Elsevier

Landscape and Urban Planning

Volume 136, April 2015, Pages 144-155
Landscape and Urban Planning

Research Paper
Modelling the anthropogenic impacts on fluvial flood risks in a coastal mega-city: A scenario-based case study in Shanghai, China

https://doi.org/10.1016/j.landurbplan.2014.12.009Get rights and content

Highlights

  • A scenario-based approach is presented to evaluate anthropogenic impacts on urban flood risks.

  • Three anthropogenic variables (land subsidence, urbanization and flood defence) are considered.

  • Land subsidence and urbanization lead to proportionate but non-linear impact on urban flood risks.

  • Flood defences offer considerable benefits in reducing the total inundated areas.

Abstract

This paper describes a novel approach to the evaluation of anthropogenic impacts on flood risks in coastal mega-cities by incorporating three anthropogenic variables (land subsidence, urbanization and flood defence) within a scenario-based framework where numerical modelling was undertaken to quantify the risks. The evolving risks at four time points (1979, 1990, 2000 and 2009) were assessed for the Huangpu River floodplain where the city of Shanghai is located. A 2D hydrodynamic model (FloodMap-Inertial) was used to estimate the flood risks associated with each scenario. Flood events with various return periods (10-, 100- and 1000-year) were designed based on a one in 50 year flood event that occurred in Shanghai on August 1997.

Results demonstrate the individual as well as the combined impacts of the three anthropogenic factors on the changing fluvial flood risks in the Huangpu River basin over the last three decades during the city's transitional economy (1979–2009). Land subsidence and urbanization were found to lead to proportionate but non-linear impact on flood risks due to their complex spatial and temporal interaction. The impacts and their sensitivity are the function of the rate and spatial distribution of each evolving factor. While the pattern of response to individual anthropogenic variables is largely expected, the combined impacts demonstrate greater spatial and temporal variation. Flood defences offer considerable benefits in reducing the total inundated areas in the Huangpu River basin over the periods considered, for all magnitude floods. This, to a large extent, alleviates the adverse impacts arising from land subsidence and urbanization.

Introduction

Flooding of coastal and fluvial systems is one of the most devastating natural hazards, causing considerable personal injury and property damage globally each year (Jongman et al., 2012, Jonkman, 2005, UNISDR, 2009). It is widely recognized that both the natural processes and anthropogenic activities have a large impact on the hydrological dynamics within floodplains, river deltas or coastal zones by altering the spatio-temporal characteristics of rainfall, underlying surface runoff and underground discharge. Increases in sea level and extreme events of rainfall and storms, which are often attributed to potential climate change, have been suggested as the primary cause of exacerbating the physical drivers of flood frequency and intensity (IPCC, 2013). Although the anthropogenic influences on flooding are less well understood than natural impacts, there is a growing awareness of their vulnerability characteristics and positive/negative feedbacks.

Rapid expansion in human settlements occupies floodplains and increases imperviousness which in turn increases peak discharge and surface runoff (Suriya & Mudgal, 2012). Moreover, as urban areas grow, the demand for water and living space gradually increases. Widespread land subsidence caused by excessive groundwater withdrawal and engineering activities (e.g. construction of high-rise buildings and underground projects) further amplifies the harmful effects of flooding by modifying the ground elevation, stream slopes and flow pathways. The negative experience of being flooded can also feed back to human's activities and social development because the human society is a self-adaptive system (Dawson et al., 2011). Indeed, for a long time, people have been seeking to alleviate the adverse consequences of flooding by structural measures (e.g. flood defences) and more recently, non-structural approaches (e.g. flood-resilient buildings and ‘green roofs’). Such measures have proved to be robust in some situations while less successful in others. Their effectiveness is being continuously evaluated in an uncertain climatic and anthropogenic future.

