to consider in designing and implementing catchment-based approaches to flood management.

But with a good understanding of how each component of the landscape works to influence the movement of water, depending on its hydrological characteristics and location in the catchment, NFM can act to slow down, rather than speed up, the movement of water through a catchment and thus significantly reduce flood peak.

In addition to being geographically complex, NFM requires the participation of the whole community and many different sectors. Implementing a coordinated programme of land use changes in a catchment is not something that can be applied by one government department, local authority, agency or community alone, it requires partnership approaches to assess and alter how land is managed across whole catchments.

But the good news is that many of the measures that make up NFM have significant ‘co-benefits’ for biodiversity, water quality and climate change mitigation. Take, for example, the creation of new woodlands in the landscape. Woodlands can reduce flood risk in a number of ways: tree canopies intercept rainwater and transpire water back into the atmosphere as vapour; high levels of organic matter in woodland soils increase the water storage capacity of the soil itself; and woodland ground flora increases surface roughness and acts to slow down overland flow, so that the flow of water to the river channel is slowed. Semi-natural woodlands offer greater scope for flood reduction than more managed plantation forests, and as we all know, are excellent and crucial habitats for biodiversity. Woodlands planted in parts of the catchment where soils generate rapid runoff into streams, such as heavy soil prone to waterlogging, are particularly beneficial for both water quality, and for flood attenuation.

As another example, drainage channels in peat bogs speed up water flow to river channels, thus exacerbating flooding. While most peatlands don’t particularly store excess flood waters, restoring them can help reduce flood peak and there are many additional co-benefits to peatland restoration such as stopping the seepage of greenhouse gasses from drained bogs; improving water quality in the catchment; and reinstating habitat for many threatened wild species.

‘Rehabilitating’ rivers and their floodplains also involves the creation of ponds and wetlands in the floodplain, reinstating wetland or woodland ecosystems, or creating floodwater storage bunds, all of which can have major benefits to water quality and wildlife.

Natural Flood Management is a suite of approaches that won’t stop flooding from happening, and because its effectiveness is cumulative and based on lots of different land use interventions, it can be challenging to model and monitor. It will not remove the need for standard, hard-engineering flood protection works, but rather will complement their effectiveness. But considering the scale of the challenges we face, and the many co-benefits for biodiversity and water quality, we simply cannot continue to ignore Natural Flood Management in Ireland.