Importance sampling for efficient modelling of hydraulic loads in the Rhine–Meuse delta

Abstract

During flood events, breaching of flood defences along a river system can have a significant reducing effect on downstream water levels and flood risks. This paper presents a Monte Carlo based flood risk framework for policy decision making, which takes this retention effect into account. The framework is developed to estimate societal flood risk in terms of potential numbers of fatalities and associated probabilities. It is tested on the Rhine-Meuse delta system in the Netherlands, where floods can be caused by high flows in the Rhine and Meuse rivers and/or high sea water levels in the North Sea. Importance sampling is applied in the Monte Carlo procedure to increase computational efficiency of the flood risk computations. This paper focuses on the development and testing of efficient importance sampling strategies for the framework. The development of an efficient importance sampling strategy for river deltas is more challenging than for non-tidal rivers where only discharges are relevant, because the relative influence of river discharge and sea water level on flood levels differs from location to location. As a consequence, sampling methods that are efficient and accurate for one location may be inefficient for other locations or, worse, may introduce errors in computed design water levels. Nevertheless, in the case study described in this paper the required simulation time was reduced by a factor 100 after the introduction of an efficient importance sampling method in the Monte Carlo framework, while at the same time the accuracy of the Monte Carlo estimates were improved.

Real-time monitoring and forecasting of dike strength

“…The use of fragility curves has been applied to flood management systems since 1991 [10] and for strategic flood risk assessments in the latest years [11][12][13]. Further developments in the methods of determining fragility curves [14], the intrinsic level of detail [15] and their integration into flood risk assessment models [16][17][18] have been researched and expanded since their conception. Trends show also an increase in the application of fragility curves for assessing operational reliability [19].…”

An advanced method for flood risk analysis in river deltas, applied to societal flood fatality risk in the Netherlands

“…Comprehensive flood risk analyses require the consideration of uncertainties in both hazards and vulnerability parameters, which is only possible in probabilistic approaches (Sayers et al, 2002;Apel et al, 2004;Gouldby et al, 2013;Vorogushyn et al, 2010;Van Mierlo et al, 2007). Probabilistic approaches are based on the awareness that most data and models are uncertain and that these uncertainties can be expressed in probability density functions.…”

“…The consequences associated with defence failure at each location are taken into For the preliminary large-scale risk assessment of England and Wales also a probabilistic approach was developed (Hall et al, 2005). Gouldby et al (2013) slightly adapted the approach and studied fatality risks. In these approaches, load reduction at downstream defence sections due to defence failures in the system was not taken into account.…”

“…In these approaches, load reduction at downstream defence sections due to defence failures in the system was not taken into account. The approach of Hall et al (2005) and Gouldby et al (2013) is suitable for coastal defence structures and for small river valleys. However, for fluvial systems such as large rivers and estuaries where the effect of water retention on flood probabilities and flood risks may be significant, this retention effect should be taken into account.…”

“…To assess societal risks of river deltas we cannot follow an approach which focuses on dike rings independently from what happens in other dike rings as was done by Jongejan et al (2012), nor can we assume full correlation between different defence sections as done for the UK (Hall et al, 2005;Gouldby et al, 2013). Instead, we need to include the effects of hydrodynamic interaction on flood probabilities to get accurate estimates of the location and number of breaches and fatalities.…”