P-045
Climate change and population depletion control over sediment hazards
and drifted wood hazards in japan
神戸大学大学院海事科学研究科○清水万莉子, Gomez Christopher, 国総研

1.Introduction
1.1
Climate change and population
depletion in Japan
In recent years, heavy rainfall events have been
increasing in japan, mostly because of climate

内田太郎

Kyusyu, (Japan) on the 5th and 6th of July 2017.
Asakura city in Fukuoka prefecture and Hita cityin Oita prefecture recorded the highest daily
rainfall.

change. Due to the effects of global warming, thetemperature of the sea can rise and the state of
the atmosphere can then become unstable,
triggering heavy rainfalls with intensities over
50mm/h. Consequently, the risk of sedimenthazards also increases. On the other hand,
population depletion in Japan is a serious
problem especially in local rural areas. The total
population is decreasing, but the ageing rate isincreasing. The present research area, Asakura
city, is also facing this combinations of problems.
The population is decreasing and the rate ofFig.1: Hourly rainfall depth and cumulative rainfall in
Asakura city on 2017.7/5~7/6

2.Method
2. 1Research location investigationelderly people is increasing. The lack ofsuccessors for the primary industry has caused
desolated landscape. In the mountainous areas,
cultivated land has left the place to forested
slopes, which unfortunately can turn into thedrifted woods easily during rainfall events.
1.2
Overview the July 2017 event and theFig.2: Field in Akatani basin at Asakura city, in Fukuoka

damageIn the Northern part of Kyushu Island, at thelimit between Fukuoka prefecture and Oita
prefecture lies the municipality of Asakura-city,where the basin topography usually protects itfrom the heavy rainfall that are usually recorded

The research location is in Asakura city,
Fukuoka Prefecture (Japan), on the basin ofAkatani river. The size of catchment area: 20.1
km², length of the river : 15.5 km ○average slope

in other part of the Island. However, an “extreme”
rainfall event was observed in the north part of- 461 -prefectureof river bed：1/35(1.6°)○Collapse area rate：
about3.1%○sediment volumeabout 2900000m³○drift woods volume ： about 39000m³ ○thenature of the soil：granite.

2.2 Data acquisition and analysis
The aerial photographs and the DEM obtained
from the LiDAR (Light Detection and Ranging)data were loaded in the GIS software QGIS foranalysis. First I did digitize 5812 trees, out of theortho-photographs. The method, unfortunately
does not account for bio debris buried in thesediments and also underestimate the numberof trees in the log-jam. Then, I measured eachtree length and characteristics (with branches
and foliage or not) Extract from the DEM data foreach tree, and exported it in Excel. (how manytrees, distance from the mouth of river, tree
length, slope data, topo data, curvature data) inorder to understand the stopping conditions of
the drifted wood as well as the distribution of
the drifted wood.
3. Result & Discussion
The statistical analysis has shown that trees
can stop both on steep slopes and on the flat areaof the valley, even if one would think that driftedwood would rather stop on flat area (Fig.3). Thisis explained by differentiations in the types oflandslides. Some of the landslides weregenerated as debris flows along thin corridors,
where only the first few centimeters of the soil
have been eroded, whereas at other locations,deeper landslides have occurred. When themass movements occurred as a debris flow, thesoil loss its structure and all the trees have been
evacuated from the area. On the contrary, inareas where the landslides are deeper, the top
soil structure was not loss and the trees did not
fully dissociate from the soil. The samelandslides but different results, it is concluded
that importance of the position in the watercatchment.
Almost of trees in Japan are coniferous trees
like cedar and cypress don’t spread its roots
deeply. They should be thinned at stated periods.
But in Japan, population depletion and aging are
advancing, so people who manage the forests
are decreasing. So drifted woods caused bysediment hazards and landslide with heavy rain

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will be more thick and long in the future. And
it applies to all over Japan.
With remote sensing system, it is easy to checksome artificial forests and catch sight of fragile
area. We can consider future measures about
how prevent drifted woods and if there are
dangerous area, we can prevent the damage
before the disaster. This isn’t the perfect tool butvery effective tool.

Fig.3: Distribution of trees per slope angles.

5.Conclusion
Drifted woods in sediment-disaster impacted
valleys are a direct impact down to the valleys
and the sea they have flown to, but a large
amount of the timber trapped in the mountains
appear to be a potential source of hazards in thefuture. As this distribution is dependent on thetype of landslides, proper sabo structures would
address present sediment and timber hazards aswell as potential drifted wood hazards.
References・JMA(2018)・GIS.(2017)・Cabinet Office.平成29年度版
高齢社会白書・九州地方整備局・福岡県（2017）
・岡本敦,(2017).平成29年度国総研講演会・Yamamoto.H, et al.
(2017).JOURNALOF SOCIETY FOR NATURAL DISASTERSCIENCE, Vol.36, No.3, p257-279 ・ Marutani.T, et al.
(2017): Journal of the Japan Society of Erosion ControlEngineering, Vol.70, No.4, p.31-42 ・ Tetsuya,K, et al.
(2018). 平成30年度砂防学会研究発表会概要集,T2-03・H. Saito, et al. (2014). v. 42; no. 11; p. 999–1002; Data
Repository item 2014350・Stefano Luigi Gariano, et al.
(2016)Earth-Science Reviews 162, 2016, p.227-252・山本晃一流木災害―発生から処理まで―,技報堂、2009