Wensum Roadside Constructed Wetlands — Measured Sediment and Phosphorus Capture in a Norfolk Demonstration Test Catchment (UK)

Diffuse agricultural pollution reaches rivers through road ditches and field drains. Within the River Wensum Demonstration Test Catchment in Norfolk, University of East Anglia researchers built three roadside constructed wetlands in September 2016 on a tributary: one linear and one U-shaped design, each acting as a settling pond.
Turbidity probes logged river conditions every 30 minutes before and after installation. Median river sediment concentrations fell by up to 14% and sediment loads by up to 82%, but the authors stress that unusually low river discharge after installation drove much of the load reduction. The U-shaped wetland, with a longer flow path, discharged less sediment than the linear one.
Over 12 months the linear wetland retained about 7,253 kg of sediment, 11.6 kg of total phosphorus, 29.7 kg of total nitrogen and 400 kg of organic carbon. Installation cost was £3,411 with £145-182 annual maintenance; the authors estimate mitigated damage costs at £553 a year, giving an eight-year payback.
Caveats: the study covers three small structures over a short period, and separate Wensum research shows sewage works are also a major phosphorus source, so wetlands address only part of the problem.

Read the full analysis: https://www.uea.ac.uk/groups-and-centres/projects/wensum-alliance/research