Major conclusions relating to the Katse Dam differential InSAR study

The Katse Dam in Lesotho, which was completed in 1997, is the deepest dam on the African continent (180 m). Lithological modelling prior to the dam´s construction had indicated that the loading effect of the reservoir on the Earth´s crust would be well within the imaging capabilities of ERS InSAR, both in terms of vertical surface movement and lateral extent. A study, funded by The South African Water Research Commission, was started in 1996, the aim of which was to derive the crustal deformation field by differential InSAR, and to compare this with hypothesised models of the Lesotho lithosphere, thereby improving the models used for seismic risk assessments in the Lesotho Highlands Water Scheme. The lack of independent height data and the alpine terrain made the removal of the topographic phase component from interferograms very difficult. Fortunately however, surprisingly good phase coherence as well as a short baseline between images acquired as far apart as three years, meant that a conclusion could be reached in spite of the topographic problems, albeit a negative one. In a short baseline (21m) interferogram generated from a pair of ERS images acquired before and after the first filling of the Katse reservoir, no clear evidence of differential movement was apparent. Conventional geodetic levelling conducted at the dam showed that the actual vertical deformation was an order of magnitude less than that anticipated through the modelling. The basic implication here being that the flexural thickness of the Lesotho crust is greater than previously thought