Green versus grey flood control: Analysis of the costs and benefits of the dependent ecosystem services

The river Dijle south of Leuven (Belgium) is one of the few remaining meandering rivers in Flanders region, and the valley area covers about 2000 ha. The area is rich in biodiversity and is protected under the European Habitat Directive. The diverse landscape makes it attractive for soft recreation, especially for urban dwellers. However, as the city of Leuven and the University campus is prone to flooding, there is need for flood protection. Two options were discussed among involved organisations: an infrastructural solution (i.e. the construction of three reservoirs plus related equipment), or a natural solution (i.e. natural flood control by using the floodplains). As hydrological models showed that both solutions could provide the same protection for a peak storm which happens only once in 100 years, the natural flood control option was selected in combination with one downstream reservoir (for safety reasons).
We tried to complete this analysis by evaluating the effect on other ecosystem services, which are affected by these two solutions. Our methodology is based on a societal cost-benefit analysis, and the necessary data were collected from literature, a Flemish model to asses monetary benefits of ecosystem services (i.e. “Natuurwaardeverkenner”) and expert knowledge. The costs of the infrastructural solution were much higher than the natural solution (4.4 million €/30 years), while the natural scenario resulted in a societal benefit between 14 and 125 million € over a period of 30 years. The ecosystem services which responded differently on the two scenarios are: denitrification, carbon sequestration, air quality improvement and recreation. The contrasting impacts are controlled by the change in water table and the modification of the landscape by the reservoirs. Considering the total societal costs and benefits, the natural scenario was far more beneficial compared to the infrastructure solution. However, such conclusions depends a lot on context of the location (e.g. legal status of the land, population pressure around the area, landscape) and cannot be generalized without verification of the costs and benefits.