An interdisciplinary and catchment approach to enhancing urban flood resilience: a Melbourne case.

This paper presents a novel interdisciplinary and catchment-based approach for exploring urban flood resilience. Our research identified and developed a diverse set of adaptation measures for Elwood, a suburb in Melbourne, Australia, that is vulnerable to pluvial and coastal flooding. We drew on methods from social science, urban design and environmental engineering to gain integrated insights into the opportunities for Elwood to increase its flood resilience and urban liveability. Results showed that an appropriate balance of social, infrastructural and urban design responses would be required to retreat from, accommodate and protect against flood risk. These would also deliver broader benefits such as securing water supplies through harvested stormwater and mitigating extreme heat through greener landscapes. Our interdisciplinary approach demonstrated the value of (i) engaging with the community to understand their concerns, aspirations and adaptation ideas, (ii) exploring design measures that densify and use urban forms in ways that implement adaptation measures while responding to local context, (iii) adopting modelling techniques to test the performance, robustness and economic viability of possible adaptation solutions, and (iv) innovating governance arrangements and principles needed to improve flood resilience in the Elster Creek catchment. Our research also provided valuable insight on how to operationalize interdisciplinary work in practice, highlighting the importance of sharing an impact agenda, taking a place-based approach, developing a common conceptual framework, and fostering a constructive team culture. This article is part of the theme issue 'Urban flood resilience'.

Regional frequency analysis of short duration rainfall extremes using gridded daily rainfall data as co-variate.

A regional partial duration series (PDS) model is applied for estimation of intensity duration frequency relationships of extreme rainfalls in Denmark. The model uses generalised least squares regression to relate the PDS parameters to gridded rainfall statistics from a dense network of rain gauges with daily measurements. The Poisson rate is positively correlated to the mean annual precipitation for all durations considered (1 min to 48 hours). The mean intensity can be assumed constant over Denmark for durations up to 1 hour. For durations larger than 1 hour, the mean intensity is significantly correlated to the mean extreme daily precipitation. A Generalised Pareto distribution with a regional constant shape parameter is adopted. Compared to previous regional studies in Denmark, a general increase in extreme rainfall intensity for durations up to 1 hour is found, whereas for larger durations both increases and decreases are seen. A subsample analysis is conducted to evaluate the impacts of non-stationarities in the rainfall data. The regional model includes the non-stationarities as an additional source of uncertainty, together with sampling uncertainty and uncertainty caused by spatial variability.

Identifying climate analogues for precipitation extremes for Denmark based on RCM simulations from the ENSEMBLES database.

Climate analogues, also denoted Space-For-Time, may be used to identify regions where the present climatic conditions resemble conditions of a past or future state of another location or region based on robust climate variable statistics in combination with projections of how these statistics change over time. The study focuses on assessing climate analogues for Denmark based on current climate data set (E-OBS) observations as well as the ENSEMBLES database of future climates with the aim of projecting future precipitation extremes. The local present precipitation extremes are assessed by means of intensity-duration-frequency curves for urban drainage design for the relevant locations being France, the Netherlands, Belgium, Germany, the United Kingdom, and Denmark. Based on this approach projected increases of extreme precipitation by 2100 of 9 and 21% are expected for 2 and 10 year return periods, respectively. The results should be interpreted with caution as the best region to represent future conditions for Denmark is the coastal areas of Northern France, for which only little information is available with respect to present precipitation extremes.

Impacts of climate change on rainfall extremes and urban drainage systems: a review.

A review is made of current methods for assessing future changes in urban rainfall extremes and their effects on urban drainage systems, due to anthropogenic-induced climate change. The review concludes that in spite of significant advances there are still many limitations in our understanding of how to describe precipitation patterns in a changing climate in order to design and operate urban drainage infrastructure. Climate change may well be the driver that ensures that changes in urban drainage paradigms are identified and suitable solutions implemented. Design and optimization of urban drainage infrastructure considering climate change impacts and co-optimizing these with other objectives will become ever more important to keep our cities habitable into the future.

Verification of flood damage modelling using insurance data.

This paper presents the results of an analysis using insurance data for damage description and risk model verification, based on data from a Danish case. The results show that simple, local statistics of rainfall are not able to describe the variation in individual cost per claim, but are, however, feasible for modelling the overall cost per day. The study also shows that in combining the insurance and regional data it is possible to establish clear relationships between occurrences of claims and hazard maps. In particular, the results indicate that with improvements to data collection and analysis, improved prediction of damage costs will be possible, for example based also on socioeconomic variables. Furthermore, the paper concludes that more collaboration between scientific research and insurance agencies is needed to improve inundation modelling and economic assessments for urban drainage designs.

Economic assessment of climate adaptation options for urban drainage design in Odense, Denmark.

