Wetlands as Nature-Based Solutions for Flood Mitigation: Insights from the Timiș River, Romania

As flood risks intensify across Europe, nature-based solutions (NBS) such as wetlands are gaining increasing attention for their potential to mitigate flooding while delivering multiple co-benefits. However, decision-making authorities often lack robust, site-specific scientific evidence to support the implementation of such measures. Flooding along Romania’s upper Timiș River poses recurrent risks to rural communities and agricultural land, prompting the Romanian public water management authority (ABA Banat), within the European LAND4CLIMATE project, to seek scientific support for evaluating NBS-based flood mitigation options.

This study assesses the extent to which a network of constructed wetlands could reduce flood risk in the Upper Timiș catchment (2,750 km²). A GIS-based multi-criteria analysis incorporating slope, soil permeability, and land-use constraints identified thirteen potential wetland sites—six side-channel wetlands, three main-channel wetlands, and four abandoned gravel pits converted into wetlands—covering approximately 0.8% of the catchment area. Using the semi-distributed SWAT+ hydrological model, four wetland implementation scenarios were developed and simulated for the 2015–2016 period, reflecting stable land-use conditions: (1) side-channel wetlands only, (2) main-channel wetlands only, (3) gravel-pit reconnection, and (4) a combined scenario including all wetland types. Model calibration (from 01-01-2012 until 31-12-2014) and validation (from 01-01-2015 until 31-12-2017) of daily discharge dynamics showed satisfactory performance (Kling–Gupta Efficiency = 0.69 vs 0.65, Nash–Sutcliffe Efficiency = 0.43 vs 0.34, Percent Bias = +13% vs +20%, respectively), supporting the use of the model for scenario analysis. Results indicate that the combined scenario achieved the strongest flow attenuation at the catchment outlet, reducing above-90th-percentile peak flows by an average of 3.1%. Individual configurations yielded more limited reductions (0.4–0.7%), although side-channel wetlands reduced tributary peak flows by up to 11%. Sensitivity analyses further revealed diminishing marginal gains from increased wetland storage unless wetland area approaches 5–10% of the catchment.

Overall, the findings suggest that under current land-use constraints, wetlands alone are insufficient to deliver substantial catchment-scale flood mitigation in the Upper Timiș. Nevertheless, they provide meaningful local attenuation and important co-benefits, including habitat creation and improved water quality. Achieving larger-scale flood risk reduction will require a significant expansion of wetland area, integration with complementary NBS (e.g. riparian reforestation), or the adoption of hybrid green–grey flood management strategies.