The widespread and devastating flooding that affected so much of the UK in December 2015 is forcing scientists, flood modellers and flood risk managers to ask some important questions. Whilst we know what caused the floods (quite simply, record amounts of rain produced record flows in rivers), there are many questions for which we are less certain of the answers. How frequent are those high magnitude floods? Will that frequency change in the future?
Flooding of the magnitude seen in December 2015 is uncommon, just how uncommon though is difficult to say. Flow records in most UK rivers extend back to around the 1960s, giving us only a 75-year historical flow record with which to predict flood frequency. For example, based on the discharge record for the River Aire in Leeds, we’ve estimated that the Boxing Day flood had approximately a 0.3% probability of occurring in a given year, or to put it another way it was a 1 in 300 year flood. Our record does not extend back 300 years, so this is an estimate based on the best available data. Is it a correct estimate? Well, we can use other evidence such as the sedimentological record and rainfall-runoff models to help us understand the likelihood of extreme flow event, but there are uncertainties in those methods as well. Therefore, on the whole we still rely on the historical record to predict future flood risk and to inform government expenditure of flood defences.
The December 2015 floods were caused by exceptionally wet weather. The Centre for Ecology and Hydrology’s summary statistics for December 2015 show in striking detail the extraordinary amount of water that fell on parts of the UK. December 2015 was the wettest calendar month on record since 1910 and set new records for the mount of rainfall in 24 and 48 hours. Many northern rivers saw their largest peak flows, with three (Eden, Lune and Tyne) setting a joint record for the highest peak discharge even measured in England (~1700 cumecs). Therefore any changes to our weather patterns, e.g. El Niño, or to the climate that affect the amount of water falling on land will affect the probability of high discharge events. Given the predictions for wetter winters and more intense storms with climate change in the UK, there is strong likelihood that extreme floods will become more frequent.
With the focus on rainfall, though, we often forget that rain does not instantly cause flooding. It interacts with the land surface, is partitioned into surface and subsurface flow paths, and is routed through the river network. Therefore anything that affects how water is stored in or transported through a catchment will also affect the probability of flooding downstream.
Our landscape has been heavily modified over thousands of years; forests and marshes have been lost and rivers straightened, widened and disconnected from floodplains. The hydrological effect of these landscape change has been the speeding up of water movement through the catchment. Thus, the natural flood management (NFM) approaches that have been highlighted in the media recently attempt to recreate some of the lost complexity. They are ’working with natural processes’ to ‘slow the flow’ of water’.
Will NFM help to reduce future flood risk? Will it help to mitigate against climate change? The jury is out on these questions; scientists are still actively researching the effectiveness of NFM. However, what is clear is that we need improved methods to predict future flood frequency which more realistically represent how catchments and river channels change over time. Afforestation of hillslopes in a steep catchment may have limited hydrological impacts initially, but these will become more pronounced as the trees mature and woody debris accumulates on land and in headwater streams. Equally important is the dampening effect that afforestation will have on soil erosion and sediment delivery to river channels, and the knock-on effect to channel capacity and conveyance. Whilst we recognise the importance of these catchment-processes in theory, they have not formed part of our routine practice for flood modelling.
Only by accepting that catchments and rivers change over time and directly incorporating uncertainty into our modelling will we be better able to predict future flood risk.
Dr. Robert Grabowski is a fluvial geomorphologist whose passion lies in better understanding how the flow of water and sediment within a catchment affects the form and behaviour of rivers. His current research investigates sediment transport dynamics in rivers channels and its impact on water quality, aquatic ecology and water levels.