Quantifying sediment pathways in anthropogenic floodplains during extreme flooding

Recent extreme floods in the UK have created extensive and expensive impacts on urban and rural communities. Much of the costs result from property damage caused by sediment in the water. Here, for example, the best estimate of total economic damages resulting from extreme flooding in the winter of 2015/16 (storms Desmond, Eva and Frank) ranges between £1.3-1.9 billion, including £308-392M for damage to residential properties (Environment Agency, January 2018). Accumulation of sediment on floodplains is a natural process but creates expensive clean up and damage to crops and property where it intersects with human activities. To better manage sediment laden flooding is a key aim of government policy for catchment-based solutions and natural flood management. Similarly, agricultural policy and best practice are increasingly seeking to keep soil on the land to help reduce off-site damage which typically far outweighs the on-site impacts of rain-driven erosion. To support these policies, which are currently undergoing revision in connection with the decision of the UK to depart the EU, requires improved understanding of where the deposited sediment is generated upstream for targeting interventions in a cost-effective manner, and how that sediment is deposited in complex human-modified floodplains. This proposal will for the first time, combine novel measures of floodplain complexity with robust estimates of sediment source, quantity and morphology. We will explore how floodplain development and infrastructure associated with farming to urban landscapes influence the processes by which mobilised sediment passing down the catchment in extreme floods is processed by the floodplains. We will use field sites in the recently heavily impacted River Severn and River Wye catchments, to generate new data in collaboration with stakeholder organisations. This new data will be used to develop new theory and evidence to support future flood and erosion risk management on the larger lowland floodplains in the UK.