Impact of rural nature-based solutions in upstream catchments on downstream flood peaks
Using modified soil parameters in plot-scale and catchment-scale models for the Geul catchment
J. Groot (TU Delft - Civil Engineering & Geosciences)
Marie-Claire Ten Veldhuis – Mentor (TU Delft - Water Systems Monitoring & Modelling)
Miriam A.M.J. Gerrits – Graduation committee member (TU Delft - Water Systems Monitoring & Modelling)
M Hrachowitz – Graduation committee member (TU Delft - Surface and Groundwater Hydrology)
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Abstract
Heavy rain events and flash floods, such as those in July 2021 in the Geul catchment, are projected to increase in both severity and frequency. In addition to traditional grey measures (e.g., dikes andreservoirs), rural nature-based solutions (NbS) that enhance the sponge function of the soil are beingconsidered. However, the effectiveness of rural NbS during extreme events remains contested.
This study simulates the potential of rural nature-based solutions for flood mitigation under extreme rainfall, with a focus on river peak flows in the Geul during the July 2021 event. To this end, the combination of a physics-based plot-scale model (SWAP) and a conceptual distributed catchment model (wflow SBM) is used. The work proceeds by: (i) comparing soil–water process representations in both models ; (ii) performing sensitivity analyses of soil parameters; (iii) designing a rural NbS scenario by modifying soil-hydraulic parameters; and (iv) evaluating the effects of these scenarios on flood hydrographs at the catchment outlet.
SWAP results show that simulated infiltration and overland flow are most sensitive to saturated water content (𝜃𝑠𝑎𝑡) and saturated hydraulic conductivity (K𝑠𝑎𝑡). A two meter sandy soil column would have accommodated the entire July 2021 rainfall, resulting in negligible overland flow. In wflow SBM, sensitivity analysis indicated that simulated flood hydrographs are most influenced by KsatVer, 𝜃𝑟, and 𝜃𝑠. Adjusting 𝜃𝑟 and 𝜃𝑠 in downstream subcatchments could significantly improve the model’s ability to match the observed hydrographs.
Scenario simulations in wflow SBM reveal spatially dependent effects. Downstream interventions have a damping effect on peak flow magnitude, whereas interventions in the upstream part of the Netherlandscan increase it. A catchment-wide rural NbS scenario reduces the downstream peak by approximately 50% and delays the peak by approximately two hours relative to the default model. This modelled delay lies within the observed five-hour offset between the Geul and Meuse peaks in July 2021, suggesting such measures are unlikely to increase flood risk at the confluence via peak synchronization.