‘Natural’ flood management would be overwhelmed by Britain’s winter super-floods
By 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 – its map of the country was a sea of orange and red symbols:
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 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.
Natural solutions?
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 re-wetted uplands.
Overwhelmed by ‘super-floods’
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, coordinated set of responses.
