Abstract
Abstract. The economic and human consequences of extreme precipitation and the related flooding of urban areas have increased rapidly over the past decades. Some of the key factors that affect the risks to urban areas include climate change, the densification of assets within cities and the general expansion of urban areas. In this paper, we examine and compare quantitatively the impact of climate change and recent urban development patterns on the exposure of four European cities to pluvial flooding. In particular, we investigate the degree to which pluvial floods of varying severity and in different geographical locations are influenced to the same extent by changes in urban land cover and climate change. We have selected the European cities of Odense, Vienna, Strasbourg and Nice for analyses to represent different climatic conditions, trends in urban development and topographical characteristics. We develop and apply a combined remote-sensing and flood-modelling approach to simulate the extent of pluvial flooding for a range of extreme precipitation events for historical (1984) and present-day (2014) urban land cover and for two climate-change scenarios (i.e. representative concentration pathways, RCP 4.5 and RCP 8.5). Changes in urban land cover are estimated using Landsat satellite imagery for the period 1984–2014. We combine the remote-sensing analyses with regionally downscaled estimates of precipitation extremes of current and expected future climate to enable 2-D overland flow simulations and flood-hazard assessments. The individual and combined impacts of urban development and climate change are quantified by examining the variations in flooding between the different simulations along with the corresponding uncertainties. In addition, two different assumptions are examined with regards to the development of the capacity of the urban drainage system in response to urban development and climate change. In the stationary approach, the capacity resembles present-day design, while it is updated in the evolutionary approach to correspond to changes in imperviousness and precipitation intensities due to urban development and climate change respectively. For all four cities, we find an increase in flood exposure corresponding to an observed absolute growth in impervious surfaces of 7–12 % during the past 30 years of urban development. Similarly, we find that climate change increases exposure to pluvial flooding under both the RCP 4.5 and RCP 8.5 scenarios. The relative importance of urban development and climate change on flood exposure varies considerably between the cities. For Odense, the impact of urban development is comparable to that of climate change under an RCP 8.5 scenario (2081–2100), while for Vienna and Strasbourg it is comparable to the impacts of an RCP 4.5 scenario. For Nice, climate change dominates urban development as the primary driver of changes in exposure to flooding. The variation between geographical locations is caused by differences in soil infiltration properties, historical trends in urban development and the projected regional impacts of climate change on extreme precipitation. Developing the capacity of the urban drainage system in relation to urban development is found to be an effective adaptation measure as it fully compensates for the increase in run-off caused by additional sealed surfaces. On the other hand, updating the drainage system according to changes in precipitation intensities caused by climate change only marginally reduces flooding for the most extreme events.
Highlights
In recent years it has been widely demonstrated that cities globally are becoming increasingly exposed to the occurrence and impacts of pluvial flooding (Barredo, 2007; Field et al, 2012)
The results of the urban development analyses show that the four cities have experienced increasing imperviousness throughout the past 30 years, which is consistent with the predominant trend towards urbanization worldwide, with absolute changes ranging from 6.6 % points in Nice to 11.6 % points in Strasbourg (Table 1)
We examine the impacts of recent urban development and future climate change in exposing urban areas to pluvial flooding for four cities in Europe using a combined remote-sensing and flood-modelling methodology
Summary
In recent years it has been widely demonstrated that cities globally are becoming increasingly exposed to the occurrence and impacts of pluvial flooding (Barredo, 2007; Field et al, 2012). It is evident that the observed changes in risk have been caused by a combination of different factors. These include ongoing climate change, leading to increases in the frequency and intensity of extreme precipitation events, general population growth and high rates of urbanization during the late 20th and early 21st centuries. The extent of urban land cover has dramatically increased while at the same time concentrations of assets and economic activities in cities worldwide have rapidly increased (Angel et al, 2011; Field et al, 2012). Urban areas are expected to become even more exposed and vulnerable to flooding in the future
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