Nature-based solutions for flood mitigation : A case study of wetland implementation along the Timiș River, Romania

Sammanfattning

Flooding along Romania’s upper Timiș River poses recurrent risks to rural communities and agriculture, yet large-scale Nature-based Solutions (NBS) have not been implemented in the catchment. This thesis investigates how constructed and restored wetlands could attenuate flood peaks. A semi-distributed Soil and Water Assessment Tool (SWAT)+ hydrological model was built for the 2 750 km² catchment using a high-resolution Digital Elevation Model (DEM), land-cover data, Food and Agriculture Organization (soil classification) (FAO) soils and 2008–2017 hydro-climatic observations. After calibration at Lugoj, the model reproduced daily discharge dynamics with satisfactory accuracy (Kling-Gupta Efficiency = 0.69, Nash–Sutcliffe Efficiency = 0.43, Percent Bias (PBias) = +13 %). A Geographic Information System (GIS)-based multi-criteria analysis, integrating slope, soil permeability and land use constraints, shortlisted thirteen potential wetland sites, six side-channel constructed wetlands, three main-channel constructed wetlands and four abandoned gravel pits converted into wetlands, covering 0.8 % of the catchment. Four implementation scenarios were simulated for 2015–2016, no relevant changes were made since then: (1) sidechannel wetlands only, (2) main-channel wetlands only, (3) gravel-pit reconnection, and (4) all wetlands from scenarios 1, 2 and 3 together. At the catchment outlet, scenario 4 lowered the fifteen flows above the 90th percentile by an average of 3.1 %, whereas single-type configurations achieved a 0.4–0.7 % reduction. Peak delays never exceeded one day at the model’s daily resolution. Sub-reach analysis revealed greater local benefits: side-channel wetlands reduced tributary peaks by up to 11 % where storage-to-discharge ratios were most favourable. Monthly flow statistics showed negligible change in medians but a slight decrease in low flows, indicating no drought-alleviation potential. Sensitivity tests demonstrated that lengthening release times yields diminishing marginal gains unless wetland area approaches 5–10 % of the catchment, consistent with literature. The results suggest that, under current land use constraints, wetlands alone cannot deliver substantial catchment-wide flood mitigation for the upper Timiș, but they can provide valuable local attenuation, besides other co-benefits such as habitat creation and water quality increases. Scaling benefits will require either a tenfold expansion of wetland area, integration with complementary NBS (e.g., riparian reforestation) or hybrid green-grey strategies. The calibrated model, suitability-mapping workflow and scenario outcomes form the base for a transferable decision-support framework for Romanian water authorities and the Horizon-Europe Land4Climate project when negotiating realistic flood-risk measures with private landowners.