Assessing the vulnerability of a Dutch river dyke to rising water levels

Dykes provide protection from the risk of flooding to approximately 60% of the Netherlands, by area. Unfortunately, most of the country’s major cities lie within this zone, placing a large proportion of the population at risk should these flood defences fail. In total the Netherlands have over 3800 km of primary flood defences protecting the coast and inland river systems with another 14000km protecting individual polders. Recent flood events in Europe caused by extreme rainfall have raised concerns about the ability of the Dutch river dykes to resist similar events. This paper describes a quantitative assessment of the vulnerability to flooding of a primary river dyke outside Dordrecht. The dyke has shown significant signs of distress over recent years developing large tension cracks during periods of drought on its leeward side, which have progressively increased in size. It is regularly inspected by the local water authority due to its concerning visible deterioration. In this study, soil uncertainty is quantified from in-situ geotechnical tests and laboratory tests. Several failure mechanisms are then considered probabilistically, namely global stability, rapid drawdown, internal erosion and overtopping. Fragility curves are generated for each failure mechanism describing how the probability of failure would change if a given flood level were to occur, event trees are used to link the individual failure mechanisms and quantify the system probability of failure. This paper illustrates the use of fragility curves for earthwork asset management and advantages and limitations of the methodology utilised are discussed.