Green versus grey solution for flood control of Leuven city (Belgium)

As the city of Leuven and the University campus is prone to flooding, there is public demand for flood protection. Two options were considered among involved agencies during the 1990’s: an infrastructural solution (i.e. the construction of three reservoirs), or a nature-based solution (i.e. using the natural floodplains of the Dijle river for flood control). The hydrological models showed that both solutions could provide the same protection for a peak storm which happens only once in 100 years. The societal costs and benefits of both solutions were however very different: the costs of the infrastructural solution were much higher than the natural solution (5.6 million€/30 years), while on the societal benefit side, the natural scenario scored much better than the infrastructural solution (difference in benefits between 29 and 330 million €/30 years). As the water table and the landscape esthetics would be different for the two solutions, also the ecosystem services would respond differently, especially denitrification, nutrient-removal, carbon sequestration, air quality improvement and recreation. Finally, the natural flood control option was implemented in combination with one downstream reservoir. This solution is also favourable for the biodiversity of the valley, although there are trade-offs.
These findings are heavily dependent on the local context, e.g. legal status of the area (in the Dijle case: protection by European Habitat Directive), presence/absence of houses and infrastructure in the flood area, geomorphology, demand for outdoor recreation, etcetera. Therefore, the findings cannot be generalized without verifying the costs and benefits.