Recent debris-flow activity on the 1913 Tsivlos landslide body (Northern Peloponnese; Greece)

Chronological data on recent debris-flow activity in the Eastern Meditteranean are fragmentary, limiting a proper assessment of future geomorphic risk. This is especially true if these processes occur on collapsed slopes that predispose their occurrence. This paper reports on the long-term debris-flow activity reccuring periodically in the body of the 1913 Tsivlos landslide in Hellenides (Greece) using variety of methods. Analysis of aerial images confirmed a shift of active channels and debris-flow fans, mainly due to the recharge of new material in the landslide source zone, the contribution of partial shallow landslides, and frequent channel avulsions. Dendrogeomorphic reconstruction of 59 trees including Pinus nigra Arn. and Platanus orientalis L. pointed to the least annual occurrence of debris flows since 2014. Our data also specified the occurrence of at least one landslide generated debris flow in 2018/2019 and one debris flow in 2019/2020 with an estimated maximum velocity of 6.3 m.s-1 and maximum discharge of 171.1 m3.s-1 in the transport zone. Our results suggest a link between increased debris-flow activity and increased occurrence of 3-day and 5-day extreme precipitation events, which were more pronounced in the 1950 s, 1990 s, and 2010 s. We conclude that debris-flow activity is likely to continue at this site and may continue to threaten infrastructure passing through the landslide body. Therefore, it is desirable to monitor the site and conduct ongoing field research to prevent future hazards from debris flows.