Monitoring the efficacy of Natural Flood Management structures on flow attenuation and flood risk reduction

Land use and management changes and landscape modifications, including urbanisation and agricultural intensification, have resulted in significant increases in flood risk across the UK in recent decades. To combat this, a shift towards catchment-based flood risk management has seen a marked rise in Natural Flood Management (NFM) schemes applied across the UK. These schemes largely represent mitigation strategies that work with natural processes to restore and augment hydrological and morphological catchment features for enhancing downstream flood resilience through the slowing, storing and filtering of runoff and flow. This has been implemented through the introduction of woody debris, afforestation of floodplains and runoff attenuation features. However, despite growing evidence highlighting their potential benefits, the function of these structures in the landscape and their effectiveness for flood risk reduction is still highly uncertain.

 

To address this knowledge gap, this study evaluates the effectiveness of a range of larger-scale floodplain and in-channel NFM features for flow attenuation and flood risk reduction.  To achieve this, a two-year field campaign was conducted in Somerset, South West England, involving the collection of continuous discharge, storage volume and local rainfall data at four sites in the Tone and Parrett catchments. The sites contained NFM structures including offline and online storage ponds and in-channel woody debris. Using these data, filling, storing and spilling capabilities were characterised through the utilisation of field-scale DEMs from Structure from Motion (SfM) and manual surveys. Storm events were separated, and key hydrograph characteristics analysed, to determine the effect of NFM structures on high flow events and the potential for flow attenuation.

 

The results indicate an increase in storage and flow attenuation as a result of the inclusion of NFM. Increases in flow lag time downstream of in-channel features were identified, relative to an upstream gauge. Longer recession limbs were also recorded downstream of storage ponds, illustrating the buffering influence of upstream structures and the consequential slowed water release downstream. Floodplain-based storage structures were found to only function optimally during the largest events, where pond filling could occur directly from the channel and flow is temporarily stored on the floodplain. These results will provide vital evidence for both local and national NFM applications.