Understanding the role of natural flood management in a ‘not-so-natural’ catchment: field observations of an NFM scheme in rural, lowland Lincolnshire

Abstract

The implementation of Natural Flood Management (NFM) measures theoretically provides an opportunity to build resilience into flood risk management systems in a way that incorporates sustainable practices and holistic management of the landscape. However, there remains a lack of clear understanding of these solutions, and in practice this can hinder the uptake of NFM;  there is a need to expand the emerging evidence base in order to quantify and demonstrate their effectiveness in a range of catchment and storm event scenarios. This study focuses on an NFM scheme implemented across a 22km2 rural, lowland catchment in Lincolnshire, UK. An additional 46,000m3 of storage has been created through the construction of five offline attenuation ponds and a number of field-edge swales alongside the channel network. Land management practices in this catchment have resulted in significant modifications to the hydrological processes through historical realignment and modification of the channel network, intensive agricultural practices and the installation of tile drainage beneath arable fields. The Swaton Eau catchment is located in a lowland area with an elevation range of less than 50m across the catchment. It is important to investigate how NFM features are expected to perform in catchments that are characterised by hydro-modifications and low gradients as there currently is a lack of research of this. An array of monitoring has been installed across the catchment in a nested structure to gather empirical evidence on the performance of the NFM scheme both at a feature scale and at a wider sub-catchment and catchment scale. Water level sensors are located throughout the channel network to track the propagation of the peak flood level. Transects of soil moisture sensors are buried in the shallow sub-surface in chosen swales to monitor short time-scale movement of water through these features. The first major test of the features occurred in October 2023 during Storm Babet and Storm Ciaran. Field evidence indicates that the flood wave was attenuated through the catchment with a less flashy catchment response observed compared to a pre-NFM event in 2012. Soil moisture data collected within the swales indicates that they are intercepting pathways of water through the catchment and preventing runoff entering the surface water drainage network.