A Post-disaster Assessment on a Storm-induced Flood and Associated Coastal Dispersal of the River-derived Suspended Radiocesium Originated from the Fukushima Nuclear Accident

摘要

Niida River, Fukushima, Japan, is well known to be a source of highly contaminated suspended radiocesium to the ocean, which is originated from the Fukushima Dai-ichi Nuclear Power Plant during the accidental leakage occurred in March 2011. We examined the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from Niida River, using a quadruple nested JCOPE2-ROMS 3D circulation model in a very high resolution configuration, coupled with a 3D multi-class sediment transport model, the iRIC-Nays 2DH river sediment model, the SWAN spectral wave model, and a static radiocesium absorption model. A particular attention was paid to the storm and subsequent flood event associated with Typhoon 201326 (Wipha) passed off the Fukushima Coast in October 2013, which provoked an enormous amount of precipitation, and subsequent riverine freshwater discharge and sediment fluxes to the ocean. With a guide of several in situ observed data, we carried out a quantitative assessment of the accumulation and erosion of the sediments and resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima Coast.