Author(s): Norio Tanaka; Hayato Kajitani; Kengo Fushimi
Linked Author(s): Norio Tanaka
Keywords: Flooding risk; Polder; Second embankments; River confluence; Historical changes
Abstract: During the Edo Era in Japan (around 150-420 years ago), river course changes were conducted in the Kanto Plain, where capital Tokyo now exists, and new paddy fields were developed. As this river course change significantly changed the flooding risk, many secondary embankments (SEs), open levees (OLs), or polders for controlling flood inundation were constructed after the river-course change. This is especially in the Arakawa River. The objective of this study was to quantify the role of the still existing SEs and OLs in terms of flood management. A two-dimensional flood model was applied to the middle reach of the Arakawa River basin to clarify the change in flooding risk area before and after the change of the Arakawa River course (Arakawa- Seisen). After the change of the river course, around a river confluence with branches (Wadayoshino River, Ichino River, Iruma River) with the Arakawa River, overtopping from river embankment was increased by backwater phenomenon. It changed the Yoshimi and Kawajima area to flood-prone areas. Although large floods had not occurred around recent 100 years till 2019, breaching of the embankment by the recent 2019 typhoon occurred in four locations in this river basin. Although the flood inundation pattern differs in the past, OLs in the Tokigawa River and Nagarakutei Embankment (part of the Kawajima Polder, which is the second embankment for Ichino River now) have proved to have still the roll to store water in the area and reduce flood inundation area downstream.