A multi‐scale framework for flood risk analysis at spatially distributed locations

This paper presents a multi‐scale framework for flood risk analysis from fluvial and coastal sources at broad (including national) scales. The framework combines an extreme value spatial model of fluvial and coastal flood hazards using the Heffernan and Tawn conditional dependence model, with a new Markov approach to representing the spatial variability of flood defences. The nested multi‐scale structure enables spatial and temporal dependence at a national scale to be combined with detailed local analysis of inundation and damage. By explicitly considering each stage of the process, potential uncertainties in the risk estimate are identified and can be communicated to end users to encourage informed decision making. The framework is demonstrated by application to an insurance portfolio of static caravan sites across the UK worth over £2bn. In the case study, the largest uncertainties are shown to derive from the spatial structure used in the statistical model and limited data on flood defences and receptor vulnerability

Large-scale stochastic flood hazard analysis applied to the Po River

Reliable hazard analysis is crucial in the flood risk management of river basins. For the floodplains of large, developed rivers, flood hazard analysis often needs to account for the complex hydrology of multiple tributaries and the potential failure of dikes. Estimating this hazard using deterministic methods ignores two major aspects of large-scale risk analysis: the spatial–temporal variability of extreme events caused by tributaries, and the uncertainty of dike breach development. Innovative stochastic methods are here developed to account for these uncertainties and are applied to the Po River in Italy. The effects of using these stochastic methods are compared against deterministic equivalents, and the methods are combined to demonstrate applications for an overall stochastic hazard analysis. The results show these uncertainties can impact extreme event water levels by more than 2 m at certain channel locations, and also affect inundation and breaching patterns. The combined hazard analysis allows for probability distributions of flood hazard and dike failure to be developed, which can be used to assess future flood risk management measures

Causative classification of river flood events

A wide variety of processes controls the time of occurrence, duration, extent, and severity of river floods. Classifying flood events by their causative processes may assist in enhancing the accuracy of local and regional flood frequency estimates and support the detection and interpretation of any changes in flood occurrence and magnitudes. This paper provides a critical review of existing causative classifications of instrumental and preinstrumental series of flood events, discusses their validity and applications, and identifies opportunities for moving toward more comprehensive approaches. So far no unified definition of causative mechanisms of flood events exists. Existing frameworks for classification of instrumental and preinstrumental series of flood events adopt different perspectives: hydroclimatic (large‐scale circulation patterns and atmospheric state at the time of the event), hydrological (catchment scale precipitation patterns and antecedent catchment state), and hydrograph‐based (indirectly considering generating mechanisms through their effects on hydrograph characteristics). All of these approaches intend to capture the flood generating mechanisms and are useful for characterizing the flood processes at various spatial and temporal scales. However, uncertainty analyses with respect to indicators, classification methods, and data to assess the robustness of the classification are rarely performed which limits the transferability across different geographic regions. It is argued that more rigorous testing is needed. There are opportunities for extending classification methods to include indicators of space–time dynamics of rainfall, antecedent wetness, and routing effects, which will make the classification schemes even more useful for understanding and estimating floods

Development of a Hydrodynamic-Based Flood-Risk Management Tool for Assessing Redistribution of Expected Annual Damages in a Floodplain

Despite spending ample resources and procedural development in flood management, flood losses are still increasing worldwide. The losses caused by floods and costs incurred on management are two components of expected annual damages (EAD) due to floods. This study introduces a generalized approach for risk-based design where a range of probable floods are considered before and after a flood mitigation measure is implemented. The proposed approach is customized from the ISO Guide 31000 along with additional advantages of flood risk visualization. A Geographic Information System (GIS)-based design of a flood-protection dike is performed to exhibit the risk redistribution. The Chenab River is selected for the existing dike system. Detailed hazard behaviour and societal vulnerability are modelled and visualized for a range of all probable floods before and after the implementation of flood-protection dikes. EAD maps demonstrate the redistribution of induced and residual risks. It can be concluded that GIS-based EAD maps not only facilitate cost-effective solutions but also provide an accurate estimate of residual risks after the mitigation measures are applied. EAD maps also indicate the high-risk areas to facilitate designing secondary measures

