Geophysical Investigation of the Landslide-Prone Slope Downstream from the Rogun Dam Construction Site (Tajikistan)

The ongoing construction and further operation of the Rogun HPP in one of the most seismically active regions of Central Asia can cause activation of dangerous processes along the slopes adjacent to the dam and reservoir. Among the gravitational mass displacements recorded near the major hydroelectric facilities, the old right-bank landslide located downstream the hydrosystem is worth of special consideration. Possible landslide activation triggered by earthquakes and/or flooding may cause collapse of unstable masses with adverse consequences for the major HPP facilities. The complex geological and tectonic conditions of the construction area, the great height of the earth-fill Rogun dam, as well as large-scale man-made impacts on the slopes surrounding major HPP facilities, including the filling of the reservoir, require a profound analysis of slope stability downstream from the dam with assessment of possible landslide risk scenarios. This report describes the main results of the comprehensive studies, calculations and dynamic modeling of stability of the right-bank landslide-prone slope, which suggest that the potentially unstable mass of strongly weathered rocks with the volume from 2 to 5 × 10⁶  m³ on a steep part of the slope may fully or partially collapse in case of combination of strong seismic action (PGA > 0.3 g) and of precipitation. In case of synchronous collapse of about five-million m³ of unstable rocks, the blockage of the Vakhsh River by a landslide dam is inevitable, with risk of flooding of the tunnels and consequential disturbance of normal functioning of the HPP.