Western Mail

‘Natural flood management would be overwhelme­d by super-floods seen across UK’

‘Leaky dams’ and tree planting are useful against smaller floods, but they won’t hold back the water after a major storm, argue professors Robert Wilby and Simon Dadson

-

AS large swathes of the UK endure the worst floods in living memory, hearts and minds are rightly focused on protecting people and property.

At one point the government’s Environmen­t Agency had issued a record 594 flood warnings or alerts.

Once the floods recede, there will doubtless be a period of reflection on what could have been done better.

It may be tempting to point the finger of blame or to promote a particular solution. But the hard truth is that there is no silver bullet for “preventing” floods.

There are common sense actions, like avoiding new developmen­t in places that are known to flood. Official statistics suggest that about 10% of new residentia­l addresses are created in these high-risk areas (classified as National Flood Zone 3).

It is also smart to protect critical infrastruc­ture like bridges or power substation­s to high standards.

Yet a 2016 UK government review of flood resilience revealed more than 500 assets vulnerable to flooding.

Other measures can help ensure that floods, when they do occur, are less devastatin­g.

These include paved floors, valves to shut off foul water, or raising electrical circuits. Who should pay for these is another matter.

However, we need to accept that the climate is changing, and with it the pattern and types of river flooding.

For instance, the Met Office has charted a steady decline in the number and severity of substantia­l snowfall events since the 1960s. Less snow means subsequent spring melting is becoming rarer.

Instead, the country is seeing more heavy rainfall, with winter records being broken on a regular basis.

Or consider how a warmer Atlantic boosted the intensity of Storm Desmond in December 2015 by 25%. Desmond set the UK’s 24hr rainfall record and caused severe flooding across much of northern England.

These consequenc­es are exactly what the climate models have been predicting for decades.

The net result is more water flowing from the headwaters of rivers in shorter periods. We are also observing simultaneo­us flooding across many river basins on a regular basis – the period since the late 1990s has been especially flood-rich.

We aren’t going to halt or reverse climate change anytime soon. However there are some technical solutions that might help reduce (note “reduce” not “prevent”) the risk of flooding.

First, we will need to build new flood and coastal defences to higher standards to cope with climate change.

Second, we’ll need state-of-the art forecasts that can zoom right in and predict the risk of flooding from street to street. These next generation systems will warn at risk communitie­s and businesses, and could help emergency services to navigate flooded road networks.

But many, including the government, are now promoting the wider uptake of “natural” flood management. This refers to various techniques intended to retain water or slow it down, or store it in floodplain­s without causing harm.

Examples of natural flood management include: soil conservati­on, which means more water soaks into the ground rather than staying on the surface; adding large wood debris to river channels and building “leaky” dams to delay the flow from upland streams; wetland creation, urban ponds, and setback of flood embankment­s to make space to store excess water.

There may also be wider environmen­tal benefits such as tree planting, habitat creation or carbon sequestrat­ion in new forests and rewetted uplands.

This all sounds very appealing and is the subject of ongoing research.

Unfortunat­ely, when we worked on the most comprehens­ive meta-analysis of natural flood management to date we concluded that such techniques are useful for reducing nuisance floods but would be overwhelme­d by the types of super-floods seen in the UK this winter.

Throw a month’s worth of rain on a saturated catchment in one weekend and no nature-based solution is going to hold back the water.

Given large UK river basins generally host various buildings, roads, many different types of fields and so on, it is also impossible to detect exactly what portion of changes in observed flood risk can be attributed to a patchwork of leaky dams, soil conservati­on and so forth.

This is also very difficult even within modelled worlds.

We don’t want to discourage natural flood management, but we need more candour about its capabiliti­es.

Given the challenges posed by climate and landscape changes, we should be drawing on the full tool kit.

Raising hopes of flood “prevention” by nature-based solutions will only lead to disappoint­ment. They have a place, but only within a much broader, co-ordinated set of responses.

Robert Wilby is Professor of Hydroclima­tic Modelling, Loughborou­gh University

Simon Dadson is Honorary Fellow of the Centre for Ecology and Hydrology, and Professor of Hydrology, University of Oxford

 ??  ?? > Members of the public are rescued after flooding in Nantgarw last week
> Members of the public are rescued after flooding in Nantgarw last week

Newspapers in English

Newspapers from United Kingdom