Planning and Implementation of the Dyke Systems in the Mekong Delta, Vietnam

Floods are a dangerous threat and an implicit risk for farming communities in rural floodplains of the Mekong Delta (MD). The Vietnamese government decided the dyke system construction to control floods and ensure safety for life and livelihood of flood affected communities. The case study of this dissertation refers to Can Tho city, an average floodplain community of the MD, covering also 9 districts. Here a comprehensive dyke system has been constructed to control flood risks. This thesis investigates the existing problems in the dyke system planning and its implementation, dyke impacts on the natural environment and socio-economic development as well as the adaptability of the flood affected farming communities in the protected floodplains. The study used both qualitative and quantitative research methods to collect information and data. Besides Global Positioning System (GPS) and a digital camera were used to identify the location of the most important samples in the field and during the fieldwork. Fieldwork itself was mainly based on the two rural communes of Thanh Thang and Thanh Phu, which are representative for the overall rural situation in Can Tho city and the MD. The study found that the Vietnamese government had made a basically correct decision concerning the necessity of flood risk control by the construction of a dyke system. Dykes have practically guaranteed safety for agricultural livelihoods of the flood affected farming communities and positively contributed to agricultural farming transformation from rice into integrated rice-fish production and rural road improvement. However, a centralized top down approach was strongly applied thus not considering the experiences and all expectations of public organizations and local people. The individuals and local organizations played a very weak role in the whole planning and implementation process. Dyke system construction consequently created conflicts between central planners and local people. In addition, the negative impacts of the dyke system have become a great threat for sustainable development in terms of water pollution, natural fish exhaustion, soil fertility reduction, erosion and in some instances, also due to increasing inundation of the fields. Thus, the dyke system construction and planning as well as implementation strategies need to be studied further to minimize the negative impacts of dyke systems and to ensure a stronger inclusion of local people and their knowledge in further planning approaches.Überflutungen sind eine permanente Bedrohung für die ländliche Kulturlandschaft, die Bewohner und ihre Aktivitäten im Deltabereich des Mekong. Die vietnamesische Regierung hat deshalb beschlossen, Deiche zur Flutkontrolle und damit auch zum Schutz der ländlichen Bevölkerung des Mekong-Delta zu bauen. Dieser Problematik widmet sich die vorliegende Dissertation am Beispiel von Can Tho city, einer typischen Gemeinde im Flutbereich des Mekong-Deltas mit insgesamt 9 ländlichen Distrikten. Die Arbeit untersucht Probleme des Planungsprozesses und seiner Umsetzung, die Auswirkungen der Deiche auf die natürliche Umwelt und die sozioökonomische Entwicklung wie auch die Anpassungsstrategien der flutgefährdeten Reisbaugemeinden im Deltabereich. Unter Verwendung qualitativer und quantitativer Untersuchungsmethoden, von GPS und digitalen Aufnahmen wurden die für die Untersuchung notwendigen Daten erhoben. Die Geländearbeit konzentrierte sich dabei auf die Kommunen Thanh Thang und Thanh Phu, beide repräsentativ für die Region Can Tho city und das Mekong-Delta. Die Studie kommt zu dem Ergebnis, dass die generelle Entscheidung der vietnamesischen Regierung zum Deichbau richtig war. Die Deiche haben insgesamt die Sicherheit der ländlichen Bevölkerung und ihrer landwirtschaftlichen Aktivitäten erheblich verbessert. Auch wurde die landwirtschaftliche Produktion in ihrem Übergang von einer Mono-Reis-Kultur zu einem integrierten Reis-Fisch-Produktionssystem befördert, ebenso der Ausbau der ländlichen Infrastruktur. Kehrseite der Medaille ist, dass die lokale Bevölkerung in das zentrale Planungssystem nur unzureichend eingebunden wurde, sodass Konflikte zwischen zentraler Staatsgewalt und lokaler Bevölkerung nicht ausblieben. Vor allem die negativen Aspekte des Deichsystems in Hinblick auf eine langfristig nachhaltige und positive Entwicklung werden geschmälert durch zunehmende Verunreinigung des Irrigationswassers, durch Rückgang der natürlichen Fischbestände, Verlust der natürlichen Bodenfruchtbarkeit, Erosion usw. Ergebnis der Studie zum Deichbau ist, dass bei künftigen Maßnahmen eine stärkere Einbeziehung der lokalen Bevölkerung und ihres Wissens dringend geboten ist, um weitere negative Effekte zu minimieren bzw. ganz auszuschließen