A tool to rapidly assess the groundwater-related failure risk of a given dike stretch with consideration of the variability in the subsoil.
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 are aware of 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 (top photo).
Groundwater flow and high pore pressures are indeed 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, visualization is difficult and field data for validation is important yet scarce (bottom photo).
To increase the knowledge on the relation between subsoil, river water level and groundwater conditions, and to introduce more realistic groundwater scenarios to dike stability calculations.
Main components of the research to improve the groundwater schematization for the dike stability estimations.
Given that the current schematization of groundwater in dike assessments is simplified, my research focusses on the temporal and spatial components of groundwater-related dike stability. Amongst these are:
- Flood wave shape is known to be 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 design flood waves.
- Establishing relations between 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 focusses on determining differences between 2D and 3D modelling, suggesting better ways of parameterizing 3D effects.
Relevant for whom and where?
Researchers working on the interface of earth sciences and civil engineering. Innovative organizations wanting to use the latest knowledge on underlying process 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. Dynamic groundwater calculations are especially important if the geology of the subsoil is relatively unknown. Moreover, changes towards finer material in either the dike or the shallow subsoil deserve consideration given the potential decrease or increase of the dike stability.
So far, our results demonstrate that high-resolution information on dike stability and a first approximation of dike failure probability is easily made by constructing a large database of stability calculations from our coupled groundwater and dike stability computer model.
Status for day-to-day practice
In an early stage of dike reinforcement design these results can help in determining the need for additional information and focusing further research, thereby increasing the cost-effectiveness of these reinforcements.
Future work will focus on creating fast high-resolution models that will improve groundwater level predictions for dike safety assessment and can be used as an interactive tool. This step needs to be taken together with water authorities for the method to be both accurate and easily applicable on case study.
Last modified: 25/06/2020
Teun van Woerkom
- 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.