Modelling hydrology and sediment transport in a drained peatland forest - Focus on sediment load generation and control after ditch network maintenance

Research output: ThesisDoctoral ThesisCollection of Articles

Researchers

  • Kersti Haahti

Research units

Abstract

Peatland drainage has been common practice in the boreal zone for centuries. In Fennoscandia, drainage has focused on improving forest production, and the maintenance of drainage conditions is an important component of operational forestry. Elevated sediment loads released after drainage have been widely recognized, but associated process understanding is still limited, especially when it comes to controlling the loads. In this thesis, a distributed process-based model is tailored and applied to describe hydrology and sediment transport in a drained peatland forest with aims to assess the generation and the control of sediment loads after ditch network maintenance (DNM). The chain of model applications revolves around a 5.2 ha forestry-drained peatland, where hydrology and sediment processes were intensively monitored after DNM. The first model development step included coupled modeling of soil hydrology and ditch network flow. This enabled the spatiotemporal assessment of flow conditions in the ditch network, which is a necessity for sediment transport modeling. Distributed modeling of catchment hydrology further showed potential to disentangle the importance of spatial factors on local soil moisture conditions, which is relevant as drainage aims to control these conditions. Extending the model to describe sediment transport in the ditch network demonstrated its utility in identifying the role of different processes and in complementing experimental data. The model described bed erosion, rain-induced bank erosion, floc deposition, and consolidation of the bed. Simulation results sug-gested that the loose peat material, produced during excavation, contributed markedly to high concentrations immediately after DNM. Erosion during spring snowmelt was driven by ditch flow whereas during summer rainfalls, bank erosion by raindrop impact was an important process. Finally, the model was applied to investigate scenarios featuring sediment control structures imple-mented in operational peatland forestry. Results suggested that bed erosion can be efficiently prevented with breaks in ditch cleaning and structures ponding water. It was proven less efficient to trap already eroded material with sedimentation ponds and pits. Furthermore, coupled modeling of soil hydrology and ditch processes showed that structures raising ditch water level had a minor effect on water table in the strips between ditches, plausibly not impairing tree growth. The process-based modeling presented in this thesis provided a yet unexplored approach to comprehensively evaluate spatiotemporal hydrological variables, sediment load generation and alternatives for sediment control after DNM in a forestry-drained peatland. Future prospects of such a modelling approach include extending the assessment to sites on shallow peat with a higher erosion risk, and the investigation of forestry management options, e.g., harvesting alternatives, on drained peatlands.

Details

Original languageEnglish
QualificationDoctor's degree
Awarding Institution
Supervisors/Advisors
Publisher
  • Aalto University
Print ISBNs978-952-60-7889-2
Electronic ISBNs978-952-60-7890-8
Publication statusPublished - 2018
MoE publication typeG5 Doctoral dissertation (article)

    Research areas

  • distributed process-based modeling, ditch cleaning, ditch erosion, forestry, hydrology, peatland drainage, sediment transport, water protection

ID: 30361553