The role of freeze-thaw action in dam reservoir cliff degradation assessed by terrestrial laser scanning: A case study of Jeziorsko Reservoir (central Poland).

Cliff recession is a combined result of wave action in the shore zone and geomorphological processes in the transformed cliff. As several processes usually take place simultaneously or consecutively, so distinguishing between the impact of each one of them individually on cliff erosion is impossible. Jeziorsko Reservoir is characterized by large fluctuations of water level in the annual cycle (several metres), hence in winter, when the water level in the reservoir is the lowest, the exposed cliff is not directly influenced by wave erosion. Sediments resulting from freeze-thaw action are accumulated at the bluff toe. However, in early spring, when freeze-thaw processes are continued, the water level in the reservoir is elevated, reaching the bluff toe. Wave erosion in that period causes removal of the material accumulated at the bluff toe and, consequently, cliff degradation. These processes interact. In 2015, the normal water level in the reservoir was lowered by 0.5 m leaving the bluff toe unaffected by wave erosion also in spring and summer months. These conditions enabled us to distinguish between the effects inflicted by freeze-thaw action and other sub-aerial geomorphological processes and wave erosion on cliff recession. This paper summarizes a series of detailed terrestrial laser scanning (TLS) measurements of morphological changes in an active cliff developing in coherent deposits in the temperate zone, resulting from freeze-thaw processes. Field research was conducted in 2014-2015 and 2018. The results of this study indicate that in coherent deposits in the temperate zone freeze-thaw action plays an important role among sub-aerial geomorphological processes such as wetting and drying of the surface sediments, linear erosion or mass movements causing cliff recession.