The outputs of this project are a better understanding of groundwater flow through and around river dikes and improved methods for estimating groundwater levels under high river water levels. We developed methods estimating groundwater flow embedded in modelling instead of analytical estimations, which can substantially improve groundwater predictions near river dikes. With the improved methods, we give examples on how to include big data in the dike safety assessment, historical dike-buildup, and phreatic groundwater because of variable river water levels.
Figure 1-Top: Seepage through the dike and the subsoil (source: Waterschap Rivierenland). Bottom: Tube that is placed in the soil to measure the groundwater (source: Waterschap Brabantse Delta) and fieldwork for data collection (Photo by Bas van der Meulen).
Motivation and practical challenge
In the Netherlands, a country in which history is intertwined with flood safety, we know the large impact high water levels on rivers can have in breaching the dikes. In contrast, the impact of high-water levels on groundwater flow is less understood and recognized by both water authorities and residents, as it is often not a visible thread. Yet, the more water the river presses through the subsoil, ditches are filled up inside the dikes, and fields may get wet on the land side without having a river flood (Figure 1, top photo).
Groundwater flow and high pore pressures are very important for various failure mechanisms of dikes, including piping and slope instability. Of course, the effect of groundwater is strongly linked to the highly variable subsoil material. Due to the limited knowledge and data availability about the subsoil, the groundwater influence is oversimplified for flood safety estimations. Moreover, as the process occurs underground, visualisation is difficult and field data for validation is important yet scarce (Figure 1, bottom photo).
To increase the knowledge of the relation between subsoil, river water level and groundwater conditions, and to introduce more realistic groundwater scenarios to dike stability calculations.
Figure 2. Main components of the research to improve the groundwater schematisation for the dike stability estimations.
Given that the current schematisation of groundwater in dike assessments is simplified, my research focuses on the temporal and spatial components of groundwater-related dike stability. Amongst these are:
- Flood wave shape is highly variable, but dike designs are created using a single characteristic flood event. This investigation shows that the hydrology in the dike and the subsoil strongly differs between flood waves, questioning the current practice of designing flood waves.
- Establishing relations bbetween material characteristics, groundwater hydrology and dike stability. Via many runs of a coupled groundwater and dike stability computer model, the aim is to find the most important factors influencing the dike stability and groundwater conditions. The results are compared against stability calculations of a river dike to prove the applicability of these relationships as a first-order screening tool for dike safety.
- Assessing subsurface variability in 3D has been common practice in geological settings, but its use in dike safety assessment (and the related groundwater hydrology) has been limited so far. This section focusss on determining differences between 2D and 3D modelling, suggesting better ways of parameterising 3D effects.
Relevant for whom and where?
Researchers working on the interface of earth sciences and civil engineering. Innovative organisations wanting to use the latest knowledge on underlying processes related to river dike safety.
Findings of this research will be applicable to dike reinforcement projects in the Netherlands.
Progress and practical application
More dynamic groundwater calculations showed that the most influential parameters of dike stability are: (1) variations in pore pressures, (2) the geometry and material of the dike and (3) material properties of the subsoil.
Dike heterogeneity because of historical dike enforcements greatly influences pore pressures in the dike. A method has been developed to create possible dike buildup scenarios, which result in different pore pressures. By assessing multiple scenarios of dike buildup and pore pressures, a probabilistic analysis of groundwater conditions can be performed.
A full probabilistic analysis should also include dynamic pore pressures as a function of dynamic river water levels. The use of time-dependent groundwater conditions causes a substantial lowering in dike failure probability compared to static groundwater levels. For details on the results, see the related outputs.
Last but not least, the historical dike near Nijmegen-Lent was used as a benchmark on how to estimate dike heterogeneity by simulating its construction history. This case study demonstrates that archaeological observations can increase the data availability of historical dikes and improve simulations of dike interior variability.
Recommendations for practice
- Use model simulations with realistic subsurface and dike buildup scenarios for groundwater level estimation.
- Time-dependent groundwater conditions because of variable river water levels provide a more realistic estimation than static groundwater conditions.
- Incorporating variability in river dike material is important for river dike phreatic level schematisation.
Last modified: 23/12/2021
Teun van Woerkom
Global Sensitivity Analysis of Groundwater Related Dike Stability under Extreme Loading Conditions
To improve the understanding of groundwater related dike-stability, we performed a global sensitivity analysis on a comprehensive hydro-stability model. Dike stability is mostly dependent on the dike slope, followed by the type of subsurface material. Interaction between the dike and subsurface material is important too, as it influences both groundwater conditions and dike stability directly.
12/12/2021 by Teun van Woerkom et al.
Bevat: Publication open access journal
- Van Woerkom, T.A.A, van Beek, L.P.H., Middelkoop, H., Bierkens, M.F.P. (2020, February 13-14). A coupled hydro-stability model for a sensitivity analysis on dike stability [Conference presentation abstract]. Managing changing rivers: NCR Days 2020, Nijmegen, the Netherlands.
- Van Woerkom, T.A.A, van Beek, L.P.H., Middelkoop, H., Bierkens, M.F.P. (2020, March 12-13). Sensitivity analysis of river dike macro-stability: It’s just hydro-logic! [Poster session]. Nederlands Aardwetenschappelijk Congres 2020, Utrecht, the Netherlands.
- Assessing lithological uncertainty in river dikes: Simulating construction history and its implications for flood safety assessment; van Woerkom et al. (in preparation).
Reflection: Data-driven dike enforcements - Constructive feedback from new and historical sources
The technical assessment of dike reinforcement projects in the Netherlands is flooded with data. Before, during and after dike reinforcements, a lot of data is collected and stored. This webinar attempts to bridge the gap between conventional and new data sources by presenting new methods for improving dike safety through data and discussing the general use of different data sources.