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

Abstract

Field investigations and analyses of modern crevasse splay deposits can both elucidate the processes of levee breaching and help to identify past crevasse splay deposits from geologic strata, thereby estimating the magnitudes of ancient river floods. In this study, we studied levee breach processes and crevasse splay deposits in order to determine the distribution characteristics of the inundation area associated with the 2019 flooding of the Chikuma River, Central Japan. The crevasse splay formed by this event can be divided into three regions: proximal, medial, and distal splays. Behind the breached levee, sandy and gravelly sediment piles (proximal splay) formed at both sides of the crevasse channel, whereas sand and mud layers (medial and distal splays) were observed over a wide area within the inundation area, extending beyond the sediment piles. The upstream gravelly sediment pile (proximal splay) was characterized by clearly bounded lower sand and upper gravel layers, reflecting the process of levee breaching: 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 crevasse splay deposits (medial splay) thinned rapidly away from the breached levee, whereas muddy crevasse splay deposits (distal splay) were thicker at lower elevations, indicating that they formed during the levee breach and stagnant stages, respectively. The distribution of the medial splay (35.7% of the inundation area) was restricted relative to that of the distal splay (~81.7% of the inundation area). This study indicates that it is important to determine the extent of muddy crevasse splay deposits from geologic strata in order to determine the inundation areas of past levee breaches.