Climate change impact assessment on urban rainfall extremes and urban drainage: Methods and shortcomings
Research Highlights
► Critical review is made of methods for assessing climate change impacts on rainfall in urban areas. ► Climate change impact estimation on extreme, local and short-duration rainfall is highly uncertain. ► Statistical downscaling and bias correction is required. ► Application, comparison and verification of different downscaling assumptions is recommended. ► Trend testing has to account for temporal clustering of rainfall extremes.
Introduction
For more than a century sewer systems have been constructed at large scale across cities worldwide. These sewer systems have reduced the vulnerability of the cities in general, but at the same time could make them more vulnerable to rainfall extremes, partly due to the lack of consideration to what occurs when the design criteria are exceeded. Next to this increase in the vulnerability, there is strong evidence that due to the global warming the probabilities and risks of sewer surcharge and flooding are changing. In their Fourth Assessment Report (AR4) the Intergovernmental Panel on Climate Change (IPCC, 2007) indeed reports for the late 20th century a worldwide increase in the frequency of extreme rain storms as a result of global warming. Based on climate model simulations with different future greenhouse gas emission scenarios, IPCC (2007) furthermore concluded that it is very likely (defined as more than 90% likelihood) that this trend will continue in the 21st century. Water managers therefore have to start accounting for these effects.
Consequently, the number of hydrological impact studies of climate change strongly increased in recent years. These studies, however, most often focus on risk of floods and droughts on river catchment scale. The number of climate change studies dealing with urban drainage impacts is still rather limited, partly because they require a specific focus on small urban catchment scales (usually less than 500 km2) and short duration precipitation extremes (normally less than 1 day). Despite the significant increase in computational power in recent years, climate models still remain relatively coarse in space and time resolution and are unable to resolve significant features at the fine scales of urban drainage systems (Fig. 1). They also have limitations in the accuracy of describing precipitation extremes due to a poor description of the non-stationary phenomenon during a convective storm leading to the most extreme events on a local scale. To bridge the gaps between the climate model scales and the local urban drainage scales and to account for the inaccuracies in describing precipitation extremes, downscaling methods and bias-correction methods are commonly used in practice.
Section snippets
methods
Evaluating regional impacts from possible climate change on urban drainage requires a methodology to estimate extreme and short-duration rainfall statistics for the time period and the geographical region of interest. For historical conditions, climate change effects can be investigated by analyzing trends in long-term historical records of rainfall. For future conditions, projected changes in rainfall statistics are based on future scenarios in greenhouse gas emissions simulated in climate
Application and verification of results of downscaling
Regardless of the downscaling method employed, verification of the downscaled climate model results under the present climate is needed. Urban drainage design and analysis are largely based on considering probabilities of reaching and exceeding certain extremes, and thus it is important that rainfall exceedance probabilities and related probabilistic results are captured well by the downscaled climate model results. Therefore comparison of quantiles in terms of intensity-duration-frequency
Methods to propagate the climate change scenarios in urban drainage models
To obtain climate change impact estimates to urban drainage, climate change scenarios are to be propagated through urban drainage models. This can be done using the output time series of the climate model as direct input for the drainage model. However, because local and small scale variables are required as input to the urban drainage model, statistical bias-correction and downscaling (or combined) have to be applied. The bias correction will avoid that for the control simulations, drainage
Conclusions
Discussions of climate change impacts on hydrology generally focus on global hydrological impacts, e.g. on floods, low flows, groundwater recharge, and droughts along major rivers. They offer little information on the small time- and space-scale hydrological impacts of relevance for urban drainage applications. The review given in this paper of climate change impacts on extreme short-duration precipitation, and sewer systems surcharge and flooding, highlighted particular difficulties at these
Acknowledgements
This review is the result of the research activities of the International Working Group on Urban Rainfall (IGUR) of the International Water Association (IWA) and the International Association for Hydro-Environment Engineering and Research (IAHR). The IGUR Working Group operates under the umbrella of the IWA/IAHR Joint Committee on Urban Drainage. Jonas Olsson acknowledges support from the Mistra-SWECIA research program.
Patrick Willems is Associate Professor in water engineering at Katholieke Universiteit Leuven (Department. of Civil Engineering, and Leuven Sustainable Earth Research Center) and part-time lecturer at the Vrije Universiteit Brussel. He is an author of more than 200 publications, about 45 in peer-reviewed international journals. He is a co-promoter of 12 PhD researchers, many of them focusing on climate change impact estimates on hydrological extremes (floods, droughts and extreme surface water
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Cited by (0)
Patrick Willems is Associate Professor in water engineering at Katholieke Universiteit Leuven (Department. of Civil Engineering, and Leuven Sustainable Earth Research Center) and part-time lecturer at the Vrije Universiteit Brussel. He is an author of more than 200 publications, about 45 in peer-reviewed international journals. He is a co-promoter of 12 PhD researchers, many of them focusing on climate change impact estimates on hydrological extremes (floods, droughts and extreme surface water pollution levels, in and outside Europe). He lectures courses on (urban) water engineering. He is currently chairman of the International Working Group on Urban Rainfall of the International Water Association (IWA) and the International Association for Hydro-Environment Engineering and Research (IAHR).
Karsten Arnbjerg-Nielsen is Associate Professor at the Department of Environmental Engineering at the Technical University of Denmark. His main fields of research are urban hydrology, rainfall modeling, and adaptation to climate change impacts. He worked as a consultant from 1998 to 2008 as project manager and Head of Innovation before returning to academia. He has published more than 20 ISI publications and is Editor of Water Science and Technology responsible for the topic climate change.
Jonas Olsson is senior researcher in hydrology at the Swedish Meteorological and Hydrological Institute (SMHI). His main fields of research are climate hydrology, statistical hydrology and dynamical runoff modeling and forecasting. His main work task is project management and supervision and he has functioned as WP-leader in several EU-projects. He has published some 45 peer-reviewed papers, is a frequent reviewer of papers and applications and has had a range of other scientific assignments. In terms of education he lectures at university courses as well as co-supervises Ph.D. and M.Sc. students.
Van-Thanh-Van Nguyen is Endowed Brace Professor and Chair of Civil Engineering Department at McGill University. He is also Director of the Brace Centre for Water Resources Management and Associate Director of the Global Environmental and Climate Change Centre. His professional contributions over the past 25 years have been mostly in Hydrology and Water Resources Management. He has served as expert consultant to various organizations in Canada and in other countries, and has been invited professors at universities in Canada, Japan, Singapore, and Malaysia. He was an active member in several professional associations (President, Hydrological Science Section, Asia-Oceania Geosciences Society, 2006–2008).