Improving Dike Slope Failure Assessments related to Groundwater Hydrology

Abstract

Dikes and dunes protect over 9 million in the Netherlands from floods due to extreme river water levels and coastal storms. According to the 2017 river flood protection assessment, all 3500 km of primary flood defenses must meet a new safety standard by 2050. Currently, 1500 km needs reinforcement. This evaluation aims to meet the new safety rules. Dike failure probability is seen as a mix of mechanisms, with dike slope instability being a key one – soil movement due to increased groundwater pressure. High water levels lead to heightened groundwater pressure through direct infiltration and subsurface flow. The current dike safety assessment uses an analytical approach for groundwater conditions, neglecting parameters like hydrological attributes, subsurface variability, and flood hydrograph shape. The diverse subsurface in the Rhine-Meuse delta impacts groundwater flow. Ignoring these parameters causes inaccurate safety estimates. The research's primary goal is to assess parameters affecting dike failure uncertainty due to groundwater variation in deltas. A groundwater model linked with a geomechanical model assessed dike slope stability, varying parameters that influence it via groundwater conditions. The significance of subsurface and geometry for dike slope stability under steady-state groundwater conditions was explored through a sensitivity analysis involving fifteen parameters related to geometry, drainage, and material properties. Dike slope and subsurface material type were pivotal factors, with complex connections influencing stability directly through geomechanical traits and indirectly through groundwater conditions. However, this assumes pressure conditions stabilize during high water events, which isn't always accurate. The significance of subsurface and geometry for dike slope stability was studied using a sensitivity analysis. Among fifteen parameters related to geometry, drainage, and materials, the dike slope and subsurface material had the greatest impact. Complex connections influence stability through geomechanical properties and groundwater conditions. Yet, this relies on pressure reaching equilibrium during high water events, which may not be consistent. Another uncertainty source is the heterogeneity in human-made river dikes and the natural subsurface. DETRIS, an object-based and process-based model, replicates material patterns in river dikes with historical cores. Incorporating heterogeneous DETRIS-simulated dikes into simulations yields a probabilistic assessment that accounts for internal dike heterogeneity. This approach reduces uncertainty by factoring in permeable layers or weak zones. In lowland deltas like the Rhine-Meuse delta, the natural subsurface's complexity includes sandy channel belts and clayey floodplain deposits. Current groundwater estimation practices oversimplify to two dimensions, but 2D models can underestimate when the river channel connects in three dimensions. A 2D groundwater model may underestimate dike slope stability compared to a 3D model. These uncertain factors merge to influence dike stability. The dike's interior composition significantly affects phreatic levels and heterogeneity. Material composition of the cover layer determines subsurface pressure. None of the factors – flood wave shape, dike interior, subsurface material, or groundwater model dimension – show a significantly greater impact on dike slope safety. Given climate change's potential extreme conditions and groundwater's role in dike failure, comprehensive guidelines for including groundwater uncertainty in safety assessments are advised.