Event-based approach for probabilistic flood risk assessment

Natural Hazards, 2006

In order to be economically viable, flood disaster mitigation should be based on a comprehensive assessment of the flood risk. This requires the estimation of the flood hazard (i.e. runoff and associated probability) and the consequences of flooding (i.e. property damage, damage to persons, etc.). Within the ''German Research Network Natural Disasters'' project, the working group on ''Flood Risk Analysis'' investigated the complete flood disaster chain from the triggering event down to its various consequences. The working group developed complex, spatially distributed models representing the relevant meteorological, hydrological, hydraulic, geo-technical, and socio-economic processes. In order to assess flood risk these complex deterministic models were complemented by a simple probabilistic model. The latter model consists of modules each representing one process of the flood disaster chain. Each module is a simple parameterisation of the corresponding more complex model. This ensures that the two approaches (simple probabilistic and complex deterministic) are compatible at all steps of the flood disaster chain. The simple stochastic approach allows a large number of simulation runs in a Monte Carlo framework thus providing the basis for a probabilistic risk assessment. Using the proposed model, the flood risk including an estimation of the flood damage was quantified for an example area at the river Rhine. Additionally, the important influence of upstream levee breaches on the flood risk at the lower reaches was assessed. The proposed model concept is useful for the integrated assessment of flood risks in flood prone areas, for cost-benefit assessment and risk-based design of flood protection measures and as a decision support tool for flood management.

2006

Flood risk is defined as the probability that floods of a given magnitude and a given loss will occur within a given time span. A complete flood risk analysis has to identify possible damage scenarios (What can go wrong?), to estimate scenario probabilities (How likely is it that it will happen?), and to assess the consequences (If it does happen, what are the consequences?). The results of flood risk analysis should be the basis for identifying locations with significant flood risk, and for deciding between flood mitigation options.

In recent years flooding and the risk associated have been rising with increased frequency in many European countries. Major floods have occurred in a wider part of Europe in recent years including the 1997 flood in Odra, 1998 and 2000 floods in the UK and most destructive flood in Central Europe in summer 2002 causing serious damages and vast economic consequences. Flood risk and vulnerability is increasing due to changes in rainfall pattern, increased frequency of extreme events, land-cover changes and development into flood-prone areas as a result of socio-economic demand. In addition change in climate is causing significant impact on the hydrological system and increasing the risk and vulnerability to flooding. Human lives, property, environment and socio-economics are at increasing risk due to flooding. River flooding has been recognised as a major natural hazard in many European countries after the recent catastrophic floods. Important lessons have been learned from recent floods and flood management has begun to focus on the practical problems, dilemmas and challenges at European scale. Basin-wide co-operation in large transnational river catchments can help to adopt sustainable flood management strategies e.g., land use regulations, upstream control/regulations etc. Risk based forecasting and dissemination tools would provide effective protection from flooding. Development of efficient operational flood warning system (with the ability to provide timely warnings to properties and lives at risk of imminent flooding) would reduce the loss of life and property due to major floods. Efficient flood warning system would be a vital part in flood risk management in areas with extensive development on the natural flood plain. Flood simulation is a major strategic planning tool for effective reduction of risk and damage due to flooding. However, there are uncertainties inherent in such prediction of flooding. Probabilistic analysis and uncertainty will play a major part in the decisionmaking process of determining the flood risk. Adopting flood defence measures at catchment, regional and national scales can improve the flood risk management. Integrated planning and management will enhance the effectiveness of flood defence infrastructure with other possible measures. Publication of this special issue will be a timely and topical contribution that will aid in the development and implementation of new technologies/ strategies to reduce the risk and vulnerability due to flooding. The combination of the papers will provide opportunities for interaction between engineers, scientists, economists, planners, regulators and decision-makers. It will also open areas/issues where further research is needed.

