Key points:

  •  The erosion assessment follows a modification of the actualistic principle. Here, proxy indicators of past processes serve as a key to the future behaviour of the erosional system.
  • Future erosion in Northern Switzerland (glacial & non-glacial) is expected to be comparable in form to Pleistocene processes, but at lower rates (no major drainage reorganisation, smaller glaciers, delay in glacial inception, and resistant rocks).
  • The comparably large uncertainty regarding future erosion was assessed with a robust, structured hybrid-probabilistic approach.
  • A residual bedrock cover (remaining overburden) of 200 m thickness inferred to be necessary to maintain self-sealing properties of the Opalinus Clay can be shown to be sustained for the most likely scenarios in all the siting regions within the next one million years. Using the 5 – 95% range as reference, the JO siting region is least robust.
  • Excavation of the repository by erosion within the next one million years can be considered extremely unlikely.

 

Erosion is the main process that can reduce the protective overburden thickness needed for sustained barrier functions (Section 6.1.2). In addition to information on the drivers of erosion (Sections 6.2 and 6.3), this section builds on Sections 3.4.5 and 3.5 and provides the key facts of past landscape evolution that allow a first-order assessment of past erosion processes and rates. To project these rates and processes under consideration of uncertainties into the future on a one-million-year timescale, a hybrid-probabilistic framework was established.

Section 6.4.1 summarises the main evidence needed to understand the Quaternary landscape evolution. It builds on site investigations (Section 2.4), but also incorporates general process understanding.

This information was used to derive probability distributions and arguments for parameters to be used in subsequent erosion modelling and assessments (Section 6.4.2). The main results and asso­ciated line of reasoning for each separate erosion process are presented briefly in Section 6.4.3.

Section 6.4.4 shows the combined results of the erosion assessment for the future one-million-year time frame. Discussed are the remaining overburden thickness for the expected geological evolution as well as, to show the robustness of the repository, the probability of repository excava­tion.

A summary of the main results is given in the conclusions of Section 6.4.5.