Landslide damming: implications for natural hazards and risks

Understanding the landslide debris emplacement kinematics and the landslide failure mechanisms is essential for the definition of hazard and for the reconstruction of risk scenarios functional to mitigation strategies. Landslides in the southwest of Iran are particularly numerous, especially in the Zagros sedimentary basins. According to some estimates based on studies and research conducted to date, between 10,000 and 11,000 years ago, a large landslide (estimated maximum volume of 44 Gm3) took place in the city of Pol-e-Dokhtar near the Lorestan region (west sector of Iran). Because this landslide has blocked the course of the Seymareh River, it is known as the Seymareh landslide. This giga-landslide is considered the largest on the Earth's surface. In the present PhD thesis, in order to find key interpretative elements and parameters for describing the longevity, the emplacement kinematics and the rupture style of the Seymareh landslide debris, morphological elements were found with high detail visible in the enormous landslide debris. After mapping the morphological elements, a zonation of the landforms in the landslide debris was performed by computing the spatial density of the main surface features including ridges, gullies and blocks. In the next step, using the zonation of the main surface features, a statistical approach was applied to zone the landslide debris in primary (original) and secondary (modified) regions which are respectively referred to the original shape of the landslide debris and to the one modified by erosions. This statistical approach in combination with kinematics indicators deduced from ridges direction and blocks distribution in primary regions provides a useful interpretation for the landslide kinematics as well as for the landslide debris emplacement. During a specific campaign, soil samples, representative of the landslide debris matrix, were also taken at different points of the debris, to be able to characterize them in the laboratory and classify them according to the USCS standard and to derive the permeability of finer portions in view of future hydraulic models aiming at quantifying the natural dam durability. The obtained results were useful for understanding the mechanism and kinematics of the Seymareh landslide debris and its evolution. They also helped us to better constrain the failure mechanism of the Seymareh landslide and will be useful for obtaining risk scenarios, quantifying the related hazard and plan mitigation strategies. Moreover, the landslide debris emplacement mechanism and kinematics deduced from kinematic indicators in the primary regions of the landslide debris as well as some outcrops of geological bedrock observed during field surveys led to the recognition of the basal surface of the landslide debris, making it possible to hypothesize the buried morphology of the paleo-valley of the Seymareh River.