Potential of a Suppletion Ditch for Restoration of Bog Woodland in De Wieden
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Currently there is a rapid decline in the expanse and biodiversity of wetlands, such as mire systems, due to human impacts. A leading cause for degradation of mires is desiccation, which through lowering water tables, affects the plant communities and biodiversity of mires. One mire system impacted by desiccation is the infiltration area of De Wieden in the Netherlands. Specifically, the bog woodland vegetation in this area is severely affected due to the high susceptibility of this vegetation to desiccation. Since bog woodland has a high national priority conservation of this habitat is important. For this reason, the scope of this thesis was a modelling study in the possibility of a suppletion ditch in providing adequate hydrological conditions. Hydrological conditions were regarded as adequate when rainwater occurred in the root-zone, soil moisture content was high and suitably high phreatic levels occurred in summer. The following research question was used to guide the modelling study: to what extent will a suppletion ditch create suitable hydrological conditions for the restoration of bog woodland based on hydrological modelling of De Wieden? Using HYDRUS-2D a model domain of a transect in De Wieden that covered 210 by 4.4 m deep was created. The material distribution of this domain was based fieldwork borehole logs, which indicated the presence of a highly permeable, water-rich slurry layer along the transect. The model was further supplemented with literature data. Calibration of the model was done based upon theoretical data sources through the change of the hydraulic conductivity of the different peat layers in the model. The model was validated using field data of Electrical Conductivity (EC) in monitoring wells along the transect and indications of vertical groundwater flow direction found in the field at the end of September 2020. Based on both the calibration and validation the model was found to represent the study area to a sufficient degree. With the developed model several scenarios runs were undertaken to indicate the potential effect of a suppletion ditch on the hydrological conditions. Each scenario implemented a different geometrical shape for the suppletion ditch. The simulation of the different scenarios indicated that drainage by the suppletion ditch lead to lower phreatic levels in the winter, although the decrease was minimal (max. 2.4 cm). In contrast to previous studies the simulations from this thesis indicate that a suppletion ditch leads to an increase up to 20 cm in phreatic level in summer. This positive effect is attributed to the high surface water level in De Wieden which provides a suitable hydraulic gradient for lateral inflow of water in summer. The suppletion ditch did lead to decreases in rainwater in the root-zone as surface water flows laterally into the root-zone. However, the extent to which rainwater is layered above surface water does increase with the implementation of a suppletion ditch which might indicate the formation of a rainwater lens. Furthermore, in the scenario where the suppletion ditch was in direct contact with the slurry layer more optimal results were found. This is attributed to the high permeability of this slurry layer which increases inflow of surface water into the system in summer. Throughout the modelling several assumptions and simplifications were made that could affect the results. Although optimal use of available data was during the calibration both the calibration and validation process could be extended to obtain a more accurate representation of the field conditions. However, the results of this modelling study may still give a good indication of the effect of a suppletion ditch on a general peatland with high water levels and a slurry layer. From this modelling study the implementation of a suppletion ditch in vicinity or within the water-rich layer in De Wieden is recommended to obtain rapid inflow of water. For the winter it recommended to limit drainage of the suppletion ditch, for example by damming, to obtain more optimal phreatic levels. Overall, the results indicate that a suppletion ditch holds potential in the restoration of bog woodland vegetation by providing more suitable hydrological conditions.