Landslide development at the shores of a dam reservoir (Włocławek, Poland), based on 40 years of research.

This article presents results of our 40-year research on dynamics of a deep rotational landslide developed in clay deposits. The landslide is located in a lowland, within the shoreline of Włocławek Reservoir, with little water level variation, in Dobrzyń on the Vistula (northern Poland). It is an old landslide, which was active also before the construction of the reservoir. Our research was initiated in 1970, i.e. when the river Vistula was dammed there. Currently the landslide covers about 20,000 m and has a volume of about 160,000 m. During the 40 years, 3 periods of increased landslide activity were observed. The first, conspicuously, nearly 40 % increase in the rate of movements within the Central Landslide was initiated in the early 1970s and lasted till the early 1980s. It coincided with the initial filling of the reservoir and the first years of exploitation of Włocławek Reservoir. The increase in landslide dynamics was a direct effect of hydrological and morphological changes associated with the damming of the river. The next 2 periods of increased landslide activity occurred: in 2001/2002 and 2010/2011. The latter was the most spectacular displacement within the Central Landslide. It was recorded between December 2010 and February 2011, and reached 10.0-19.4 m, after unusually long-lasting, heavy rainfall in the summer and autumn of 2010, followed by snowmelt and unfavourable changes in air temperature. Since March 2011, no movements within the landslide have been recorded. [ABSTRACT FROM AUTHOR]