Spatial Distribution of Drifted-wood Hazard following the July 2017 Sediment-hazards in the Akatani river, Fukuoka Prefecture, Japan

Abstract

In recent years, heavy rainfall leading to floods, landslides and debris-flow hazards have had increasing impacts on communities in Japan, because of climate change and structural immobilism in a changing and ageing society. Decreasing rural population lowers the human vulnerability in mountains, but hazards can still leave the mountain to the plains and sea, potentially carrying drifted-wood. The aim of the paper is to measure the distribution of wood-debris deposits created by the 2017 Asakura disaster and to rethink the distribution and spatial extension of associated disaster-risk zoning. For this purpose, the authors: (1) digitized and measured the distribution of drifted-wood, (2) statistically analyzed its distribution and (3) calculated the potential impact force of individual drifted timber as a minimal value. The results have shown that there is a shortening of the wood debris as they travel downstream and that the geomorphology has an important control over deposition zones. The result of momentum calculation for different stems’ length show spatially differentiated hazard-zones, which limit different disaster-risk potentials. From the present finding, we can state that we (1) need to develop separate strategies for sediments and wood debris (2) and for wood hazards, zonations can be generated depending on the location and the size of the deposited trees that differs spatially in a watershed.

Large wood remobilization in Asakura (North Kyushu, Japan): Adapting strategies to climate change and rural population depletion

“…On July 5 th -6 th 2017, heavy rainfalls fell in North Kyushu, in the Asakura area. Daily rainfall reached 516 mm, when monthly rainfalls only reached 600 mm to 800 mm for the wettest months of the last 40 years [7]. In turn, this event generated more than 1,500 mass movements.…”

“…In Northern America and in Europe, research on driftwood benefit for the ecological system and the environment has progressed in the recent decades [1,2], but this approach has seen less traction IOP Publishing doi:10.1088/1755-1315/1314/1/012063 2 in East Asian nations with short floodplains and dense population, as the disaster-risk still trumps ecological priorities. Those risks are often structural (bridges, weirs… [3,4,5], but the absence of any reliable warning system (except the radar-based in-flow detection system proposed by Gomez et al, [6]), has led to catastrophic impacts [7]. In the present contribution, the authors are only interested in the large wood travelling in river floods.…”

“…In tectonically active countries, earthquakes can also be a source of driftwood [8]. Besides purely natural processes, anthropogenic modification of the landscape [9] and landcover change due to population depletion in rural areas are also increasing sources of large wood [7].…”

Detection and Velocimetry of Floating Wood for Flood Disaster Risk Management Using Electromagnetic Imaging

“…The characterization and volume estimation of the wood in the floodplain and in the waterways is mostly occurring in the aftermath of wood travel and transport. It is done for wood trapped in water reservoirs behind dams using data from volume removed by the hydro-electric companies [7]; using historical and time-lapse cameras [7], using geometric calculations from field photographs of the deposits [8], and using digitization from post-flood deposits [9] . Even field research on the flowage of wood is dominated by work on deposited material [10], and the entrainment [11] and movement data are often obtained from laboratory and computer simulations [12,13,14].…”

From the Hydroclimatic Disaster to the Forced (Re)construction: Case Study of the Akatani Watershed in Japan

“…The region registered an unusual amount of daily rainfall, reaching 516 mm in 24 h in Asakura city [1]. Between 1977 and 2017, precipitation rates exceeded 600 mm on a monthly scale on nine occasions [2]. However, the J17 precipitation rate was exceptional, with an estimated occurrence probability higher than 1/200 [3], leading to thousands of landslides in the mountainous areas of subwatersheds on the right bank of the Chikugo River (CH) [1].…”