# Landscape coevolution of human-geomorphic system in northern Kyûshû (Japan) induced by the July 2017 heavy rainfall

Vincent Siccard, Mélody Dumont, Candide Lissak, Christopher Gomez, Gilles

Arnaud-Fassetta

► To cite this version:

Vincent Siccard, Mélody Dumont, Candide Lissak, Christopher Gomez, Gilles Arnaud-Fassetta. Land-scape coevolution of human-geomorphic system in northern Kyûshû (Japan) induced by the July 2017 heavy rainfall. European Geosciences Union, Apr 2025, Vienna, Austria. 10.5194/egusphere-egu25-12564. hal-05056558

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# EGU25-12564: Landscape coevolution of human-geomorphic system in northern Kyûshû (Japan) induced by the July 2017 heavy rainfall

**Vincent Siccard**, Université Lyon 3 Jean Moulin, Environnement, Ville, Société (EVS) UMR 5600 CNRS (France). vincent.siccard@univ-lyon3.fr

**Mélody Dumont**, Université Paris Cité, PRODIG UMR 8586 // Kobe University, Laboratory of Sediment Hazards And Disaster Risks (France - Japan)

**Candide Lissak**, Université de Rennes, Inserm, EHESP, Irset (Institut de recherche en santé, environnement et travail) - UMR_S 1085, F-35000 Rennes. (France)

**Christopher Gomez**, Kobe University, Laboratory of Sediment Hazards And Disaster Risks (Japan)

**Gilles Arnaud-Fassetta**, Université Paris Cité, PRODIG UMR 8586 (France)

<figure><img src="image_3.png" /></figure>

EVS
Environnement
Ville Société

cnrs

UMR 5600

<figure><img src="image_4.png" /></figure>

UNIVERSITÉ
DE LYON

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

Sediment Hazards And Disaster Risks

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

irset
Institut de recherche en santé
environnement et travail
Location :

- Northern Kyûshû (South Japan)

- Municipality of Asakura

- Chikugo watershed

- Souzu and Shira subcatchments

<figure><img src="image_9.png" /></figure>

Source: ESRI Images, GSI
Vincent Siccard, 2024
<figure><img src="image_10.png" /></figure>

**Topography:**

- Low mountain area

- Small size watershed (< 4 km²)

**Study area geology:**

- **Sangu-Suo Metamorphic rocks (schist)**

Souzu = 56%

Shira = 13%

- **Granodiorite**

Souzu = 27%

Shira = 65%

**Regional geological setting:**

- Non-alkaline mafic volcanic rocks (30%)

- Sangu-Suo Metamorphic rocks (Schist = 26%)

- Marine and non-marine sediments (22%)

- Terrace and fan (7%)

- Granodiorite (7%)

- Non-alkaline pyroclastic flow volcanic rocks (5%)

- Granite (1,5%)
## 1.1) Localized heavy rainfall

<figure><img src="image_11.png" /></figure>

### 5-6 July 2017:

* Stationary front during the rainy season

* Asakura's station in lowland: 24h cumulative rainfall: **516 mm**; 1h rainfall intensity **129.5 mm/h**

* Hundreds of landslides

* 42 casualties

* More frequent and intense heavy rainfall in a climate change context (MEXT et JMA, 2020; MLIT, 2020)

Estimated probability of occurrence greater than **1/200**. (Dumont et al., 2023; MLIT, 2017)

* **Low Frequency / High Magnitude (LFHM) event.**
## 1.2) Hundreds of landslides triggered

<figure><img src="image_12.png" /><figcaption>I. Landslide induced by bank erosion</figcaption></figure>

<figure><img src="image_13.png" /></figure>

**Shallow landslide:** « Hydrologically triggered shallow landslides typically occur when the shear strength is rapidly reduced due to rising pore water pressure above a hydrologic discontinuity in the regolith, caused by the infiltration of heavy rainfall into the soil layer (Sidle and Bogaard, 2016). High intensity rainfall is the main trigger for driving this process (Caine, 1980; Iverson, 2000). » (Sato et Shuin, 2025)

