Coast. Eng. J. 2015.57. Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 06/03/15. For personal use only. H. Oumeraci et al.A brief overview of the joint research project XtremRisK is given. The project has been focusing on developing/improving/expanding the knowledge, methods and models with respect to (i) physically possible extreme storm surge for current conditions and scenarios for climate change, (ii) failure mechanisms of flood defenses, (iii) assessment of intangible losses (social and ecological) and their integration with direct/indirect economic losses, (iv) reliability analysis of flood defense systems and (v) sourcepathway-receptor (SPR)-based integrated flood risk analysis involving both tangible and intangible losses and its implementation for two selected pilot sites (representative for an open coast and an urban estuarine area in Germany). The key results are briefly summarized and the lessons learned for future flood risk studies are finally drawn.

Failures of flood defenses have been one of the major reasons in the past leading to flooding of the hinterland behind flood defenses along rivers and at the sea. It is therefore inevitable to investigate the reliability of such defenses for extreme events as have occurred in the past and are discussed to happen more frequently in the future and due to climate changes. The first subproject in XtremRisK (SP 1) and the related papers in this issue "A multi-method approach to develop extreme storm surge events to strengthen the resilience of highly vulnerable coastal areas," Coast. Eng. J., this special issue; Wahl, T. et al. [2015] "Statistical assessment of storm surge scenarios within integrated risk analyses," Coast. Eng. J., this special issue; Tayel, M. and Oumeraci, H.[2015] "A hybrid approach using hydrodynamic modelling and artificial neural networks for extreme storm surge prediction, Coast. Eng. J., this special issue] have investigated the components of storm surges and their statistical occurrence, also in relation to the wave parameters. These results can now be used as input for investigating the reliability of flood defenses and provide an overall failure probability for different types of defenses and different failure modes. This paper therefore summarizes the key findings of the "risk pathway" analysis of XtremRisK Subproject 2 (SP 2) which comprise a reliability analysis 1540005-1 M. Naulin, A. Kortenhaus & H. Oumeraciand breach modeling of coastal and estuarine flood defenses using storm surge scenarios and sea states, including their occurrence probabilities provided by XtremRisK SP 1. The paper discusses the key results, the progress, and challenges in reliability analysis and breach modeling of flood defenses. The developed and advanced methods were applied to pilot sites in Hamburg (Elbe Estuary) and the Island of Sylt (North Sea). These pilot sites are mainly protected by linear flood defenses such as sea dikes, estuarine dikes, coastal dunes, and flood defense walls. Results have shown that under extreme conditions many dikes may fail simply from wave overtopping and even overflow but also from dike breaching due to the severe loading of the dike slopes when heavy overtopping and overflow occurs. The inflowing water volumes were calculated based on time-dependent water levels and then used for inundation modeling of the hinterland in Subproject 3 (SP 3) of XtremRisK. Furthermore, the limit state equations for wave overtopping and overflow had been adapted to time-dependent simulations. An importance factor was introduced for the probability of breaching of sea dikes leading to significantly different failure probabilities. The length effect considering the different homogeneous segments in the dike ring of Hamburg-Wilhelmsburg was estimated using an upper and lower bound approach showing the importance of the segmentation of the dike ring.

Extreme storm surges have frequently led to major damages also along the German coastline. The joint research project 'XtremRisK' was initiated to improve the understanding of risk-related issues due to extreme storm surges and to quantify the flood risk for two pilot sites at the open coast and in an estuarine area under present and future climate scenarios. In this context, an integrated flood risk analysis is performed based on the source-pathway-receptor concept under consideration of possible tangible and intangible losses. This paper describes the structure of the project, the methodology of the subprojects, and first results. Moreover, integration approaches are discussed. The results of 'XtremRisK' will be used to propose flood risk mitigation measures for the prospective end-users.

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Extreme storm surges can cause failures of flood defences resulting in severe flooding of the hinterland and catastrophic damages. In order to quantify the risk of flooding, an integrated risk analysis is being performed within the German 'XtremRisK' project wherein one subproject failure probabilities of flood defences are determined. In this paper, the failure probability calculations of flood defence structures and systems under the loading of extreme storm surges are discussed. Moreover, the analysis of dike breaches and breach development is briefly introduced. Preliminary results of the failure probabilities and the breach modelling are presented using the example of the estuarine urban area of Hamburg, Germany. These results are put in context of an integrated risk analysis approach for extreme storm surges.