Urban populations are rapidly increasing, with 6.7 billion people (68.4% of the global population) projected to live in cities by 2050, compared to 4.4 billion today. Anthropogenic changes to, and control of, hydrological processes in urban areas, coupled with ongoing climate change enhance global urban populations being increasingly at risk of flooding and water scarcity, sometimes in the same city at different times of year.

Traditional drainage systems have left urban infrastructure vulnerable to flooding, particularly due to the increased frequency and intensity of extreme precipitation brought on by climate change. More urbanised communities with impermeable areas increased the flooding risks to people. For example, approximately 1.81 billion people (23% of the global population) are currently exposed to 1-in-100-year return period flooding.

As such, a paradigm shift in urban water management is necessary to enhance the societal and ecosystem services urban freshwaters provide whilst reducing hazards. This shift is currently taking place, with a move away from grey infrastructure and piped drainage, and towards Nature-Based Solutions (NBS) that work alongside and alleviate pressure on grey infrastructure, promoting more natural hydrological processes within urban areas, as emphasised by the 2022 report from the Intergovernmental Panel on Climate Change (IPCC) Working Group II.

In China, the National Government is concerned about the future of climate resilience and urban ecosystem services to promote - the "Sponge City Program" (SCP) initiated in 2013 and progressed with amendments and legislated the National plan of promoting the SCP in 2014, a decade before. Hence, the effectiveness of SCP facilities in the context of future climate change has also not been sufficiently studied. Current knowledge and practice of SCP facilities are limited in their ability to address climate challenges and climate change adaptation.

Since then, we have focused on assessing SCP facilities on climate change adaptation and quantifying the environmental effects of both SCP and traditional facilities through lifecycle assessment. We also try to appraise the performance of SCP facilities under existing climatic conditions and anticipated future climatic scenarios for addressing climate challenges.

Hence, in this study, we have argued that future SCP interventions are urgently addressing the local challenges and be tailored to site-specific temporal, spatial, and functional dimensions, for example, we need to do the "fit-for-purpose" SCP practices and look at their physical and human interventions and their functions for the various Chinese cities. Indeed, SCP has been defined as “place-based partnerships between people and nature” for climate change adaptation, implemented by considering the local conditions and providing community-led climate resilience practice. 

Our overarching aim of this study is to explore the effectiveness of SCP facilities that may change with climate change. We want to know how increasing the coverage of bioswales may offset urban runoff reductions in efficiency under various rainfall patterns in the future by considering the projected water quantity and quality for these Chinese SCP-enhanced Bioswales (see Figure 1).

In this paper, we focus on adopting the SWMM model and evaluate the performance of bioswales under various future climate change scenarios with three major targets here:

1. To evaluate different rainfall intensities (i.e., 2, 5, 10, 20, 50, and 100 years return period) and precipitation patterns (i.e., early peak, central peak and late peak) affect the performance of bioswales;

   2.To quantify the performance of existing SCP facilities in the study area under different climate change scenarios (i.e., rainfall intensity and consecutive dry days);

   3. To identify suitable coverage of bioswales within the study area for climate change adaptation.

Our study is the first and pioneered study looking at the effectiveness of bioswales under contemporary and future conditions, taking an urban community in Ningbo, a Chinese pilot Sponge City as an example (see Figure 2 below).

 We have found that:

• The overall performance of existing SCP facilities such as bioswales, rain gardens, permeable pavements, etc., on their runoff volume reduction and nutrient removal are quite similar under various climate change scenarios at the rainfall peak.
• The effects of climate change are thought to be more severe than these SCP facilities can mitigate in our case, Ningbo, E Coast of China.
• To address the impact of climate change over 20 years from 2020 to 2039, increasing the community's bioswales coverage to 2% is a viable solution for the removal of nutrients from urban runoff, whereas expanding the community's bioswales to at least 4% would effectively address both flooding and water quality issues simultaneously.

In summary - this study presents evidence that sheds light on how well NbS performs in reducing urban runoff in the context of climate change. Future studies would be more meaningful if the coverage of NbS could be designed with cost-effectiveness in mind and comparative analyses among different NbS options would provide a more comprehensive understanding of their efficacy. 

For detailed information - you are very welcome to read our manuscript that was published in the Scientific Reports