The recent and ongoing flooding of urban areas in northern England and the Scottish borders has in part been caused by unprecedented rates of rainfall. However, this is only one of the factors that contribute to the development of a flood wave passing down a river. Centuries of alteration to how our river catchments (or drainage basins) function has undoubtedly exacerbated the risk of downstream flooding in every river area in Britain.
Rivers are inherently variable, but those with floodplains tend to flood in these areas every couple of years with deeper more extensive flooding occurring less frequently. Sometimes riverbeds also deepen as part of a natural adjustment and in these cases flooding would also be less frequent. Most often, though, floodplain inundation has been reduced deliberately through dredging, embanking, straightening and clearing, which concentrates flood flows in a main channel.
These alterations to the landscape have fundamentally altered surface and subsurface flow processes. They include a strategic network of drains and ditches cut by landowners and managers (often with government grants) to improve floodplain land for agriculture to prevent surface flooding and waterlogging, and dredging and embanking which helps to contain flow within a river. Improved drainage in our upland moors through the cutting of a network of drainage channels means that upland precipitation or snow melt is also now concentrated into channel networks more quickly and flows downstream into river valleys faster.
All of these interventions move water more quickly downstream instead of capturing it in the soil or naturally inundating valley bottom areas where it flows more slowly. Government plans to relax current rules will see this drainage network extended and improved to the detriment of the main river flood regime and those living and working downstream.
Natural Management
Natural flood risk management includes measures to store and slow flow in catchments more effectively with the aim of reversing the long-term trend of moving water downstream as quickly as possible and restoring some functionality to beleaguered uplands and floodplain areas.
It would make sense to try to restore some of the storage function that catchments used to perform before they were so expertly drained. Keeping water in the catchment area for longer means that the flood wave in the river will be longer but at a generally lower level than we currently see during heavy rainfall. This effect has been known from early studies in catchments such as Plynlimon in Wales, and Coalburn and Balquidder in Scotland.
The recent floods in the north-west were linked to record rainfall figures – Cumbria, for example, saw 341.4mm of rain in 24 hours. The flooding that occurred in Carlisle in December 2015 surpassed the 2005 event which generated a flood peak that exceeded the one in 200-year event that is the standard of protection for the city and followed extreme rainfall in the River Eden catchment area.
Rapid deployment of an sUAV (a Small Unmanned Air Vehicle) after the event captured imagery and evidence of the flood’s extent. It revealed that the entire floodplain along the main river and its tributaries was active during the heavy rainfall, along over 60km of surveyed watercourse. This has obvious implications when evaluating the value of natural flood management measures for an event of this magnitude.
