The Disaster Protection System of Mountainous Rivers in Japan: The Example of the Akatani Watershed’s Reconstruction

Abstract

On 5–6 July 2017, an unstable atmospheric condition caused an unusual concentration of rainfall above the Northern part of Kyushu Island, triggering a set of hydro-meteorological hazards. Within the affected area, the mountainous subwatershed of the Akatani River was significantly impacted by numerous landslides combined with debris flow and floods. National and local agencies deployed a plan of reconstruction to restore the floodplain and protect inhabitants. Regarding the hydrosystem in the Akatani watershed, this reconstruction project mainly focuses on the restoration of damaged protection systems and the construction of new infrastructures. Thus, this paper aims to explain the restoration plan of the Akatani River in terms of the strategic Japanese River System Sabo and then as a model of a national-scale spatial plan. It draws on (i) a literature review based on the historical evolution of Japanese protection systems and the River Sabo System; (ii) field surveys in 2019, 2022 and 2023, in conjunction with (iii) interviews with local, regional, and national officials; and (iv) a Geographical Information System analysis of previously and newly built protection systems through aerial photograph interpretation and geospatial data. Sabo works implemented in the Akatani watershed illustrate the engineering vision of Japanese river management. They also constitute a comprehensive system and include a downstream–upstream logic which echoes that of the River System Sabo. In addition, the disaster of July 2017 and the government’s response emphasize the continuous adaptation and improvement of the Japanese disaster management system, which mitigates severe disasters.

Check dams overtopping scenarios from catastrophic landslides in mountains’ headwaters: case study in Kobe City, Japan

“…Under vegetation cover, the erosion was recorded at the three other location to be 5.7 g/m 2 /month, 7.4 g/m 2 /month and 0.3 g/m 2 /month. In the light of these results, and considering a material density of 2.1 to 2.5 g/cm 3 , it is safe to rule out the progressive accumulation of sediments from background erosion, as a threat to the safety and usability of check dams.…”

“…In Japan, a narrow coastal plain separating the sea and steep mountain or volcanic slopes -which occupy 70% of the land surface -has resulted in the need of controlling sediment hazards originating from the mountains (Siccard et al, 2022) using check open and closed-dams made of steel frame, slit, culvert and steel and concrete dams (Dumont et al, 2023). These structures are mainly designed to mitigate debris-ows, which are concentrated mixtures of debris and water owing downslopes (Hungr et al, 2001).…”