Quantitative assessment of subsurface architecture and variability for better estimations of probability of river channel and bank failure.
Fieldwork and subsurface data retrieval which is primary input for subsurface characterizations and reconstructions (Photos by Bas Knaake).
Motivation and practical challenge
Several dike failure mechanisms are related to the subsurface buildup that shows a great spatial variability (see photos). However, current risk assessments rely on over simplified subsurface subsoil properties. Owing to its depositional history, the subsurface shows systematic patterns. Subsurface characterization and reconstructions can provide important contextual information to reduce uncertainties in subsurface parameter estimates.
As an earth scientist, I am very interested in the sedimentology and genetic history of the subsurface and associated processes. This project offers a great opportunity for me to integrate my own knowledge and interests to understand how the subsurface spatial distribution is related to several river channel and bank failure mechanisms. This additional knowledge provides important insights into the geologic boundary conditions and relevant properties that are also relevant when designing flood defences.
This project integrates existing information and knowledge on the subsurface characteristics of the Rhine-Meuse delta in the Netherlands and its relation to failure mechanisms.
Main components of the research (source: photos bank failure by Deltares, left-figure by Kim Cohen and right figure adapted from Gouw, 2007).
Our research focuses on past occurrences of failure mechanisms such as (1) channel scour and (2) river bank failure. We particularly focus on the relation with subsurface architecture (former river channels in the bottom figures). This project will assess channel scour locations for all the major river branches of Rhine-Meuse delta. For bank failures we will make use of an extensive historical database.
We compare locations where a process occurs to existing geological mappings, such as locations of former river channels (bottom-left figure). Furthermore, we use data such as subsurface models of TNO-Geological survey to document and map regional and local buildup of the subsurface. These comparisons give important insight into relevant geological buildup and architectural elements for a failure mechanism.
Next, we focus on the characterization of deposits and the geotechnical properties for different architectural elements such as fluvial channel belts (bottom-right figure). This characterization can help in better prediction of potential risk areas for a failure mechanism which ultimately leads to more efficient design of flood defences.
Relevant for whom and where?
The history and origin of the subsurface can help advisors and management organizations to identify potential risk areas for a failure mechanism when planning or implementing river measures such as dike reinforcements.
This project will assess the major river branches of the Rhine-Meuse delta until the Western Scheldt. On the map, Dordtsche Kil branch, near the city of Dordrecht which is referred as example on the progress results.
Progress and practical application
The locations of scour holes in the Rhine-Meuse delta show a clear link with subsurface architecture and associated depositional history. Therefore, the downstream area is characterized by high variability in erodible and non-erodible materials. As a result, a sharp increase in scour hole frequency is observed. This relation is especially clear for the Dordtsche Kil branch, near the city of Dordrecht, where subsurface architecture strongly influences the river bathymetry. Due to its depositional history, the northern part of the branch is characterized by an erosion-resistant, stiff clay layer which is locally dissected by former river channels. Instead, the southern part consists of much more homogeneous sand. Resulting from this subsurface architecture, the bathymetry of the northern part of the branch is characterized by local scour holes and the southern part is much more homogeneous in depth.
Status for day-to-day practice
For interventions on the river channels, such as dredging, detailed knowledge on the location and depth of erosion resistant layers is very important. Local perturbations of such layers can result in unwanted side effects in the form of scour holes and may reduce the life span or stability of nearby structures.
Future work will consist of quantifying geotechnical properties of deposits within different depositional environments and the spatial distribution thereof. This will help to perform delta scale assessments of failure hazard based on the characteristics.
Last modified: 25/06/2020
- S.M. Knaake, M.W. Straatsma, Y.Huismans, K.M. Cohen, E. Stouthamer, H. Middelkoop (2020). Relating scour hole development to subsurface heterogeneity of the Rhine-Meuse delta, the Netherlands. NCR Abstract.
- S.M. Knaake, M.W. Straatsma, Y.Huismans, K.M. Cohen, E. Stouthamer, H. Middelkoop (2019). The influence of subsurface heterogeneity on scour hole development in the Rhine-Meuse delta, the Netherlands. NCR Abstract.