In this master thesis the current Dutch method of combining failure probabilities at different spatial scales is researched. This method is used in the Dutch flood risk analysis of earthen dikes to assess the probability of flooding. Currently, probabilities of flooding for geotechnical failure mechanisms are obtained which are not considered to be realistic. To obtain more realistic results the following steps are researched; the assessment of cross-sections, scaling to dike sections and combining sections to a dike trajectory. The increase of failure probability over increasing length, known as the length-effect, is of importance in the current Dutch method of combining failure probabilities. This research shows the conservatism of the assessment of cross-sectional failure probabilities and offers possibilities to reduce the length-effect within and between dike sections to obtain more realistic results.