The recent rise in short, localized high-intensity torrential rainfall events leading to flooding from rivers, has generated a growing number of disasters affecting urban areas. Japan's railways restrict train operations according to precipitation and river water levels, however, these traffic control methods have not yet been adapted to cope with such short, intense downpours. A real-time rainfall runoff-flooding simulation method, which takes into account predicted rainfall therefore needs to be developed. This paper reports on evaluation results obtained to validate a simulation method.
<p>The impact of freeze-thaw processes and subsequent runoff affecting the <sup>137</sup>Cs flux and concentration in sediment discharge were revealed in bareland erosion plot following the Fukushima nuclear power plant accident by detailed monitoring and laser scanner measurement on the soil surface. We found that surface topographic changes due to the frost-heaving during the winter-spring period, and rill formation during the summer. We also found the evident seasonal changes in <sup>137</sup>Cs concentration; high during the early spring and gradually decreased thereafter, then surface runoff from the plot frequently occurred during spring and autumn when rainfall was high and reached a maximum in summer. From these results, the higher <sup>137</sup>Cs concentration in spring was caused by a mixture of unstable surface sediment following freeze-thaw processes and then transported in the early spring, but erosion amount is not significant because of the less rainfall event. The sediment with a lower <sup>137</sup>Cs concentration, which was supplied from the rill erosion and its expansion, was wash-offed during the summer, contributing most of the flux from erosion in bareland in Fukushima region. In case, &#160;heavy rainfall occurs in the early spring, caution is required because high concentrations of cesium may flow down into the river.</p>
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