**Deep-seated landslide:** « The initiation of hydrologically triggered deep seated landslides is driven by increasing the positive pore water pressure on the potential sliding surface due to the groundwater level rising after prolonged heavy rainfall; however, the comprehensive mechanism has not been fully determined (Prokesova et al., 2013). » (Sato and Shuin, 2025)

### Mostly Shallow landslide

**Few deeper landslide** often in headwater with deeply weathered granodiorite and schist. (Jiban, 2017)

### 2 deep-seated landslide

Landslide concentration where 12-hour rainfall exceeds 400 mm (MLIT, 2020).

Large amount of landslide deposit remain on site. (Jiban, 2017)

⇒ Cascading effect / bank erosion and scouring

⇒ **Hundreds of sediment sources** suddenly opened

⇒ **Sedimentary pulsation**
## 2.1) Significant landscape changes

### Souzu watershed (2,8 km²):
1 casualty

**Before 2017 :**

* Landslide area = 0,08% of the watershed

**After July 2017 event:**

* Landslide area = 10,5 % (294 725 m²)

* Flooded area = 17%

### Shira watershed (3,9 km²):
1 casualty

**Before 2017 :**

* Landslide area = 0,05%

**After July 2017 event:**

* Landslide area = 7,3 % (284 576 m²)

* Flooded area = 4,7%

⇒ Fluvial metamorphosis (Chorley & Kennedy, 1971)

Before July 2017

<figure><img src="image_14.png" /></figure>

N

After July 2017

<figure><img src="image_15.png" /></figure>

N

Vincent Sicardi, 2014
Source: Google Earth. Imagery

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>
## 2.2) High sediment production

Sediment production at regional scale (MLIT, 2020):

* $\Rightarrow$ Landslide area = 4.44 km²

* $\Rightarrow$ Sediment production > 10 000 000 m³

* $\Rightarrow$ Sediment discharge into Chikugo ≈ 2 000 000 m³

### Souzu watershed sediment production:

≈ 550 000 m³ (MLIT, 2020);

≈ 503 000 m³
(GIS Based estimation)

* $\Rightarrow$ ≈ 196 000 m³/km²

* $\Rightarrow$ Sediment deposits in valley > 389 000 m³ (measured on 2/3 of the main valley)
(Komuro et al., 2018)

### Shira watershed sediment production:

≈ 590 000 m³ (MLIT, 2020);

≈ 599 000 m³
(GIS Based estimation)

=> ≈ 151 000 m³/km²

<figure><img src="image_18.png" /></figure>

Sediment production and potential sediment fluxes during the LFHM event on 5 July 2017

<figure><img src="image_19.png" /></figure>

Vincent Sicard, 2024
Source: GSI, Geomorphological classification modified from GSI

Vincent Sicard, 2025
Source: 2017 landslides inventory from GSI, 1m DEM MLIT

<figure><img src="image_20.png" /></figure>
## 2.3) High sediment fluxes generated by concomitant heavy rainfall and sediment production

**Souzu watershed: sediment fluxes have reinforced flooding by clogging the channelized river**

* **Lower valley pre-2017:** inhabited places and agricultural production

* **Lower valley post-2017:** sediment filling over 1-3m; destroyed houses

<figure><img src="image_21.png" /></figure>

© Ushiyama

<figure><img src="image_22.png" /></figure>

Height 1-3m

© Nakano, Flickr
## 3.1) Removal of sediment deposits in valley floors: exemple of Souzu watershed

Post 2017

<figure><img src="image_23.png" /></figure>

Post 2017

<figure><img src="image_24.png" /></figure>

Post sediment removal

<figure><img src="image_25.png" /></figure>

Post sediment removal

<figure><img src="image_26.png" /></figure>

### Souzu watershed:

* Sediment production:

≈ 550 000 m³ (MLIT, 2020);

