Sediment-related disasters in the Nakajo River watershed in Sakae Village, Nagano Prefecture triggered by the earthquakes in Northern Nagano Prefecture on March 12, 2011

Abstract

On March 12, 2011, the North Nagano Prefecture Earthquake and subsequent aftershocks triggered several sediment-related disasters, including two large-scale landslides that dammed the Higashi-irisawa River, a left tributary of the Nakajo River in Sakae Village, Nagano Prefecture. The volume of the landslides was about two hundred thousand cubic meters and one million cubic meters. In this report, we summarize the characteristics of the earthquakes, landslides and subsequent debris flow, and emergency works following the disasters. Based on boring surveys of the sediment accumulated on the streambed and other field measurements, we discuss the geological features and the hydrologic response in the borehole and the dammed pond.