Sustainable stormwater management under the impact of climate change and urban densification

Highlights

• Stormwater management practices help to adapt to extreme rainfall events.

• Green roofs, rain gardens and porous pavement are helpful to cope with heavy rains.

• Creation of living space can be combined with water resilient planning.

• Compaction by additional storeys with green roofs preferable over extra buildings.

• Reduction of climate change impact of heavy rainfall with nature-based solutions.

Abstract

The demand for living space is rising in growing cities. To restrict urban expansion in the outskirts, a common strategy is to densify existing neighbourhoods. Densification implies the increase of water impervious area which increases the vulnerability to flooding during extreme precipitation events. Sustainable urban drainage systems are considered as a strategy to handle stormwater runoff locally and thus relieve the sewage system. This study investigates the combined quantitative hydrological impact of densification and sustainable stormwater management measures in a residential neighbourhood in Munich, Germany. The living-lab approach pursues the application of nature-based solutions in a real planning case to achieve positive climate effects while densifying the neighbourhood. The study is based on single event simulations of three return periods with the physically based software PCSWMM. The events are implemented for both current and climate change precipitation intensities of the RCP 8.5 projection for 2040–2069. Three scenarios are implemented: a status quo, a business as usual scenario (additional buildings without compensation measures) and a best-case scenario (one additional floor with green roofs disconnected from the sewers in combination with rain gardens and porous pavements on the land parcels). The comparison between the different scenarios focuses on three main aspects of the water balance, namely, infiltration, runoff and storage. The results show that measures for sustainable stormwater management are crucial elements to cope with an increasing number of heavy precipitation events due to climate change. The best-case scenario significantly outperforms the other two concerning water infiltration, surface runoff and storage. Most notably is the impact of climate change projection rainfall intensities for 2040–2069. The outcomes for these intensities clearly show the positive impact of sustainable water-sensitive design. The results demonstrate that it is in fact possible to enhance the water balance and gain new living space simultaneously if a sustainable urban planning strategy is implemented that includes future-oriented stormwater management.

Introduction

Today, about 50 percent of the world’s population lives in cities (United Nations, 2019). With growing urbanisation worldwide, it is predicted that about two thirds of the global population will live in urban environments by 2050 (ibid.). Munich, Bavaria, Germany, where the study area of this work is located, is one of few significantly growing big cities in Germany. Current population of 1.5 million is predicted to increase by 18.8% to reach 1.85 million inhabitants by 2040 (Landeshauptstadt München, 2019, Landeshauptstadt München, 2013).

Densification is a common approach to cope with the rapid growth. Densification as a planning strategy aims to constrain the expansion of built-up area on agricultural land and natural landscapes in the outskirts by creating additional living space within already built areas (Sörensen et al., 2016). In Munich, where land for further expansion is generally scarce, potential for the implementation of densification lies within the reuse of derelict land, e.g. of disused railway areas and infill development in existing residential areas (Landeshauptstadt München, 2013, Landeshauptstadt München, 2011). However, densification leads to further sealing of surfaces and the loss of green spaces with negative environmental impacts (Pauleit et al., 2005). Particularly temperature and wind flow patterns can be influenced (Fontenelle et al., 2015, Lemonsu et al., 2015). Moreover, the amount of rainfall that can infiltrate is reduced while stormwater runoff to the sewer is increased (Haase, 2009, Kokkonen et al., 2018, Zhang et al., 2015). Yet, densification can also be understood as a chance for sustainable urban development as densifying neighbourhoods promote resource and transport efficiency and undeveloped land out of town can be preserved as natural environment (Emilsson and Sang, 2017).

Climate change puts further pressure on cities. According to the IPCC it is very likely that extreme precipitation events intensify in continental Europe, especially in winters (Kovats et al., 2014). Therefore, the need for climate adaptation and mitigation is increasing (Alexander et al., 2019, Leandro et al., 2020, Kovats et al., 2014). These challenges request a change in urban water management and urban planning. In order to restore a city’s natural hydrology, integrated urban infrastructure solutions are needed that enable the city to act as a “sponge” (Griffiths et al., 2020). This design aims to create specific flooding spaces, storage capacities and to use rainwater as a resource (International Water Association, 2016; Nguyen et al., 2019). The main objective of stormwater management measures like green roofs, permeable paving or rain gardens is to reduce stormwater runoff and increase retention and infiltration capacity (Davis and Naumann, 2017, UNEP, 2014, Berghage et al., 2009, Malaviya et al., 2019). These measures are also referred to as nature-based solutions in the following text, as this concept includes climate change mitigation and adaptation by supporting, amongst other things, stormwater retention (Pauleit et al., 2017). However, with increasing integration of sustainable urban drainage systems, today the focus lies also on recreational and aesthetical concerns, water quality, ecology of receiving waters, restoration of the flow regime, the use of the stormwater resource, resilience and microclimate, just to name the most relevant elements (Davis and Naumann, 2017, Fletcher et al., 2015).

Thus, the potential of stormwater management measures is multifarious and offers various chances for adaptation and reduction of vulnerability of cities. However, there are still knowledge gaps about their effectiveness regarding adaptation and mitigation of climate change (Kabisch et al., 2017). Long-term impacts and potentials still need to be systematically investigated in order to translate the knowledge into management approaches and policy instruments (Kabisch et al., 2017). With respect to urban densification projects, which are indispensable in rapidly growing cities where there is pressure for action to create more living space, there is still little knowledge about the capacity of nature-based solutions to compensate the increase of impervious surfaces. Particularly the frequency and magnitude of changing precipitation patterns through climate change increases the need to understand their impact on stormwater infrastructure (McGrane, 2016). It needs to be determined to what extent sustainable rainwater management measures can compensate for increased imperviousness of urban areas.

The objective of this study is to investigate the hydrological impact of densification under altering return periods of heavy precipitation events through climate change in a residential area in Munich, Germany. The research approach of the study follows the concept of a Living Lab, which includes a concrete planning case with its real framework conditions. Thus, the effectiveness of individual rainwater management measures is not evaluated generically. The impact of heavy rainfall events on infiltration, runoff and storage of the area for three scenarios that exemplify the current situation, a business as usual scenario and a best-case scenario is examined. The scenarios are examples for specific planning cases which could typically be implemented in this specific settlement type in order to densify the area. The focus lies on the assessment of the performance of the stormwater management measures and the influence of additionally sealed surfaces for the named specific scenarios. The study investigates whether the gain of living space can be combined with the enhancement of the water balance in the neighbourhood. The study area of this work has a moderate density with lots of green space which largely remains even after densification. The implemented measures comprise green roofs, rain gardens and porous pavements as nature-based solutions.