1. The Formation and Behaviour of Natural and Artificial Rockslide Dams; Implications for Engineering Performance and Hazard Management

The formation and behaviour of natural and artificial rockslide dams are reviewed to update the well-known work of Costa and Schuster [1]. Rockslide dams block surface drainage to form upstream lakes. They may occur naturally due to landslides or as a result of engineered rock slope failure. As evidenced by the 2010 Hunza event (Pakistan), the stability of rockslide dams is a major consideration in landslide risk assessment in mountain terrain, particularly with respect to the possibility of a destructive downstream flood resulting from a breach of the dam. The damming of a river by a rockslide may require immediate engineering response to mitigate the hazard. However, failure by breaching is less frequent than long-term stability. These issues are examined with reference to nine case histories of rockslide dams and rockslide-dammed lakes; Gohna (1894), Rio Barrancas (1914), Condor-Sencca (1945), Mayunmarca (1974), La Josefina (1993), Tsao-Ling (1999), Yigong (2000), Tangjiashan (2008), and the Hunza (2010). The case histories also illustrate the utility of digital terrain data (especially the SRTM-3 data set obtained in February 2000) and remote sensing imagery to obtain accurate estimates of the impoundment volumes and other geomorphic data on rockslide-dammed lakes. Methods of estimating peak breach discharge and downstream flood effects exist but are still largely empirical in nature. Measures to mitigate hazard associated with rockslide-dammed lakes include the construction of a spillway over the rockslide debris, a by-pass tunnels through the abutments of the debris dam, the implementation of dam and lake-level monitoring and failure warning systems to mitigate downstream damage. A review of some well-documented examples show that these measures have had been applied with mixed success in the past. Natural rockslide dams are commonly used for foundations for conventional constructed dams. Artificial rockslide dams are created by rock slope failure induced by large-scale explosion (blast-fill dams). The largest blast-fill dam yet constructed is the Medeo Dam, a debris flow retention structure near Alma-Ata, Kazakhstan. Rockslide dams and their geomorphic effects may create an important legacy in the landscape through massive accumulations of lake sediments, impact on river channels, and effects on river long-profiles.