On Thursday morning, there was a knock on my office door – “The BBC wants to talk to someone about snowmen” – “okay…” I reply, not quite sure where this is going. Five minutes later I am on the phone to a reporter.
It had been in the newspaper that morning about reducing flood risk by building snowmen. The Environment Agency was concerned about a rapid warming in the weather melting the snow and this released water creating a flood risk. The proposed solution from the Environment Agency was based on the fact that snow in large piles, either stacked up or as a snowman, takes longer to melt and hence would slow the delivery of the water to the rivers (see BBC News article). My first thought on this was that we would need to build a huge number of snowmen to capture enough water to have an effect. Having travelled from Durham to London and back this week, it was clear how much of the country was covered in snow. I was about to calculate the number of snowmen required to capture a significant fraction of the water (Prof. Kevin Hiscock at UEA has done this for the River Wensum catchment and estimated 6 million snowmen in that area alone) before I started to think about the problem from another perspective.
‘Rain on snow’ events are a particular flooding threat. The snow on the landscape is the accumulated precipitation for a few weeks. If this accumulated water in the snow is melted by a rapid warming and a heavy rainstorm, large amounts of water are released in a short amount of time. The problems are also increased by the fact that the ground may still be frozen, decreasing the infiltration and hence increasing the amount of the water reaching the rivers.
However, the idea of the snowmen might not as crazy as it might sound. The key points to consider are:
- Where the snowmen and snow piles might be built?
- How these locations are connected to the rivers?
- Where are the vulnerable people located?
A typical scenario could be that people will build snowmen at their house and move the snow from hard standing areas (driveways and patios) to lawns. This scenario is beneficial since it moves the water from the areas that are not able to let the water infiltrate into the soil to areas where it can. Also, these hard standing areas, and the urban environment in general, has been engineered to transport water rapidly way to the rivers through the drainage system. This means that the amount of water that could be transported along these rapid pathways is decreased and hence the chances of rapid flooding are also decreased. The snow that melts and infiltrates into the soil will take longer to reach the river. The final point is that locking up the water and slowing the melt within the urban environment reduces the risks at the location where there is the greatest vulnerability – where people are living. We normally focus on river flooding but water from rainfall and rapid snowmelt (pluvial flooding) poses a significant problem and affects people over shorter spatial distances. Therefore, local actions could be effective.
Since the idea was raised, the forecast changed from a shift from -13C to +13C in 24 hours to a more gradual warming over five days, the EA retracted the advise and the need for snowmen decreased and their role was changed from ‘flood managers’ back to being a children’s game. Could building snowmen and stacking snow have a significant effect on flood risk? Well, in the UK context, it would require a certain set of circumstances to have any effect and would be limited to a small local area. The news stories around this have made people think about how snow melt could affect flooding and if that connects people to the environment and makes them think about hydrology with a smile, then it is all good.