Surface runoff in urban areas: The role of residential cover and urban growth form

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Highlights

  • High-density settlements generate more surface runoff within the neighborhood.

  • Compact growth reduces total surface runoff at the city scale.

  • Trade-offs in runoff generation were found between neighborhood and city scales.

Abstract

In the context of rapid urbanization and climate change, understanding the impact of urban dynamics on surface runoff is important. For future urban planning, there is a particular knowledge gap regarding the factors that affect surface runoff to improve urban resilience to local flooding. This study explored the impacts of different urban dynamics on surface water runoff considering the surface cover characteristics of different residential types (i.e., low- and high-density settlements) in the city of Munich, Germany. The surface cover characteristics were quantified using the ‘i-Tree Canopy’ method based on high-resolution aerial images. The surface runoff was simulated by the Soil Conservation Service Curve Number (SCS-CN) method. Meanwhile, the impacts of different urban dynamics on surface runoff were compared by incorporating a scenario-based urban dynamic modeling approach. The results suggest that, compared with low-density settlements, high-density settlement areas tend to generate more surface runoff within the neighborhood because they have considerably less vegetative surfaces and more built ones. As the development of low-density settlements leads to a greater loss of green and open spaces in the surrounding peri-urban areas, however, the compact growth scenarios characterized by high-density settlements have advantages over low-density scenarios in terms of overall surface runoff reduction at the city scale. Thus, the trade-off between the neighborhood and city scale should be considered when promoting nature-based solutions for surface runoff mitigation. Moreover, the approach presented here can be used as an effective planning tool supporting various urban contexts.

Introduction

Over the last few decades, rapid urbanization with its’ increasing ecological footprint has drawn public as well as academic attention (Elmqvist et al., 2013). Along with the social and economic opportunities to the local communities, land-use changes with urbanization introduce considerable negative environmental consequences, for instance the loss of biodiversity (Yan et al., 2019), air pollution (Wang et al., 2020), greenhouse gas emissions (Pan et al., 2019), urban heat island effect (Rahman et al., 2020), and stormwater runoff (Sterling et al., 2012). Among the negative effects of urbanization, extensive area of hardscape significantly transforms the hydrological response and has become a critical issue (Zhang et al., 2018b).

The process of rapid urbanization involves considerable land use conversions of vegetative surfaces (e.g., agricultural lands, forests, grasslands, and wetlands) into built-up urban areas leading to an increasing proportion of impervious surfaces (Kim et al., 2017; Li et al., 2018; Tang et al., 2005). Vegetative surfaces can intercept significant amounts of incoming rainfall (Armson et al., 2013; Zhang et al., 2018a) and direct a significant proportion of the rainfall towards subsurface water flow (Rahman et al., 2019a). The infiltrated water can partly be used again for evapotranspiration (Rahman et al., 2019b). A greater cover of impervious surfaces inhibits infiltration and significantly increases surface runoff volume and peak discharges, consequently raising the risk of urban flooding (Li et al., 2019; Schoener, 2018; Yao et al., 2017). Although urban surface runoff increases due to the expansion of impervious urban surfaces, it can be mitigated to a significant extent by incorporating permeable urban surfaces (Lim and Welty, 2017; Miller and Hess, 2017). In particular, with the increasing frequency of rainfall extremes due to global climate change, urban flooding risk is most likely to be intensified in the future (Hirabayashi et al., 2013; Wu and Lau, 2016; Zhou et al., 2019). Therefore, more efforts are needed to understand the influence of future urbanization on surface runoff and to propose planning strategies towards the improvement of urban resilience to flooding (Debbage and Shepherd, 2018; Ohana-Levi et al., 2018).

There is a growing body of literature that investigates the impacts of urbanization on urban hydrological systems and processes (Chen et al., 2017; Prosdocimi et al., 2015; Zhang et al., 2018b). For example, Miller and Hess (2017) identified the impact of urbanization on runoff generation from storm events along a rural-urban gradient in the south of the UK and observed clear differences in hydrological response between rural and urban catchments. Using multiple runoff metrics, Zhang et al. (2018b) explored urbanization impacts on runoff regimes in the Qing River catchment located in the northern part of Beijing metropolitan area, China and reported that runoff depth metrics increased due to the transition from dry agriculture to impervious urban surfaces. Moreover, the proportion of impermeable and permeable surface covers of different residential types (high- and low-density settlements) is an important factor that determines the impacts of urbanization on surface runoff because residential areas are a dominant urban land use (Miller and Hess, 2017). For instance, Sjöman and Gill (2014) examined the impacts of different residential types with distinct surface cover characteristics on runoff generation in three major urban areas in southern Sweden. Their findings indicated that high-density residential areas with a higher proportion of impervious surfaces tended to generate more surface runoff compared to low-density areas. Nonetheless, the combined effect of different residential types when associated with different urban development schemes on surface runoff at the city scale should be further explored.

