Landscape coevolution in a Japanese human-geomorphic system induced by the July 2017 heavy rainfall in northern Kyûshû

On 5-6 July 2017, during the rainy season, heavy rainfall battered northern Kyûshû (Japan), breaking previous records, with 516mm in 24 hours and a maximum intensity of 129.5 mm/h at the Asakura station in the lowlands. In mountainous areas, the total rainfall was between 600 and 800mm in 24 hours. As a result, hundreds of landslides were triggered, mobilizing large quantities of sediment and driftwood, with a regional estimation of respectively 10 million and 210 000 cubic meters (KRDB, 2017). These floods caused 42 casualties and extensive material damage, as well as multi-meters deposits in several valley bottoms. This is a «Low Frequency / High Magnitude» (LFHM) event with an estimated probability of occurrence of 1/200 (KRDB, 2017).

This event produced major landscape changes mainly linked to the triggering of landslides, which constitute a sudden and massive opening of sedimentary sources whose contribution to the sediment cascade is 1) facilitated by the very high general sediment connectivity and 2) goes beyond the 2017 event.

To mitigate the vulnerability of populations, Japan has undertaken major countermeasures works to stabilize slopes and develop rivers (check dams, sedimentation basins, recalibration), relying heavily on engineering and concrete. The outcome is profound geomorphological changes, reinforced by human intervention through the development and management of these hydrosystems, which modify 1) the sediment cascade and the connectivity settings from a structural and process-based perspective and 2) the specific socio-ecological system of these rural mountain areas called «Satoyama» (Indrawan et al., 2014). The main example is the change of function of areas, changing from a living space with agricultural production area to a sedimentary storage area. Therefore, these elements illustrate a complex process-response feedback loop in a coupled humain-geomorphic system (Poeppl et al., 2017).

In a climate change context, heavy rainfall events are expected to increase, including in East Asia and the Japanese archipelago (MEXT JMA, 2020; MLIT, 2020). Related hazards such rapid-moving landslide is predicted to increase along with the population exposed to landslide risk (Gariano and Guzzetti, 2016).

This raises the question of the relaxation time of geomorphological systems between two disturbance phenomena like LFHM events, in a context of sometimes increased sensitivity. The torrential rain of 10 July 2023 is a significant example in our case, as it was the 2nd record for maximum hourly intensity with 72.5 mm/h and third for cumulative rainfall over 24 hours with 290.5mm (behind 2018 with 295.5mm) (JMA, 2024). The newly-built countermeasures were therefore put to the test and proved their effectiveness overall, but the increased sensitivity of these systems combined with the magnitude of the 2023 LFHM event exceeded the countermeasure system's absorption capacity calibrated for a specific return period. As a result, most of the check dams have been filled up massively, leading to flooding and massive sediment deposits in several valley bottoms, which provides some insight into the real extent of human-induced disturbance to the sediment cascade.