The Hydrological Effect of Urban Nature-based Solutions on Catchment Scale

Abstract

Climate change imposes an increasingly big challenge worldwide regarding floods and droughts. The Netherlands is no exception to this, as it has been increasingly hit by these phenomena in recent years, especially in the south of the country. The July 2021 flood in the Geul catchment intensified discussions on climate resilience. It prompts consideration of transforming the Dutch landscape into a more sponge-like system. Urban areas, identified as both problem areas and potential solutions within the catchment, stand out, since these areas are highly vulnerable and amplify climate change effects. Implementing Urban Nature-Based Solutions (UNBSs) emerges as a promising approach to address these challenges, potentially offering a solution to enhance climate resilience and mitigate the vulnerabilities of urban areas.
This research addressed the challenge of flood protection in the Geul catchment. It focuses on studying the impact of UNbSs and developing a methodology to select, model, and assess their performance in the catchment. This has been done by answering the following questions:

Which Urban Nature-based Solutions are suitable for the Geul catchment?
What is the hydrological effect of these Urban Nature-based Solutions locally?
What is the hydrological effect of Urban Nature-based Solutions at catchment scale?

This research followed a three-step workflow in line with the research questions. Firstly, the study focused on an assessment of neighbourhood types in the catchment and selecting suitable UNbSs for the Geul catchment. Based on this assessment, UNbS measures were chosen for their compatibility with these neighbourhoods. Next, this research analyzed the local effects of the implementation of these measures into the chosen neighbourhoods using the Climate Resilient City Tool. The final step involved an assessment of the hydrological impact of UNbSs on catchment scale. This was achieved by converting the previous results to wflow parameters.
The research succeeded in establishing a workflow for modeling UNbS impact at the catchment scale. It involved selecting UNbSs for three neighborhood types, resulting in the selection and implementation of green roofs, water roofs, permeable pavement, retention ponds, removing pavement to plant green and bioswales.
Locally, the study found that permeable pavement and bioswales were most effective for increasing storage capacity, evapotranspiration, and groundwater recharge. However, the overall order of magnitude for all measures remained consistent across neighbourhoods. Considering the total storage capacity increase, the order of magnitude was found to be between 34- and 39-mm storage equivalent over the total surface area of the neighbourhoods.
On a catchment scale, UNbS implementation resulted in a discharge reduction ranging from 1.71% to 3.10%, with a more pronounced effect upstream. Three neighbourhood scenarios exhibited minimal differences, all below 0.1%, which is considered insignificant. Absolute discharge reduction consistently followed patterns across low and high discharge periods, with more substantial reductions during peak discharges. However, the percentage difference between original and altered discharge was found to be similar across all periods, making this trend less evident.
Based on these results, the research proposed the following five recommendations regarding future research and implementation: 1) improve model transparency and sensitivity analysis, 2) consider practical implementation challenges, 3) refine typology mapping and data, 4) evaluate Nature-based Solutions in diverse landscapes and 5) promote the role of UNbSs in climate resilience.