Monitoring the efficacy of nature-based solutions for diffuse pollution mitigation in a lowland catchment

Abstract

Natural Flood Management (NFM) is an increasingly used Nature-based Solution (NBS) for flood risk reduction in the UK. It has been suggested that such NBS may also be able to deliver multiple benefits concerning wider ecosystem services such as improvement of stream water quality. This thesis aims to address the gap in the NFM and NBS evidence-base concerning their potential to deliver multiple benefits through the mitigation of diffuse pollution in a lowland agricultural catchment. Hydrological and water quality monitoring was carried out over multiple years in two small headwater sub-catchments of the Evenlode catchment, a largely rural upper tributary of the river Thames. A number of interventions including online ponds, offline storage areas, and instream leaky barriers were implemented within these sub-catchments as part of the Littlestock Brook NFM pilot scheme, one of the first of its kind in the Thames Basin. Online pond interventions were able to reduce concentrations of dissolved nutrients (nitrate by 5 % and soluble reactive phosphorus by 29 %) and suspended sediment by 32 % during baseflows. During storm events, online ponds were able to attenuate sediment delivery downstream, however they also showed potential to act as sources of sediment in higher-magnitude events. Rapid sediment accumulation rates diminished storage capacity in the upstream-most pond and highlighted the need for frequent sediment removal to maintain functionality. Offline ponds with a primary purpose of temporarily storing water during events were found to provide storage of sediment and nutrients, trapping 47.9 t of sediment over 2-3 years. When combined with accumulations in the online ponds, this was the equivalent of 14.7 % of the suspended sediment yield from the catchment over the same period. Accumulation rates were influenced by hydrological connectivity, with enhanced sediment, phosphorus, and organic carbon capture in offline ponds that filled via overbank flows induced by leaky barriers. Instream monitoring showed that attributing changes in suspended sediment loading to the implementation of NBS was challenging at a sub-catchment scale due to hydroclimatic variability and land cover change. Therefore, future research should aim to adopt long-term, multi-scale monitoring approaches to better assess the efficacy of such interventions for diffuse pollution mitigation. NFM and NBS provide a valuable suite of options for catchment management, however their efficacy is dependent on their design, connectivity, and maintenance. It is recommended that future NBS research focus on upscaling combined effects of interventions to determine the extent to which implementation is needed to improve downstream water quality.

More information

Identifiers

ORCID for John Nicholas Peter Robotham: orcid.org/0000-0002-1223-8345

ORCID for Peter Shaw: orcid.org/0000-0003-0925-5010

Catalogue record

Export record