Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.3.4.3 Stormwater Regulation and Sanitation ====

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Urban parks and open spaces, forests, wetlands, green roofs and engineered stormwater treatment devices help manage stormwater and wastewater by reducing the volume of stormwater runoff, reducing surface flooding, and reducing contamination of runoff by pollutants ( ''robust evidence'' , ''high agreement'' ). Engineered devices include bioswales, rain gardens, and detention and retention ponds, and are becoming common and standard approaches to mitigate the negative effects of impervious surfaces on stormwater quality and surface flooding in cities ( [[#Zhou--2014|Zhou, 2014]] ; McPhillips et al., 2020). Allotment gardens, street trees, green roofs and urban forests may also help reduce runoff and provide a stormwater retention service (Pennino, McDonald and Jaffe, 2016; Berland et al., 2017; Gittleman et al., 2017). Modelling and empirical studies show that NBS at small spatial scales lead to improvements in water quality and reduction of peak flows (Moore et al., 2016; Keeler et al., 2019; Webber et al., 2020). Peak flow reductions are greatest for small rain events. For example, D-Ville et al. (2018) observed a 30–70% reduction in peak flow for the 1-in-30 year storm, but performance reduces for more intense rainfall or if saturated (Garofalo et al., 2016). Employing NBS to reduce flooding on roads can be an important adaptation mechanism for reducing the impact of flooding events on traffic flows (Pregnolato et al., 2016).

During periods with intense precipitation, low-lying urban parks and open space, engineered devices and wetlands can play an important role in reducing stormwater runoff volumes by providing places for water to be stored and infiltrate during heavy storms (Moore et al., 2016). However, the magnitude of the runoff reduction service will depend on the total area of green infrastructure, vegetation type and its position on the landscape. There is less evidence of the effectiveness of NBS at larger temporal and spatial scales (Pregnolato et al., 2017; Jefferson et al., 2017). The performance of NBS depends on the degree to which their extent and spatial configuration in the city are optimised to capture runoff ( [[#Fry--2017|Fry and Maxwell, 2017]] ). Investing in a diversity of NBS types may be important to maximise stormwater management and flood regulation, as different types of engineered NBS have different strengths and weaknesses.

Overall, NBS are attractive adaptation options for stormwater management and to reduce impacts of pluvial and fluvial flooding in cities (Rosenzweig et al., 2018a) compared, and in combination, with grey infrastructure. Cities with combined sewer infrastructure are ''likely'' to see benefits from NBS due to reductions in stormwater quantity and reduced sewage overflows. Cities where a large proportion of residents lack access to piped infrastructure and drink surface water may see large benefits, especially to human health, from NBS investments (Keeler et al., 2019). Where future large-scale upgrades or installation of grey infrastructure will be necessary, new and growing cities may have more opportunity to realise large net benefits from investments in NBS. Older cities, and new, rapidly urbanising areas that lack large-scale water infrastructure may see the greatest benefits from enhanced NBS, relative to cities where heavy investments infrastructure upgrades have already been made. Cities facing climate changes that include more frequent or extreme precipitation may also see large water quality benefits from investment in NBS (Keeler et al., 2019). Overall, there is increasing evidence that NBS for addressing stormwater is cost effective (Bixler et al., 2020; Kozak et al., 2020; Mguni, Herslund and Jensen, 2016), especially in cities facing a need to update current infrastructures.

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