KEM-16b Towards a relation between subsidence of various origins and damage

This research is a follow up of the KEM-16a project regarding a toolbox for subsidence. In KEM-16 work was performed in two pilots regarding the relation between multiple causes of deep subsidence and the relation between deep and shallow causes of subsidence in the period 2020-2022. The relation between subsidence and building damage was postponed to a follow up. Since 2022 other research is ongoing to describe the relation between subsidence and building damage (GEMMA projects by the Commission Mining damage (CM). In order to prevent double work this follow up project is redefined along two lines:

1.      To elaborate the pathways linking surface water level change to groundwater level change to damage to buildings (link chain).

2.      To improve/advance the knowledge base of pertinent links of the chain with (model-based) research. And to address the question how validation can be achieved.

In the Deltares KEM-16 report a start was made to elaborate the link between “groundwater level lowering due to surface water level lowering”. Preliminary results were presented, and a listing was provided of required and desired further steps. The general aim of this work is to develop (authoritative) tables which provide practical information on GWT lowering due to surface water level lowering, for application in the context of damage risk of buildings due to differential sagging. The modelling should establish a meaningful classification of conditions that can be linked to distinct GWT lowering impacts of SWL lowering. The information in the desired tables is expected to be in the form of impact factors (ratio of GWT lowering to SWL lowering).

Specific research questions are:

1.      Elaboration of the link chain between damage-risk and surface water level change. The aim of this task is to create an accessible overview of the damage-risk chain following surface water level change.

2. Extend the knowledge base for key elements in the link between groundwater level lowering due to surface water level lowering, including climate forcing, phreatic storage, the role of ditches.

Finally, a synthesis is aimed for where all results of modelling are evaluated and summarized in the desired practicable tables listing subsidence impact factors for relevant settings and conditions.

KEM RESEARCH QUESTION

The KEM 16b project activities and results are documented in 2 deliverables:

1.      KEM-16b PROJECT REPORT 1, elaborating on the pathways linking surface water level change to groundwater level change to damage to buildings (the link chain).

2.      KEM-16b PROJECT REPORT 2, improving/advancing the knowledge base of pertinent links of the chain with (model based) research and to address the question how validation can be achieved (the model chain).

Related to the first objective, the project has provided an accessible overview of the cause-effect link chain between surface water level change and building damage. Existing guidelines for quantitative assessment of the chain links are reviewed and three knowledge gaps that contribute most to uncertainty in the assessment of building damage (risk) due to SWL change are identified.

Related to the second objective, the project has performed an in-depth groundwater modeling study aimed at expanding the understanding of the relationship between surface water level lowering and groundwater table lowering. It also offers a more effective framework for assessing groundwater table reductions resulting from surface water level changes, compared to what has previously been available. The project has developed and utilized a modeling tool that represents the key processes and characteristics governing both the unsaturated and saturated soil zones. Based on simulation results, a concise synthesis of impact factors was listed in tables for relevant settings and conditions. Due to data not being available, only a preliminary step could be taken in validation of the modeling tool and the selected parameterization methodology based on historical GWT time series.

Importantly, a conclusion of the modeling study for authorities is that the 1987 guidelines from CBA provides impact factor ranges that are too small.

As a recommended follow-up, the project has identified plans to raise SWLs near Marum, Groningen, in December 2026, thus yielding an opportunity for a validation study of the model chain.

The project also recommends developing a user guide, with examples, to facilitate the use of the new tool that has been developed, as well as further research to investigate conditions that result in a greater groundwater table lowering than surface water level lowering.

The project was evaluated by the KEM scientific expert panel.

KEM PROJECT EVALUATION

It is concluded that both reports are of high quality, reflecting an effectively executed project. The work includes appropriate references to existing literature, and presents a feasible modelling approach, with well-considered assumptions and simplifications aligned with the project's scope. Limitations of the study and in the interpretation of results, given the available data and model assumptions, are comprehensively addressed and clearly articulated. Overall, all the research activities expected by KEM were addressed in a very good manner, and sound conclusions were drawn from the study.

The recommendations brought forward are supported by KEM, as they are investigations that are needed to validate and facilitate the use of the developed methodology. This will in turn contribute to a better foundation for revision and development of guidelines for quantitative assessment of the link chain between building damage-risk and SWL change in terms of a practical set of tables that can be used in the context of building damage (risk) assessment.