Journal of the American Water Resources Association, 1989

The probability distributions of annual peak flows used in flood risk analysis quantify the risk that a design flood will be exceeded. But the parameters of these distributions are themselves to a degree uncertain and this uncertainty increases the risk that the flood protection provided will in fact prove to be inadequate. The increase in flood risk due to parameter uncertainty is small when a fairly long record of data is available and the annual flood peaks are serially independent, which is the standard assumption in flood frequency analysis. But standard tests for serial independence are insensitive to the type of grouping of high and low values in a time series, which is measured by the Hurst coefficient. This grouping increases the parameter uncertainty considerably. A study of 49 annual peak flow series for Canadian rivers shows that many have a high Hurst coefficient. The corresponding increase in flood risk due to parameter uncertainty is shown to be substantial even for rivers with a long record, and therefore should not be neglected. The paper presents a method of rationally combining parameter uncertainty due to serial correlation, and the stochastic variability of peak flows in a single risk assessment. In addition, a relatively simple time series model that is capable of reproducing the observed serial correlation of flood peaks is presented.

Osterreichische Wasser- und Abfallwirtschaft, 2008

The aim of flood risk analyses is to quantitatively assess the flood risk. In its broad sense, flood risk is defined as the probability of adverse consequences due to inundation. Flood risk analyses are typically dealing with extreme events and failure scenarios which have hardly been (or not at all) observed. Therefore, flood risk assessments are accompanied with considerable uncertainty. Today, it is not standard practice to explicitly analyse the uncertainty bounds of flood risk estimates. This paper discusses the value of uncertainty analysis for flood risk analysis. We argue in favour of uncertainty analyses in flood risk assessments, since uncertainty considerations (1) improve risk analyses, (2) help to confirm or falsify risk analyses, and (3) support decision-making.

2005

Risk analysis will become more important in formulating cost-effective solutions to flood problems. For this purpose, we need reliable estimates of flood frequencies in terms of inundation depths, as well as the possible damage that may result from the flooding. This article deals with the first aspect. Hydrologists are using many different modelling techniques for estimating the flood frequency. These methods can be divided into two categories: event based design and continuous simulation approaches. In the first approach, the probability of extreme water levels is derived directly by simulating a large number of storm events, for which the probability has been determined in advance. Each event is characterized by a combination of stochastic variables that may contribute to flooding, such as rainfall volume, antecedent conditions, rainfall pattern, event duration, etc. An important feature of this approach is that it is assumed that the probability of the simulated storm event is e...

2013

In recent years there have been a number of advances in modelling flood events. This has been matched by simultaneous advances in probabilistic techniques to evaluate loading and response. The reliability of a flood risk assessment, however, remains constrained by the ability to determine the likelihood of defence failure and test multiple failure and load scenarios. In addition, conventional analysis has difficulty resolving flood risk resulting from multiple breach/failure locations and load combinations. Therefore, it often remains focused on the impact of a breach on discrete areas of the flood plain; rather than considering the potential damage resulting from a broader perspective of multiple failures arising in any given ‘system’ of defences. As a result, the improvements in flood inundation modelling and probabilistic analysis techniques have been largely incompatible in practice. For ‘simple’ flood plains protected by a limited number of discrete defences subjective techniqu...

Proceedings of the ICE - Water Management, 2008

Serious flooding in the UK over recent years has increased the attention on urban flooding and highlighted the need to better understand and manage urban flood risk. Further to this, water companies are under increasing pressure to reduce the number of flooding incidents as a result of inadequate network capacity and asset failure.

FLOODsite was the largest ever EC research project on flood risk management, with an EC grant to the budget of nearly €10 Million complemented by supporting national funds. The project, which started in 2004 and was completed in February 2009, involved over 200 researchers from 13 countries including many of Europe’s leading research institutes and universities. The project was interdisciplinary integrating expertise from across the environmental and social sciences, as well as technology, spatial planning and management. All professionals involved in flood risk management practice now need to consider the question of what actually constitutes “integrated” flood risk management and to prepare society at large for the change in policy from one of flood defence to flood risks being managed, but not eliminated. The circumstances in which the research will be implemented are changing with firstly the entry into force of the European Union Directive on the assessment and management of fl...