Identifying barriers for nature-based solutions in flood risk management: An interdisciplinary overview using expert community approach.

The major event that hit Europe in summer 2021 reminds society that floods are recurrent and among the costliest and deadliest natural hazards. The long-term flood risk management (FRM) efforts preferring sole technical measures to prevent and mitigate floods have shown to be not sufficiently effective and sensitive to the environment. Nature-Based Solutions (NBS) mark a recent paradigm shift of FRM towards solutions that use nature-derived features, processes and management options to improve water retention and mitigate floods. Yet, the empirical evidence on the effects of NBS across various settings remains fragmented and their implementation faces a series of institutional barriers. In this paper, we adopt a community expert perspective drawing upon LAND4FLOOD Natural flood retention on private land network (https://www.land4flood.eu) in order to identify a set of barriers and their cascading and compound interactions relevant to individual NBS. The experts identified a comprehensive set of 17 barriers affecting the implementation of 12 groups of NBS in both urban and rural settings in five European regional environmental domains (i.e., Boreal, Atlantic, Continental, Alpine-Carpathian, and Mediterranean). Based on the results, we define avenues for further research, connecting hydrology and soil science, on the one hand, and land use planning, social geography and economics, on the other. Our suggestions ultimately call for a transdisciplinary turn in the research of NBS in FRM.

Modeller subjectivity and calibration impacts on hydrological model applications: an event-based comparison for a road-adjacent catchment in south-east Norway.

Identifying a 'best' performing hydrologic model in a practical sense is difficult due to the potential influences of modeller subjectivity on, for example, calibration procedure and parameter selection. This is especially true for model applications at the event scale where the prevailing catchment conditions can have a strong impact on apparent model performance and suitability. In this study, two lumped models (CoupModel and HBV) and two physically-based distributed models (LISEM and MIKE SHE) were applied to a small catchment upstream of a road in south-eastern Norway. All models were calibrated to a single event representing typical winter conditions in the region and then applied to various other winter events to investigate the potential impact of calibration period and methodology on model performance. Peak flow and event-based hydrographs were simulated differently by all models leading to differences in apparent model performance under this application. In this case-study, the lumped models appeared to be better suited for hydrological events that differed from the calibration event (i.e., events when runoff was generated from rain on non-frozen soils rather than from rain and snowmelt on frozen soil) while the more physical-based approaches appeared better suited during snowmelt and frozen soil conditions more consistent with the event-specific calibration. This was due to the combination of variations in subsurface conditions over the eight events considered, the subsequent ability of the models to represent the impact of the conditions (particularly when subsurface conditions varied greatly from the calibration event), and the different approaches adopted to calibrate the models. These results indicate that hydrologic models may not only need to be selected on a case-by-case basis but also have their performance evaluated on an application-by-application basis since how a model is applied can be equally important as inherent model structure.

Quantifying the hydrological impact of simulated changes in land use on peak discharge in a small catchment.

A physically-based, distributed hydrological model (MIKE SHE) was used to quantify overland runoff in response to four extreme rain events and four types of simulated land use measure in a catchment in Norway. The current land use in the catchment comprises arable lands, forest, urban areas and a stream that passes under a motorway at the catchment outlet. This model simulation study demonstrates how the composition and configuration of land use measures affect discharge at the catchment outlet differently in response to storms of different sizes. For example, clear-cutting on 30% of the catchment area produced a 60% increase in peak discharge and a 10% increase in total runoff resulting from a 50-year storm event in summer, but the effects on peak discharge were less pronounced during smaller storms. Reforestation of 60% of the catchment area was the most effective measure in reducing peak flows for smaller (2-, 5- and 10-year) storms. Introducing grassed waterways reduced water velocity in the stream and resulted in a 28% reduction in peak flow at the catchment outlet for the 50-year storm event. Overall, the results indicate that the specific effect of land use measures on catchment discharge depends on their spatial distribution and on the size and timing of storm events.