Browsing by Author "Stenberg, Leena"
Now showing 1 - 5 of 5
- Results Per Page
- Sort Options
- Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance
School of Engineering | Doctoral dissertation (article-based)(2016) Stenberg, LeenaIt 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 hydro- graph 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. - Flow simulation in a ditch network of a drained peatland forest catchment in Eastern Finland
Insinööritieteiden korkeakoulu | Master's thesis(2014-01-27) Haahti, KerstiThe suspended solid (SS) load of drained peatland forest sites after ditch network maintenance causes one of the largest strains on the water system by forestry. Understanding the hydraulic processes in newly maintained ditch networks is necessary for quantifying SS load generation and transport in source areas. In this thesis, a hydraulic unsteady flow model that would meet the needs of flow simulation in a drainage network on a forested peatland site, was built. The algorithm proposed by Zhu et al. (2011) was applied to simulate flow in a network of channels. In the iterative procedure flow in the channels of the network was solved separately using the Saint-Venant equations and the flow depths at the junction-points were corrected using the characteristic method. The algorithm was programmed using the numeric computing environment MATLAB by MathWorks. According to the hydraulic conditions produced by the model, erosion risk within the network was evaluated. The model was applied to a newly maintained ditch network on a drained peatland catchment Koivupuro (5.2 ha) in Eastern Finland. The ditch network consisted of 15 branches, altogether 1.6 km in length, and 8 junctions. Flow simulation in the small ditches of Koivupuro with mostly very low flow rate (0.0005–0.025 m3/s) introduced its challenges. The magnitude of flow resistance depended highly on the flow conditions. Evidently no one single Manning’s coefficient could be applied for the full range of flow conditions or even for the whole network as differences in discharge were large even within the network. The variability of flow resistance introduced an inevitable challenge to flow computation over a longer time period in a network of such small scale. The assessment of absolute erosion risk was uncertain because of the variability of flow resistance and the uncertainties related to the onsite water depth measurements. However, spatial differences in erosion risk could be derived. Based on bed shear stress evaluation the highest relative erosion risk was found in the southern end of the first and second ditch, close to the outlet of the catchment. Along these reaches flow was relatively high and the bed slope steep. In accordance with previous studies on drained peatlands, the results suggested only highest peak flows have a role in SS generation and transport in the Koivupuro ditch network. - Flow simulation in a ditch network of a drained peatland forest catchment in Eastern Finland
School of Engineering | Master's thesis(2013) Haahti, KerstiThe suspended solid (SS) load of drained peatland forest sites after ditch network maintenance causes one of the largest strains on the water system by forestry. Understanding the hydraulic processes in newly maintained ditch networks is necessary for quantifying SS load generation and transport in source areas. In this thesis, a hydraulic unsteady flow model that would meet the needs of flow simulation in a drainage network on a forested peatland site, was built. The algorithm proposed by Zhu et al. (2011) was applied to simulate flow in a network of channels. In the iterative procedure flow in the channels of the network was solved separately using the Saint-Venant equations and the flow depths at the junction-points were corrected using the characteristic method. The algorithm was programmed using the numeric computing environment MATLAB by MathWorks. According to the hydraulic conditions produced by the model, erosion risk within the network was evaluated. The model was applied to a newly maintained ditch network on a drained peatland catchment Koivupuro (5.2 ha) in Eastern Finland. The ditch network consisted of 15 branches, altogether 1.6 km in length, and 8 junctions. Flow simulation in the small ditches of Koivupuro with mostly very low flow rate (0.0005-0.025 m3/s) introduced its challenges. The magnitude of flow resistance depended highly on the flow conditions. Evidently no one single Manning's coefficient could be applied for the full range of flow conditions or even for the whole network as differences in discharge were large even within the network. The variability of flow resistance introduced an inevitable challenge to flow computation over a longer time period in a network of such small scale. The assessment of absolute erosion risk was uncertain because of the variability of flow resistance and the uncertainties related to the onsite water depth measurements. However, spatial differences in erosion risk could be derived. Based on bed shear stress evaluation the highest relative erosion risk was found in the southern end of the first and second ditch, close to the outlet of the catchment. Along these reaches flow was relatively high and the bed slope steep. In accordance with previous studies on drained peatlands, the results suggested only highest peak flows have a role in SS generation and transport in the Koivupuro ditch network. - Hydrology of Drained Peatland Forest: Numerical Experiment on the Role of Tree Stand Heterogeneity and Management
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-10-16) Stenberg, Leena; Haahti, Kersti; Hökkä, Hannu; Launiainen, Samuli; Nieminen, Mika; Laurén, Ari; Koivusalo, HarriA prerequisite for sustainable peatland forestry is sufficiently low water table (WT) level for profitable tree production. This requires better understanding on controls and feedbacks between tree stand and its evapotranspiration, drainage network condition, climate, and WT levels. This study explores the role of spatial tree stand distribution in the spatiotemporal distribution of WT levels and site water balance. A numerical experiment was conducted by a three-dimensional (3-D) hydrological model (FLUSH) applied to a 0.5 ha peatland forest assuming (1) spatially uniform interception and transpiration, (2) interception and transpiration scaled with spatial distributions of tree crown and root biomass, and (3) the combination of spatially scaled interception and uniform transpiration. Site water balance and WT levels were simulated for two meteorologically contrasting years. Spatial variations in transpiration were found to control WT levels even in a forest with relatively low stand stem volume (<100 m3/ha). Forest management scenarios demonstrated how stand thinning and reduced drainage efficiency raised WT levels and increased the area and duration of excessively wet conditions having potentially negative economic (reduced tree growth) and environmental (e.g., methane emissions, phosphorus mobilization) consequences. In practice, silvicultural treatment manipulating spatial stand structure should be optimized to avoid emergence of wet spots. - Lammin Pääjärven ja Lahden Vesijärven vesitase: maankäytön vaikutus valuntaan ja haihduntaan
Helsinki University of Technology | Master's thesis(2007) Stenberg, Leena