Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance

• Published in: Ecological Engineering
• Main Author: Stenberg, Leena
• Other Authors: Finér, Leena, Prof., Natural Resources Institute Finland, Insinööritieteiden korkeakoulu, School of Engineering, Rakennetun ympäristön laitos, Department of Built Environment, Koivusalo, Harri, Prof., Aalto University, Department of Built Environment, Finland, Water and Environmental Engineering, Aalto-yliopisto, Aalto University
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Aalto University 2016
• Subjects: Environmental science; ditch network maintenance; erosion; peatland drainage; sediment transport; eroosio; kiintoainekuormitus; kunnostusojitus; suometsät; Boreal Environment Research
• Online Access: https://aaltodoc.aalto.fi/handle/123456789/23390

It is common practice to drain peatland forests in order to obtain better conditions for tree growth. Ditch network maintenance (DNM), the cleaning of existing ditches and digging of new supplemental ditches, is needed every few decades. DNM causes some of the most harmful environmental effects of forestry due to the sediment load induced by the increased erosion in the ditch network. The main objectives of this thesis are: 1) to identify the key mechanisms inflicting erosion and sediment load following DNM, 2) to compare pin meter measurements and terrestrial laser scanning (TLS) for roughness assessment and the change detection of peatland forest ditch topography, and 3) to discuss the practical implications of the results. Two experimental areas were included in the thesis. In Santamäensuo, bank erosion induced by seepage was studied in a short-term experiment post-DNM by using artificial irrigation and pin meter measurements to quantify the changes (erosion and deposition) in the topography of a cleaned ditch bank. In Koivupuro, erosion mechanisms were monitored for two years following DNM by applying pin meter measurements and TLS for the change detection of ditch topography, as well as simultaneously measuring discharge and suspended sediment (SS) load at the main catchment and sub-catchment outlets. A paired catchment method was applied using a nearby reference catchment to estimate the impact of DNM on the SS load. Reference catchments were also utilized to assess the changes in unit hydrographs caused by DNM. The results indicate that several interacting processes and mechanisms affect erosion and sediment generation from the ditch network. Subaerial processes, such as frost and desiccation, prepare the bank for erosion. Bank erosion had an important role in producing sediment in the network, while the role of bed erosion was more modest. In the area of thin peat layer, erosion was the highest during the winter-spring period, while in the peat ditches, most of the erosion occurred during the summer ...