Leveraging hillslope lengths for the design of peatland restoration

Peatlands are a crucial component of the environment, supporting biodiversity and providing essential ecosystem services. Despite their importance, these ecosystems are highly vulnerable, and many have been extensively drained for peat exploitation. In these cases, restoration through ditch removal is considered an effective nature-based strategy for climate adaptation and flood mitigation. Within this context, the present thesis investigates the relationship between drainage network structure and mean hillslope length ⟨l⟩ in four Norwegian peatland catchments. The case studies span various regions of the country and are analyzed using high-resolution Digital Terrain Models and analytical tools. For each catchment, the mean hillslope length is evaluated analytically for removal sequences proceeding from the headwaters to the outlet (Upstream–Downstream), from the outlet to the headwaters (Downstream–Upstream), and for random removal sequences. A simple economic evaluation of different restoration alternatives is also discussed, where the benefit is identified by the increase in ⟨l⟩ within the focus peatland. The results demonstrate that the analytical model provides a satisfactory approximation of the DTM-derived data, showing flexibility and precision in predicting variations in ⟨l⟩ for both Up–Downstream and Down–Upstream removal sequences, which define the envelope of all strategies. Results indicate that the contributing area of a ditch is the fundamental criterion determining its beneficial effect in terms of hillslope length and flood-mitigation capacity, and ditches with larger contributing areas should therefore be prioritized in restoration. When the restoration goal is to achieve a target ⟨l⟩, the Downstream–Upstream removal strategy consistently minimizes ditch removal, making it the most economically convenient option. In the case of a super-linear benefit function with respect to increases in ⟨l⟩, removing all ditches with sufficiently large contributing areas under the Down–Upstream scenario maximizes net value. The developed models can support peatland restoration planning and, more broadly, the analysis of the relation between drainage network configuration and mean hillslope length in river basins.