Climate change is likely to influence the water cycle by changing the precipitation patterns, in some cases leading to increased occurrences of precipitation extremes. Urban landscapes are vulnerable to such changes due to the concentrated population and socio-economic values in cities. Feasible adaptation requires better flood risk quantification and assessment of appropriate adaptation actions in term of costs and benefits. This paper presents an economic assessment of three prevailing climate adaptation options for urban drainage design in a Danish case study, Odense. A risk-based evaluation framework is used to give detailed insights of the physical and economic feasibilities of each option. Estimation of marginal benefits of adaptation options are carried out through a step-by-step cost-benefit analysis. The results are aimed at providing important information for decision making on how best to adapt to urban pluvial flooding due to climate impacts in cities.

Decision strategies for handling the uncertainty of future extreme rainfall under the influence of climate change.

Several extraordinary rainfall events have occurred in Denmark within the last few years. For each event, problems in urban areas occurred as the capacity of the existing drainage systems were exceeded. Adaptation to climate change is necessary but also very challenging as urban drainage systems are characterized by long technical lifetimes and high, unrecoverable construction costs. One of the most important barriers for the initiation and implementation of the adaptation strategies is therefore the uncertainty when predicting the magnitude of the extreme rainfall in the future. This challenge is explored through the application and discussion of three different theoretical decision support strategies: the precautionary principle, the minimax strategy and Bayesian decision support. The reviewed decision support strategies all proved valuable for addressing the identified uncertainties, at best applied together as they all yield information that improved decision making and thus enabled more robust decisions.

Past, present, and future design of urban drainage systems with focus on Danish experiences.

Climate change will influence the water cycle substantially, and extreme precipitation will become more frequent in many regions in the years to come. How should this fact be incorporated into design of urban drainage systems, if at all? And how important is climate change compared to other changes over time? Based on an analysis of the underlying key drivers of changes that are expected to affect urban drainage systems the current problems and their predicted development over time are presented. One key issue is management of risk and uncertainties and therefore a framework for design and analysis of urban structures in light of present and future uncertainties is presented.

Feasible adaptation strategies for increased risk of flooding in cities due to climate change.

Northern Europe is one of the regions where more frequent and more severe storms and storm surges are expected due to climatic changes. In order to maintain an acceptable risk of flooding suitable adaptation strategies must be defined and implemented. Optimum solutions demand collaboration of different professionals and thus simple graphical means must be employed to illustrate the economic impacts of the change in risk of flooding. A case study indicates that urban drainage infrastructure capacity should be upgraded while there is currently no economic incentive to improve protection against sea surges.

Significant climate change of extreme rainfall in Denmark.

During recent years, practitioners of urban drainage have complained that they felt that surcharging occurred more frequently. Therefore, a study was initiated focussing on the variations in extreme rainfall during the last two decades. In Denmark, a total of 41 rain gauges with a high resolution in time and volume have an observation period close to 20 years. The rainfall observed at these gauges was selected for this study. Three variables were analysed for statistically significant trends during the observation period: Maximum average 10 min intensity, maximum average 6h intensity and the total volume of individual rain events. For the 10 min maximum intensity there is a statistically significant trend towards more extreme and more frequently occurring rain storms. The trend is pronounced in the eastern part of the country and below statistically significant levels in the western part of the country. For the 6 h maximum intensity and total volume of events the trends are less pronounced. The findings are confirmed by comparison to physically based climate models and studies based on large regions.

An enquiry into rainfall data measurement and processing for model use in urban hydrology.

Rain data are collected all over the world because water is of paramount importance to all human life. WMO has provided standards for collection and standardized data processing of daily rainfall measurements. Currently no such standards are available for gauges with a resolution suitable for urban hydrology, where the resolution in time must not exceed a few minutes. The Group on Urban Rainfall under the International Water Association has made a comparison of national standards by means of a survey of 77 questions sent to 44 countries. The paper discusses the first results of the answers of the survey. Currently tipping bucket gauges are the dominating method of obtaining high resolution rain data, but the numbers of weighing gauges and radar measurements are rapidly growing. It is necessary to try to increase the awareness of documentation of current standards and to agree on standards for measurements and data processing on an international level in the future.

Dissemination of regional rainfall analysis in design and analysis of urban drainage at un-gauged locations.

A research program in Denmark on statistical modelling of rainfall has resulted in a model for regional distribution of rainfall extremes. The results show that extreme rainfalls critical to the hydraulic function of urban drainage systems and the pollution discharge are subject to a significant regional variation of extreme rainfalls throughout the country. This has implications for design and analysis of all practical problems related to urban drainage, since the rainfall data so far recommended as input to engineering analyses underestimates the problems. Consequently, the Danish Water Pollution Control Committee has issued a statement recommending a new engineering practice. The dissemination of the research results proved to be difficult due to lack of understanding of the concepts of the new paradigm by practitioners. The traditional means of communication was supplemented by user-friendly spreadsheets and easy access to rainfall data as well as giving courses on the new paradigm. This has eased the implementation of the new concepts greatly.