Changes in the historical river course and related flood risk in the Arakawa River basin in Japan and the role of still-existing secondary embankments in the recent 2019 flooding event

Abstract

During the Edo era (1603–1867) in Japan, river courses were changed in the Kanto Plain, where Tokyo—the capital of Japan now exists. Because these river course changes significantly modified the flooding risk, many secondary embankments, discontinuous open levees, and polders (ring levees) for managing flood inundation were constructed in the basin. After the change in the river course around the river confluence with branches, overtopping from the river embankment occurred due to backwater phenomena. It made the Yoshimi and Kawajima areas flood-prone. Although large floods did not occur during the last 100 years after the construction of high continuous levees and upstream reservoirs, the embankment was breached by the 2019 Typhoon Hagibis in seven locations in this river basin. Although the flood inundation mechanism differed in the past, two old secondary embankments constructed in the Edo era—the Nagarakutei Embankment and Iimori Embankment proved to still be able to stop the inundation current and store water in the upstream area in 2019. They reduced the flood inundation area and depth behind the embankments. Historically, many of the discontinuous levees were changed to continuous levees after the Edo era. However, under the increase in floods exceeding the design level due to climate change, the system of water management by discontinuous levees and secondary embankments should be paid attention to as flood mitigation measures using the function a watershed has, and the old wisdom will be further improved in association with land-use management of a high-flood risk area.