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.

On 5–6 July 2017 (J17), an unusual series of rainfalls induced a concentration of precipitations in Northern Kyushu, Japan, reaching 516 mm in 24 h in Asakura City, a first in its history since the beginning of observation in 1976. This triggered unprecedented hydro-meteorological hazards (landslides, debris flows, and floods) in forested mountainous areas, such as in the Akatani watershed, where the estimated discharge reached 520 m³/s at its outflow. It induced numerous deaths, structural damages, destruction of river channels, and deposition of sediment in flood plains. If smaller-scale hazards have usually driven authorities to build protection systems in the watershed, the J17 crisis called for a full remodeling of it, interrupting waterways and reshaping slope shapes and structures. Considering the means deployed by the central government for this reconstruction, the J17 event triggered a full “re-construction” of rivers, modifying the hydrosystem’s functioning at the watershed scale. Thus, our objective is to show that after the relative stability of Akatani watershed’s hydrosystem over 75 y, the exceptionality of the J17 crisis forced us to rethink the watershed’s organization, using this event as the reference for post-disaster reconstruction. The research relies on field surveys in 2019 and 2022, interviews of officials, and GIS analysis based on historical aerial photograph interpretation and geospatial data. It revealed that (a) the geometry of post-disaster channels was completely redesigned to match a new reference and (b) sediment control structures were multiplied to restructure slopes, breaking the slope angles into subwatersheds. The Akatani watershed case then illustrates the full range of structural measures developed by Japanese engineering to reduce hydrological risks.

Le Japon est soumis à des aléas gravitaires et hydrologiques dus au passage de pluies d’été et de typhons entre les mois de mai et octobre. Malgré le développement d’un système de gestion et de protection, le pays enregistre toujours des dommages matériels et parfois humains. Les pluies torrentielles des 5-6 juillet 2017 ont déclenché de nombreux glissements de terrain et crues torrentielles dans la région montagneuse du nord de Kyūshū. En réponse aux perturbations induites dans les bassins versants, l’État déploie des projets de restauration d’urgence basés sur un large éventail de structures d’ingénierie civile. Depuis plusieurs décennies, le bassin-versant étudié (Akatani) connaît des phénomènes de dépeuplement et de vieillissement de la population, impactant progressivement les pratiques rurales et le paysage. La catastrophe de juillet 2017 et les projets de restauration associés représentent des accélérateurs de la mutation de ces territoires. Cette contribution cherche à montrer et expliciter ces transformations à travers l’étude des principaux éléments constitutifs des espaces ruraux de basse montagne. L’exploitation de données gouvernementales, d’images satellites et d’un suivi de terrain annuel depuis 2022 nous permettent d’appréhender les changements induits par la catastrophe et la restauration sur le réseau hydrographique, le peuplement et le parcellaire agricole. Ainsi, la place laissée aux hameaux se réduit au profit du réseau hydrographique et des structures de protection, tandis que le parcellaire agricole connaît un remembrement, augmentant la superficie des parcelles et simplifiant leur forme.