Abstract
Evolution and aggradation rates of floodplains on a centennial to millennial-scale were investigated by analyzing landform features and borehole core sediments. This study was performed at the floodplain along the former course of the Ibi River flowing through the Nobi Plain, central Japan, because the microtopography in the floodplain can be easily identified. The sedimentary environment changed from a delta to a floodplain after 3,200calBP, and a floodbasin environment continued for nearly 2,000 years. However, an avulsion of the Ibi River that happened approximately 400 years ago caused channel erosion at the point that became a river course and a rapid aggradation of natural levee deposits along the channel. Additionally, it was suggested that a part of the artificial ringed levee (“Wajyu” in Japanese) distributed along the channel originated from the natural levee which was formed in relation to the avulsion. Although the millennial-scale floodplain aggradation rate was about 3mm/yr, the aggradation rate shows a large variation depending on the sedimentary environment. The rate is estimated to be more than 10mm/yr on the natural levee and 1 to 2mm/yr in the floodbasin.
