Summary
The proposed Wellenberg site for a radioactive waste repository is located between Altzellen in the Engelberger valley and the Oberrickenbach valley, in a thick Valanginian marl series. The marl is generally overlain with unconsolidated rocks but reaches to the surface in some places. In contrast to the situation in the Oberbauenstock region this marl complex is not protected by an overlying erosion resistant series and exhibits a marked relief (the altitude difference between the base of the Engelberger valley and the Eggeligrat is around 650 m). The question therefore arises with respect to the Wellenberg site; to what extent will the marl (i.e. the repository host rock formation) be removed by erosion processes during the 100,000 year interval under consideration and what overburden will remain at the end of this period?
This report, NTB 93-34, presents the results of an investigation of the long term behaviour of the proposed site in respect of those processes of erosion and deposition which can lead to changes in the terrain surface and its location relative to the repository. A wide range of possible scenarios, encompassing different developments of climatic conditions during the 100,000 year period of interest, was investigated. In addition to the continuation of the present climate and the occurrence of a new ice age on the scale of the Würm glaciation the consequences of altered climatic conditions (semi-arid, arid and warm-humid) on erosive removal of the repository overburden were considered.
In a first step the development of the Engelberger valley, which forms the base for local erosion, during the period of interest was analysed. Local erosion at the Wellenberg site was then estimated, taking account of the effects and limits imparted by this base. The results for the various scenarios are presented in the form of sections along the Engelberger valley and the Secklis Bach (the valley from Oberrickenbach to Schüpfentobel) together with the cross-section Engelberger valley - Eggeligrat - Oberrickenbach, which passes through the repository. The choice of parameter values ranges from slightly conservative to conservative. As a result of the accumulation of processes the overall deductions can, however, be considered as conservative to very conservative.
After 100,000 years the remaining overburden along the section Engelberger valley -Altzellen -Eggeligrat -Oberrickenbach will be at least 300 m (compared with 500 m at present) for the scenarios: 1) "Continuation of present climate"; 2a/2b) ”Ice age lasting 60,000 years followed by return to present climate"; 4) "Arid climate"; 5a) "Warm-humid climate". For the scenarios 3) "Semi-arid" and 5b) "Warm-humid with consequent changes in vegetation" the overburden thickness will be reduced to 100 m after 100,000 years. It should be noted that an increase in the local erosion rate will develop in the latter 2/3 of the period when Lake Lucerne (Vierwaldstättersee) has been filled by sediment, thus possibly enhancing the rate of lowering of the erosion base of the Engelberger valley.
Within the 100,000 year period of interest none of the scenarios considered leads to the exposure of the repository.
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