Oral communications at the 13th International Sednet Conference, Lisbon
1 st communication :
Partner : Scottish Canals and University of Strathclyde
Authors/presenting : Alasdair Hamilton, Keith Torrance, Peter Birch, Richard Lord
Audience type : Technical
Audience : 120
Title of teh communication : Towards net-zero sediment management of inland waterways - comparing embedded and embodied carbon emissions for dredging and reuse scenarios
Abstract : Introduction: Dredged sediments from inland waterways typically contain higher levels of organic matter than marine sediments (averaging 12 % solid organic matter in UK canals). Ongoing degradation releases CO2 and the potentially more potent greenhouse gas CH4 while the sediment is still in situ. After disturbance and dredging this may continue at differing rates with different proportions of methanogenesis and oxidation, depending on the type of dredging, disposal or reuse options chosen. As a consequence, attempts to manage and reduce the greenhouse gas emissions from dredging and sediment removal or relocation should consider the fate of this embedded carbon in the form of organic matter in addition to the embodied carbon resulting from operational activities and transport fuel use. The aim of this paper is to compare the embedded and embodied carbon for the annual canal dredging activity in England and Wales, together with scenarios for the likely effects of typical dredging and sediment reuse or disposal options.Methods: The annual operational carbon emissions (embodied carbon) for the Canal & Rivers Trust provided by their national dredging team framework contract are compared with the estimated organic carbon content of the c 100,000 tonnes which are dredged annually (embedded carbon). Using the most common dredging and reuse scenarios, qualitative estimates of the relative emissions during and after dredging operations are suggested.Results: Fuel use carbon emissions including diesel plant operation, plant mobilization, personnel travel and disposal haulage (for 55% of arisings) are estimated as 1340 tonnes CO2 equivalent. Assuming 38% dry matter content, 12 % total organic matter on a dry basis and 58% carbon content of the organic matter, the 100,000 tonnes of wet sediment contain 2645 tonnes of organic carbon. If fully oxidized this would equate to 9706 tonnes of CO2, so roughly 7 x the operational CO2 emissions, far greater than this if the long term storage conditions of the sediment promote methanogenesis. These calculations indicate the importance of considering the fate of embedded carbon within the dredged sediment if estimating the whole life emissions of a dredging project.Discussion: Dredging options for UK canals include reallocation by ploughing, cutter suction and hydraulic transfer, or most commonly, pontoon-mounted excavation for barge transfer and road haulage. Arguably these represent successive increases in the expected operational emissions. Reallocation, dewatering and disposal result in similar or enhanced rates of emissions from oxidation of embedded carbon, with landfilling likely to enhance methanogenesis. However, nature-based solutions reusing sediment for soil creation and revegetation can be shown to lead to storage and future increases in soil carbon, so could be used to offset the operational emissions of dredging activity. In conclusion, using low carbon or renewable energy will not in itself achieve Net Zero dredging, so sediment managers must consider the fate of embedded carbon after dredging and reuse or disposal.
Link : https://sednet.org/events/sednet-conference-2023/
2d communication :
Partner : Scottish Canals and University of Strathclyde
Authors/presenting : Alasdair Hamilton, Keith Torrance, Richard Lord
Audience type : techincal
Audience : 120
Title of teh communication : Lessons from pilot-scale sediment reuse projects on theScottish canal network
Abstract : Beneficial reuse of dredged sediment from canals, harbours, and waterways would appear to be a straightforward opportunity to transform a waste product into a valuable resource, in line with developing a circular economy. In practice there are multiple technical, regulatory, and economic barriers to their beneficial reuse. Two pilot studies were undertaken in Scotland in conjunction with scheduled maintenance dredging on the Scottish canal network to investigate the practical difficulties to sediment reuse on a 1,000 - 15,000 m3 scale. Projects were undertaken on the Forth and Clyde Canal at Bowling, Dumbartonshire and on the Caledonian Canal at Laggan, Highland Region. At Bowling, sediment characteristics favoured reuse as a topsoil following bio-conditioning in a cell constructed for this pilot. After only 5 months in this cell, samples met the specifications of BS 3882:2015 and was suitable for landscaping on an adjacent construction site.By virtue of its location within in a steep valley and the underlying geology, sediment entering the Caledonian Canal is granularly coarser than that of lowland canals. Consequently, dredged sediment from this canal was better suited to reuse as aggregates and as components for concrete. At Laggan, suction dredging was used to remove over 8,500 m3 of material, which was delivered as a slurry to a series of lagoons. As the slurry washed through the lagoons, sediment particles were separated by gravity resulting in clean sand, with gravel fractions removed by screening. These fractions were mixed with Portland cement to manufacture one tonne concrete blocks for erosion control. The residual silty deposit in the lower cell met the criteria for topsoil and was used to restore the site to its original grade. Meanwhile, sand in the upper lagoon contained <10 % of <0.063 mm, so met the requirements for reuse as fine aggregate in asphalt (BS EN 13043:2002).The pilot studies demonstrated the viability of reusing dredged sediments for infrastructure projects. However, they highlighted the importance of minimizing transportation and handling of wet sediment to the viability of sediment reuse.
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