Аннотация: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. The upstream part of the basin is located in the area with very high level of
radionuclide contamination after the accident at Fukushima Dai-ichi NPP. Field and GIS
methods were used, including direct measurement of the depth of fresh sediment and its area,
with soil descriptions for the typical floodplain sections, measurement of dose rates,
interpretation of space images for a few time intervals (before and after flood event) with the
following evaluation of spatial changes in deposition for different floodplain sections. In
addition, results of quantitative assessment of sedimentation rates and soil radionuclide
contamination were applied for understanding the effect of extreme flood on alluvial soils of the
different sections. It was established that the maximum sedimentation rates (20-50 cm/event)
occurred in the middle part of the lower reach of the Niida River and in some locations of the
upper reaches. Dose rates had reduced considerably for all the areas with high sedimentation
because the top soil layers with high radionuclide contamination were buried under fresh
sediments produced mostly due to bank erosion and mass movements.
