water Article Global Sensitivity Analysis of Groundwater Related Dike Stability under Extreme Loading Conditions Teun van Woerkom 1, * , Rens van Beek 1 , Hans Middelkoop 1 and Marc F. P. Bierkens 1,2   Citation: van Woerkom, T.; van Beek, R.; Middelkoop, H.; Bierkens, M.F.P. Global Sensitivity Analysis of Groundwater Related Dike Stability under Extreme Loading Conditions. Water 2021, 13, 3041. https://doi. org/10.3390/w13213041 Academic Editor: Renato Morbidelli Received: 21 September 2021 Accepted: 26 October 2021 Published: 1 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Physical Geography, Faculty of Geosciences, University of Utrecht, P.O. Box 80115, 3508 TC Utrecht, The Netherlands; R.vanBeek@uu.nl (R.v.B.); H.middelkoop@uu.nl (H.M.); m.f.p.bierkens@uu.nl (M.F.P.B.) 2 Unit Soil and Groundwater Systems, Deltares, P.O. Box 85467, 3508 AL Utrecht, The Netherlands * Correspondence: t.a.a.vanwoerkom@uu.nl Abstract: With up to 15% of the world’s population being protected by dikes from flooding, climate- change-induced river levels may dramatically increase the flood risk of these societies. Reliable assessments of dike stability will become increasingly important, but groundwater flow through dikes is often oversimplified due to limited understanding of the important process parameters. To improve the understanding of these parameters, we performed a global sensitivity analysis on a com- prehensive hydro-stability model. The sensitivity analysis encompassed fifteen parameters related to geometry, drainage conditions and material properties. The following three sensitivity settings were selected to characterize model behavior: parameter prioritization, trend identification and interaction qualification. The first two showed that dike stability is mostly dependent on the dike slope, followed by the type of subsurface material. Interaction quantification indicated a very prominent interaction between the dike and subsurface material, as it influences both groundwater conditions and dike stability directly. Despite our relatively simple model setup, a database containing the results of the extensive Monte Carlo analysis succeeded in finding most of the unsafe sections identified by the official inspection results. This supports the applicability of our results and demonstrates that both geometry and subsurface parameters affect the groundwater conditions and dike stability. Keywords: groundwater; flood defenses; sensitivity analysis; dike stability; flood safety 1. Introduction Over 45 major flood events occurred in Europe between 1950 and 2005 that each resulted in more than 70 fatalities or a collected economic damage of EUR 7.6 × 10 10 [1]. As a result, many flood prone areas have an extensive network of artificially elevated levees or dikes, which, along Europe’s major rivers, add up to a length of approximately 60,000 km [2]. To ensure the safety of people living behind dikes, continuous maintenance and reinforcements are needed to warrant the stability of dikes and their proper functioning during high water events. Climate change, e.g., earlier snow melt or an increase in extreme precipitation events in the upstream drainage area [3], poses a new threat that may increase the risk of a society to flooding [4]. To maintain safety levels under changing climatic conditions, major investments are needed for dike maintenance and reinforcement, of which the costs for the latter are in the order of EUR 1–20 million per kilometer [1]. Improved knowledge of the processes during and following a high-water event that can result in dike failure is crucial for more cost-effective dike reinforcements, which may reduce the total expenditures on dike reinforcements substantially and can support more societally acceptable flood defense measures [5]. Many dike failure mechanisms are related to local groundwater conditions and pore pressures in the dike body. In response to elevated river stages, changing groundwater conditions may increase the pore pressure and, thus, reduce the effective normal strength, while, at the same time, the lateral load of river water pushing against the dike is increased. Water 2021, 13, 3041. https://doi.org/10.3390/w13213041 https://www.mdpi.com/journal/water