Recently, research focus has also been devoted to investigating the relationship between surface runoff and different urban growth forms (Brody et al., 2011; Kokkonen et al., 2018). Much of the current discussion regarding urban growth form has focused on the controversy between the urban sprawl and the compact growth model. Featured by large expanses of low-density or single-use development, urban sprawl is one prevailing form of urbanization worldwide; however, regarded as unsustainable due to the extensive land take and related environmental degradation (Ewing, 1997; Xu et al., 2018). As an alternative to urban sprawl, compact growth is characterized by high-density and mixed-use urban development and deemed to be capable of counteracting the negative effects of urban sprawl (Milder, 2012).

Thus far, the effects of urban growth form on urban surface runoff are still inconclusive. On the one hand, urban sprawl can increase runoff due to the vast expansion of impervious urban surfaces and the fragmentation of drainage networks as a result of low-density development (Brody et al., 2011), whereas the low-density development incorporates more green spaces may potentially provide added benefits in urban water storage and infiltration (White, 2008). On the other hand, compact growth can mitigate the impact of urbanization on runoff by constraining the spatial extent of urban expansion, while a higher proportion of impervious surfaces associated with high-density development is most commonly linked with a greater surface runoff and a higher risk of flooding (Brody et al., 2011; Tratalos et al., 2007). Therefore, comparative studies that take the impacts of both urban growth forms (i.e., urban sprawl and compact growth) on surface runoff into consideration are required to shed new light on the relative advantages and disadvantages of different urban growth forms in terms of urban runoff mitigation.

Against this backdrop, this paper seeks to fill the current knowledge gaps and contributes to the literature by exploring the impacts of different urban growth forms on surface runoff in the growing city of Munich, Germany by accounting for the surface cover characteristics of different residential types. To this end, the surface cover characteristics of different residential types are assessed based on high-resolution aerial images. Then, the impacts of different urban dynamics on surface runoff are evaluated by combining surface runoff simulations with a scenario-based urban dynamic modeling approach. The specific objectives are to examine the effects of different residential types on surface runoff at the neighborhood scale and to explore the potential trade-offs of different urban growth forms between the neighborhood and city scale in terms of runoff mitigation.

Section snippets

Study area and data acquisition

The study was carried out in the city of Munich (48°3ʹ30ʺ−48°15ʹ0ʺ N, 11°21ʹ30ʺ−11°43ʹ30ʺ E, at 519m asl). The climate of the city is characterized as humid continental with the highest precipitation occurring during warm summers (June to August). As the capital city of Bavaria, Munich is the third largest city in Germany. With an average population density of 4,668 inhabitants per km2, this city is home to approximately 1.45 million residents (Xu et al., 2018). As projected by the Bavarian

Surface cover and runoff coefficients of different residential types

There was a clear difference in the proportions of land surface cover between the two residential types, as shown in Fig. 3. The dominant land surface covers in the low- and high-density settlements were lawns and buildings, respectively. In the low-density settlements, vegetative surfaces (lawns and tree cover) accounted for more than a half of the total area, whereas built surfaces (buildings and impermeable pavement) constituted almost two-thirds of the total area in the high-density

Impacts of different residential types on surface runoff at the neighborhood scale

The low-density settlements were comprised mostly of row housing, single-family housing, and detached houses, which were typically developed with private front and/or backyard gardens (Haase et al., 2019; Pauleit and Duhme, 2000). Compared to low-density settlements, there were considerably more built surfaces (buildings and impermeable pavement) and correspondingly fewer vegetative surfaces (lawns and tree cover) in high-density settlements which were mainly consisted of multistory housing and

Conclusions

The impact of urban dynamics on surface runoff is likely to be intensified with rapid urbanization globally. Accordingly, understanding the influences of different urban dynamics on surface runoff is crucial for the formulation of planning strategies that aim to improve urban resilience to flooding. By using an integrated approach of surface cover quantification, runoff simulation, and scenario-based urban dynamic modeling, this study investigated the surface runoff under different urban

CRediT authorship contribution statement

Chao Xu: Conceptualization, Methodology, Formal analysis, Funding acquisition, Writing - original draft. Mohammad Rahman: Conceptualization, Formal analysis, Writing - review & editing. Dagmar Haase: Methodology, Writing - review & editing. Yiping Wu: Validation, Writing - review & editing. Meirong Su: Formal analysis, Funding acquisition, Project administration, Writing - review & editing. Stephan Pauleit: Resources, Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are grateful to the anonymous reviewers for providing constructive comments and suggestions to improve this article. This work was supported by the National Key R & D Program of China [No. 2016YFC0502800], the National Natural Science Foundation of China [No. 71673027], the Natural Science Foundation for Distinguished Young Scholars of Guangdong Province [No. 2017A030306032], GDUPS (2017), China Postdoctoral Science Foundation [No. 2019M663665], and the Scientific Research

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