A pilot study for the nature-based conditioning of dredged urban canal sediments and their beneficial reuse

Torrance, Keith and Lord, Richard and Hamilton, Alasdair and Jakstys, Ignas (2021) A pilot study for the nature-based conditioning of dredged urban canal sediments and their beneficial reuse. In: Joint SedNet-Navigating a Changing Climate online Workshop, 2021-02-10 - 2021-02-11, Moved online due to Covid.

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Introduction: Only a small fraction of the estimated 200 million cubic metres of sediments that are dredged each year from ports, canals and waterways are repurposed. As part of the SURICATES [1] Interreg NWE462 project, a pilot study was conducted in 2019 to address the technical and economic challenges to adaptive sediment reuse, using passive dewatering and revegetation to accelerate sediment conditioning. Figure 1 Falkirk South site with the Union Canal in the background Methods: A 800 m2 treatment area was constructed on a former open cast coal mine site adjacent to the modern section of the Union Canal near Falkirk, Scotland constructed in 2001 as part of the Millenium Link. Six cells were designed that received approximately 1,018 m3 of wet sediment dredged from the Union Canal by a pontoon mounted excavator, which was emplaced over three weeks during July and August 2019. The project aim was to condition sediment for reuse in engineering projects along the canal.Canal sediments sampled in-situ prior to dredging consisted of silty sand and fine sands (10µm – 120 µm), with up to 23% loss on ignition at 550 ºC (equivalent to c.13% total organic carbon). This is consistent with leaf litter and other natural inputs from the canal system. Conditioning of wet canal sediment into a material that can be beneficially reused requires both dewatering from its initial dry weight of around 20% to around 65%. De-watering progress was monitored over a period of 6 months using pre-installed monitoring wells and fiberglass sleeves for use with a Delta-T PR2/6 [2] time-domain reflectometry moisture probe. This probe allowed moisture content to be measured at six depths within the sediment. Soil texture, surface features, and re-vegetation were also documented over the course of the pilot trial.Results: Progressive evidence of sediment de-watering was measured and observed over the course of the trial. This was evidenced by a reduction in the measured moisture content, falling water levels in the wells, and the formation of a crust on the sediment with polygonal desiccation cracks. Our observations also suggest that crustal formation is a largely irreversible process as sediment dewatering rebounds rapidly after heavy rainfall events. Some surface oxidation of organic matter was suggested by colour changes. Figure 2. Monitoring well levels in Cell F1.Discussion: The pilot study reinforced the importance of establishing effective drainage pathways, ideally at both the base of the cells and on the sediment surface to manage runoff and dewatering. Despite an exceptionally wet summer, the material was suitable for excavation and reuse as soil by February 2020. Due to the short residence period, weather events, flooding, and germination issues with the batch of seed used, the effects of plant growth were not observable, other than for voluntary revegetation. Lessons from this pilot study have been applied to two full-scale studies at Bowling and another site on the Forth & Clyde Canal that began in 2020.References: [1] SURICATES - Sediment Uses as Resources In Circular And Territorial EconomieS. Interreg, North-West Europe. https://www.nweurope.eu/projects/project-search/suricates-sediment-uses-as-resources-in-circular-and-territorial-economies/[2] Delta-T Devices, Burwell, Cambridge https://www.delta-t.co.uk/.