Hybrid measures for flood risk management: combining hydrological investigation and public perception survey

Climate change is expected to affect the frequency, magnitude and seasonality of various precipitation-related hazards, including flooding as one of the costliest hazards in Europe. As natural hazards have a significant impact on infrastructure, human lives, and habitats, it is clear that adaptation measures aimed at both prevention and mitigation need to be considered to address climate change. Green (referred to as Nature-based) measures are currently being promoted by the European Union, but in some planning contexts these measures may not be fully capable of coping with predicted future climate hazards, especially in the case of extreme events. Furthermore, the implementation of such measures is often met with resistance from planning departments and decision makers due to institutional dependencies created by the use of grey infrastructure measures in the past. In addition, scepticism about the effectiveness of green measures goes hand in hand with a preference for grey measures. Hybrid measures do have a prevailing green visual look, they can fulfil some ecosystem services, but they require substantial technical equipment for implementation and may present a feasible complementary measure in planning context with limited space or already existing infrastructure. These solutions therefore combine parts of grey and green measures and present an alternative that can reflect the diversity of environmental conditions.

This study evaluated the effectiveness of selected green (e.g. urban trees, rain gardens), grey (e.g. drywells, permeable sidewalks) and hybrid (e.g. green roofs, stormwater tree tranches) measures on flood risk using hydrological modelling with the HEC-HMS software. This study was carried out in order to define the most effective and suitable flood protection measures for the selected case study, the Glinšcica river basin in the municipality of Ljubljana in Slovenia. Based on the hydrological modelling performed, rain gardens were found to be the most effective measure in terms of reducing peak runoff and runoff volume for the Glinšcica river model. Both green roofs and stormwwater cisterns also showed relatively good results compared to the other measures. The hydrological study was combined with a public perception survey in which we investigated the acceptance, feasibility and effectiveness from the perspective of the Slovenian public. Therefore, we were interested in whether there are differences in the perception of the selected green, grey and hybrid flood risk management measures. We were also interested in which of the contextual (e.g., flood exposure and experience) and compositional (e.g., socio-demographics) factors influence public perception of the acceptability, feasibility, and effectiveness of these measures.

Acknowledgment: The research was conducted within the project [Evaluation of hazard-mitigating hybrid infrastructure under climate change scenarios] co-granted by Slovenian Research Agency (J6-4628) and Czech Science Foundation (22-04520L).