Abstract
Erosional and sedimentary features associated with flooding have been documented in both modern and past cases. However, only a few studies have demonstrated the relationship between these features and the corresponding hydraulic conditions that produced them, making it difficult to evaluate the magnitude of paleo-flooding. This study describes the characteristics associated with inundation depth and flow direction, as well as the erosional and sedimentary features resulting from the disastrous flooding of the Kinu River, central Japan, in September 2015. Water levels rose rapidly due to heavy rainfall that eventually overtopped, and subsequently breached, a levee in Joso City, causing destructive flooding on the surrounding floodplain. Distinctive erosional features are found next to the breached levee, while depositional features, such as a sandy crevasse-splay deposit are found further away from the breach. The deposit can be divided into three units based on sedimentary facies. The vertical and lateral changes of these sedimentary facies may be the result of temporal and spatial changes associated with flow during the single flooding event. These observations and quantitative data provide information that can be used to reveal the paleohydrology of flood deposits in the stratigraphic record, leading to improved mitigation of future flooding disasters.
Highlights
The disastrous flooding along the Kinu River in September 2015 was the result of intensive rainfall in central Japan
Bedforms, and erosional scours (Fig. 4B–D) indicated an ESE-SSE flow direction that was perpendicular to the Kinu River flow in the area next to the breached levee (Fig. 3A)
Flow direction deduced from leaning plants and sedimentary features provides a way to reconstruct the dispersing process of the inundation flow
Summary
The disastrous flooding along the Kinu River in September 2015 was the result of intensive rainfall in central Japan. This study documents the patterns of erosional scour and sedimentary deposition that occurred on the floodplain during the September 2015 flooding of the Kinu River in central Japan. The link between their formation and hydraulic conditions was not addressed To investigate this link, we surveyed the inundation depth and flow direction shortly after the flood waters receded, and documented the erosional and depositional features at neighboring stations. The Kinu River breached the levee and generated a large-scale inundation near the study area during the 1949 flood. All the floodplain located between the Kinu and Kokai Rivers was inundated and the flood affected a 40 km[2] area around Joso City[14] (Fig. 1C). The study area is located near the breached levee in the Misaka area in Joso City (Figs 1C and 3)
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