The History of Safety Factors for Dutch Regional Dykes

Regular dyke assessment is part of reducing the risk of flooding in the Netherlands. 18 000 km of dykes are assessed at regular intervals, of which 14 000 km are classified as regional dykes and their main aim is to defend polders from inundating. The methods of assessing regional dykes are strongly inter-twined with the methods of assessing primary dykes; however, regional dykes differ due to lower consequence levels and significantly shorter individual dyke lengths. Initially, local experience was relied upon for maintaining dykes, prior to the utilisation of soil mechanics calculations to determine the safety/stability of the dykes. Over the years, the approaches have been developed leading to different assessment criteria. This paper aims to give insight into the development of the assessment criteria for regional dykes in Dutch norms/guidelines since the devastating storm surge of 1953, starting with the probabilistic assessment of water heights and global factors for slope stability, through to the adaption of statistical models which enabled the use of partial factors in dyke assessment. Partial factors in assessments allow for improved and more detailed knowledge to limit the uncertainties and lead to more detailed assumptions in the calculations. The introduction of risk-based design enables assessment criteria based on the expected damage due to inundation. The paper discusses assumptions, levels of safety and information required to complete the assessment. Including consequence based assessment and risk based design, leads to a larger possible range in the required factors of safety.Geoscience and EngineeringCivil Engineering and Geoscience