Although there are still debates as to whether the number of flood disasters driven by climate variation has increased, in the Asia context, there is an increasing consensus that more intense precipitation events are being observed and recorded in the regional level over the last few decades (e.g. Zhai, Zhang, & Pan, 2005), causing persistent, if not increasing events of flooding at a local level (Wu, Yu, Wilby, & Chen, 2012). Despite the uncertainty in the frequency of flood hazard occurrences, exposure to flooding has undoubtedly increased as a result of more concentrated settlements on floodplains vulnerable to flooding from various sources. Coastal mega-cities are in particular vulnerable to flood risks given their typical low-lying nature, vulnerability to sea-level rise and/or storm surge and increased concentration of population. A number of studies have examined the anthropogenic impacts on the hydrological responses of flooding using distributed models and scenario analysis, focusing in particular on the process of urbanization and/or land cover change (e.g. Du et al., 2012, Suriya and Mudgal, 2012, Xiang and Clarke, 2003, Yin et al., 2014). However, only a limited few studies have attempted to quantitatively explore the individual and combined impacts of anthropogenic factors by means of hydrodynamic modelling and/or comprehensive urban-development scenarios.

In this paper, we used a 2D hydrodynamic model (FloodMap, Yu and Lane, 2006a, Yu and Lane, 2006b, Yu and Lane, 2011) to quantify flood responses to three major anthropogenic variables (i.e. urbanization, land subsidence and flood defence) in the city of Shanghai, illustrating their underlying influences on flood risks during the city's transitional economy between 1979 and 2009. The main aims of this study are to: (1) develop a scenario-based framework for determining the potential interactions between anthropogenic variables; (2) identify and compare the spatio-temporal characteristics of flooding under different anthropogenic scenarios; and (3) understand and quantify the effects of human induced drivers on flood risks. The remainder of this article is organized as follows. Section 2 introduces the materials and methodology, including the study area, scenario design, data availability, flood modelling and evaluation metrics. The results and discussion are presented and discussed in Sections 3 and 4. Section 5 concludes with the key findings, possible adaption measures and future research directions.

Section snippets

Study area

Shanghai is a coastal mega-city located in the frontal alluvial plain of the Yangtze River Delta, with a total area of 6340 km2. It has a mild and low-lying topography. The average elevation is about 4 m above the Wusong Datum, with most of the region below the high tide level. This area experiences a northern subtropical monsoon climate with four distinctive seasons, receiving most of its cyclonic storms and rainfall during the flood season (June–September). The mean sea level of Shanghai has

Results

The impacts of land subsidence and land use change on fluvial flood risks are investigated by assessing the characteristics of: (i) the time series of inundation (Section 3.1); and (ii) the maximum inundation (Section 3.2). Three metrics are used to quantitatively evaluate the relative impacts of different factors, including the total inundation area, F statistics and RMSD.

Discussions

The findings reported in Section 3 suggest that the anthropogenic impacts on fluvial flood risks in Shanghai are determined by the interplay between individual factors. Although urbanization coupled with land subsidence have significantly aggravated the risks, the evolving flood defence system to some extent mitigates the negative effects over the last three decades, restricting risks to regions that are of relatively less economic importance. The following sections will discuss the key

Conclusions

This paper describes a novel approach to the evaluation of anthropogenic impacts on flood risks by incorporating three anthropogenic variables within a scenario-based framework where numerical modelling is undertaken to quantify the risks. Distributed data on land subsidence rate, urbanization rate and flood defence systems are considered in the analysis. A number of major conclusions can be drawn. First, changes in land subsidence and/or urbanization may lead to proportionate but non-linear

Acknowledgements

This paper was supported by the National Natural Science Foundation of China (Grant nos: 41201550, 41371493, 71373084), the Humanities and Social Science Project of Education Ministry (Grant no: 12YJCZH257), Innovation Program of Shanghai Municipal Education Commission (Grant nos: 13YZ061, 13ZZ035), the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (Grant no: SKLEC-KF201407), Key Project of Young Talent Foundation from Zhejiang Gongshang University (Grant no:

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