ИСТИНА

Extreme flood events are very typical of small mountain river basins in different parts of tropical and sub-tropical landscape zones. They usually result in both intensive erosion and sediment deposition on the river valley bottom. Also, sedimentation rates considerably influence on alluvial soil formation and contamination, depending from the sources of sediment. Detailed study of different sections of floodplain was undertaken in the Niida River basin (Fukushima Prefecture) after an extreme flood event which occurred in the middle of September 2015, when 385-456 mm of precipitation fall down for the 6 days with maximum 200 mm/day in the end of typhoon. The objectives of the study are: to evaluate the erosion/deposition processes and its spatial changes in different sections of the river floodplain, with particular attention to the low reaches of the Niida River, and to assess changes in alluvial soil contamination in comparison with the level of contamination before the extreme flood. The detail assessment of geomorphological and radioecological consequences of the extreme flood in the Niida River basin allows to conclude that different exogenic processes (landslides, scree, bank erosion etc.) were the main sediment sources for the river sediment discharge. Maximum channel deformations were observed within the lower reach of the Niida River because of extremely high energy of the stream with maximum water and sediment discharges. However, sedimentation rates on the floodplain of the upper reaches of the Niida River were also one order higher under the mean annual rates. Finally, extreme event promoted to the serious reduction the dose rate of the river bottom except some high floodplain levels where sedimentation rates were negligible.