High-resolution 3D geological modelling for water safety: the story of the Lek River dike

A large, densely populated part of the Netherlands is located below mean sea level. The risk of flooding is therefore omnipresent. Dikes are vital for safety, and local waterboards apply high standards when monitoring and maintaining the dikes. The waterboard ‘Hoogheemraadschap de Stichtse Rijnlanden’ (HDSR) has launched a maintenance program to investigate and reinforce the 55km long northern Lek River dike between Schoonhoven and Amerongen. As the dike’s strength depends on both its design and its subsurface, detailed knowledge of the local geology is essential. Along the Lek, the subsurface is characterized by a heterogeneous composition of the shallow subsurface and HDSR requires a higher level of detail than provided by the currently available 3D voxel model GeoTOP (Stafleu et al., 2012. TNO 2012 R10991). We constructed a 3D geological model, based on the GeoTOP workflow, for three sub-trajectories beneath the dike with a voxel size of 25x25x0.25m. The lithology of each voxel is modelled based on borehole descriptions, cone penetration tests (CPT) and paleogeographic maps. Applying machine-learning to retrieve lithological data from CPT’s increased the data density and allowed for high-resolution modelling. The new high-resolution model is now used by the waterboard to: - identify dike-trajectories that need further investigation - design location-specific and fit-for-purpose dike reinforcement measures - explain proposed measures to local stakeholders. The first two applications potentially reduce costs significantly; whereas the third application aids creating social foundation for reinforcement measures. Most importantly, the new high resolution model helps HDSR to enhance safety behind the dikes.