Flood hazard mitigation value of upstream landscape elements

Flooding of rivers or from surface runoff is a reoccurring issue in extensive areas of Europe. The likelihood of flood events in these areas will most likely increase in the future due to global warming-related increases in the frequency and magnitude of extreme precipitation events. Climate-smart upstream land management is increasingly acknowledged as a way to mitigate downstream flood risk. Vegetated landscape elements (vLEs) like hedgerows and grass buffer strips are recognised to be inherent components of climate-smart agricultural land use systems. However, the effectiveness of nature-based solutions for flood risk management, including vLEs, is still debated. Evidence of the importance of the presence and characteristics of vLEs for the hydrological response on the catchment scale is lacking. The absence of exhaustive and up-to-date inventories of vLEs and their traits, and the lack of appropriate rainfall-runoff models that can take them into account can partly explain this knowledge gap.

With this research, we contributed to the understanding of the importance and hydrological functioning of vLEs in agricultural watersheds located in the loess belt of Belgium. To do so, we first developed a multi-step workflow for the automated identification and classification of vLEs present in a typical Western European agricultural landscape based on high-resolution airborne LiDAR point cloud data. Our method showed high accuracy (overall classification accuracy between 0.92 and 0.97) for the identification of vLEs and these vLEs could subsequently be classified into hydrologically relevant classes. With a limited amount of reference objects, our workflow can be applied in other areas.

To assess the extent to which individual vLEs and their characteristics affect the frequency and severity of floods in agricultural catchments, we modelled the impact of a range of vLE scenarios using a distributed hydrological model implemented in the Landlab modelling framework. Our modelling results confirmed that vLEs contribute to lowering flood hazard. Higher initial soil wetness levels were demonstrated to result in more and faster discharge, regardless of the presence and characteristics of the vLEs present in the landscape. Further, it was shown that runoff is controlled by the density of landscape elements and their upstream area.

To maximise the benefits of vLEs for runoff mitigation, we integrated the Landlab hydrological model with a spatial heuristic optimisation framework for identifying priority parcel boundaries for vLE implementation with the aim of reducing discharge volume at the outlet of an agricultural watershed. To reduce the computation time, we evaluated new and existing methods to upscale hydro-physical parameters to a coarser resolution and assessed whether sub-pixel vLEs can still be correctly accounted for in the upscaled hydrological model. The upscaled model performed best when differences in flow lengths were taken into account for estimating the upscaled Manning's roughness coefficients. The impact of vLEs on the discharge volume was captured most accurately when the hydro-physical parameters of the sub-pixel vLEs were obtained by upslope area-based weighting, highlighting again the importance of the upslope area. The optimisation results demonstrated that discharge volume can be effectively reduced when vLEs are implemented based on the priority ranking of parcel boundaries. Compared to vLEs of the same length and type positioned along randomly selected parcel boundaries, optimised vLEs are associated with a reduction in discharge that is up to 63% higher. Compared to the existing vLE configuration present in the watershed, this reduction is 76% higher. Priority locations were mainly positioned in and along preferential flow paths and were associated with a high upslope area.

This study provides insights and tools for support of decisions aimed at the reduction of flood hazard at the outlet of agricultural watersheds. They have the potential to effectively contribute to economically and societally relevant innovations in governance and rural planning and for the insurance industry.