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

Leena Stenberg

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

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 time. Erosion inside the ditch network was significantly higher than the SS load at the catchment outlet, indicating that there is a vast potential of easily transportable sediments deposited at the ditch bed. Vegetation started to develop in the first summer following DNM stabilizing the banks and reducing sediment transport. The hydrograph analysis showed that DNM increased the discharge peaks and shortened the mean transit times in the catchment. The timing of the peak was more influenced by the proportion of the drained area of the catchment. There were differences in the results, but both methods (pin meter measurements and TLS) indicated that more erosion and higher roughness occurred in the ditch with a thin peat layer, especially in the fine-textured mineral soil part of the ditch bank, which emphasizes the importance of avoiding cleaning areas of thin peat where possible.
Translated title of the contributionKunnostusojituksen vaikutus eroosioon ja kiintoaineen kulkeutumiseen metsäojitetuilla soilla
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Koivusalo, Harri, Supervising Professor
  • Finér, Leena, Thesis Advisor, External person
Publisher
Print ISBNs978-952-60-7053-7
Electronic ISBNs978-952-60-7054-4
Publication statusPublished - 2016
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • ditch network maintenance
  • erosion
  • peatland drainage
  • sediment transport

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