Features of crevasse splay deposits and sedimentary processes associated with levee breaching due to the October 2019 flood of the Chikuma River, Central Japan

Field investigations and analyses of modern crevasse splay deposits can both elucidate the levee breaching processes and help to identify past crevasse splay deposits from geologic strata, thereby estimating the magnitudes of ancient river floods. The crevasse splay deposits formed by the 2019 flooding of the Chikuma River in Japan can be divided into three regions. Behind the breached levee, gravelly and sandy sediment piles (proximal splay) formed at both sides of the crevasse channel, whereas sandy and muddy deposits (medial and distal splays) were observed over a wide area within the inundation area. The upstream sediment pile was characterized by lower sand and upper gravel layers, reflecting the levee breaching process: the outer sandy soil of the artificial levee began to be scoured by external erosion, followed by the erosion of the inner gravelly soil. The sedimentary characteristics of the proximal splay deposits appear to have been strongly controlled by the local environment but are useful for inverse analysis of the progressive process of past and future levee breaches. Sandy deposits (medial splay) thinned rapidly away from the breached levee, whereas muddy deposits (distal splay) became thicker as the elevation decreased, in addition to the distance from the breached levee, indicating that they formed during the levee breach and stagnant stages, respectively. The distribution of the sandy deposits (35.7% of the inundation area) was restricted relative to that of the muddy deposits (~ 81.7% of the inundation area), indicating that the extent of muddy crevasse splay deposits is important to determine the inundation areas of past levee breaches.