Reliability-Based Flood Defense Analysis in an Integrated Risk Assessment

Abstract

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.

Failure of Grass Covered Flood Defences with Roads on Top Due to Wave Overtopping: A Probabilistic Assessment Method

“…Such climate change effects increase the risk of flooding of low lying, highly populated areas around world [5,6]. In the last decades, flood defence design has moved towards a risk-based approach [7,8] in which it is possible to express the design parameters uncertainty as probabilistic distributions [8]. Such methods allow inclusion of the expected future effects due to climate change as modifications in the parameter statistical distributions.…”

XtremRisK — Integrated Flood Risk Analysis for Extreme Storm Surges at Open Coasts and in Estuaries: Methodology, Key Results and Lessons Learned

“…For each section, 35 limit state equations (LSEs) were required and used to perform the reliability analysis. The segmentation of the flood defenses and the LSEs, including all the relevant input parameters are described in Naulin et al [2015] in more detail. Most of the LSEs considered were taken from previous studies, including some modifications and further developments in this project [Naulin et al, 2015].…”

“…failures of moveable barriers due to human and organization errors). Despite these simplifications and the application of only conventional fault trees, the results have clearly revealed the relative contributions of each failure mode to the probability of the top event (flooding) [Naulin et al, 2015;Kortenhaus, 2003]. These also provided valuable indications on the priority issues which need more elaboration and more research efforts.…”

“…It is therefore indispensable to carefully check the results of any probability assessment of flood defenses and make sure that the aforementioned aspects are properly considered, hence leading to results which are conservative. The approach as described in Naulin et al [2015] may serve as an example and starting point for any probability assessment in coastal or estuarine areas.…”

XtremRisK — Today's and Future Coastal Flood Risks Under Extreme Events

“…Joint probabilities were estimated for different storm surge scenarios taking into account the water level but also its duration and wave parameters. Naulin et al [2015] have looked into various aspects of risk pathways, comprising failure modes for different types of flood defences (dikes, dunes, storm walls, and single point structures), breach models, time-dependency of failure modes considering the time history of storm surges, and length effects of flood defence lines, amongst others. Results have been generated for all pilot sites predicting the initiation conditions for flood inundation models for each of the storm surge scenarios (extreme events) considered.…”