≈ 503 000 m³ (GIS Based estimation)

≈ 196 000 m³/km²

* Sediment deposits in valley floor:

• > 389 000 m³
(Komuro et al., 2018)

• Measured on 2/3 of the main valley

⇒ removal of hundreds of thousands of m³ of sediment deposits

⇒ Partially reused in reconstruction work
### 3.2.1) High responsiveness and intervention post-event through engineering: check dam system

Classification of Sabo dams according to the MLIT (Dumont et al., 2023)

* **Sabo system:** when designed by the Japanese Ministry of Land, Infrastructure and Transport (MLIT)

* **Check dam system:** for similar objects not built by MLIT

* **System of structural countermeasures** to mitigate sediment production from hillslopes to transport in rivers

* **Modification of the sediment cascade**, the structural and process-based connectivity

<figure><img src="image_27.png" /><figcaption>**Open check dam: steel frame structure dam**</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Close check dam: 3 dimensional dam**</figcaption></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>
### 3.2.2) High responsiveness and intervention post-event through engineering: slope works

<figure><img src="image_31.png" /></figure>

© M. Dumont, 05/2024

© V. Siccard, 2023

A wide range of slope works from soft to hard engineering:

- Vegetation and Biological Soil Crust (BSC)

- Geotextile and Tarpaulin

- Terrace and gabion work

- Wire mesh

- Shortcrete grid beam

- Concrete cover

- Concrete cover with anchor and soil nailing

<figure><img src="image_32.png" /></figure>

© V. Siccard, 2023

<figure><img src="image_33.png" /></figure>

© V. Siccard, 2023
### 3.2.3) High responsiveness and intervention post-event through engineering

**Check dam system** in Souzu watershed: 11 dams built after 2017 (8 before 2017)

**Slope works:** mainly hard engineering

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

Souzu

Vincenç SCARD. 2024
Source: DEM 1 m DSM MULT
### 3.2.3) High responsiveness and intervention post-event through engineering

#### Check dam system of Souzu watershed during 2023 rainy season

Sediment production + remobilisation of older deposit

**Funasoko check dam N°2 => ≈ 20 000 m³ of 59 000 m³ (upstream erosion)**

Efficiency of the sediment capture countermeasures but higher sediment fluxes production => Dam maintenance is crucial for maximum efficiency

<figure><img src="image_36.png" /><figcaption>Pre 2023</figcaption></figure>

<figure><img src="image_37.png" /><figcaption>Pre 2023</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>Post 2023</figcaption></figure>

<figure><img src="image_39.png" /></figure>
<figure><img src="image_40.png" /><figcaption>3.3) Dismantling of deep-seated landslide and land use changes in Otoishi watershed</figcaption></figure>

Source: Pléiades © CNES 2024
(Distribution Airbus DS)

Source: GSI 2009

Source: MLIT GSI 2017

10/2017

04/2024
3.3) Dismantling of deep-seated landslide and land use changes in Otoishi watershed

<figure><img src="image_41.png" /></figure>

Source: GSI 2009

<figure><img src="image_42.png" /></figure>

Source: Pléiades © CNES 2024
(Distribution Airbus DS)

<figure><img src="image_43.png" /></figure>

V. Siccard, 2025

04/2024
# Landscape (co)evolution due to a serie of feedbacks in those coupled human-geomorphic system

## Before 2017: Satoyama (里山) landscape

- Socio-ecological system (Indrawan et al., 2014)

- Agroforestry practices and associated spatial organization

- Management by local populations

## 2017 post-event: radical changes in the Satoyama landscape linked to the propagation of hydro-sedimentary fluxes

- Flooding and fluvial metamorphosis

- Destruction of buildings and infrastructure

- Structural and process-based changes of the sediment cascade and connectivity

## Post 2017 reconstruction: A strong post-event intervention through the development of structural countermeasures system and land use changes

- Aligning the social use with geomorphological dynamics

- Sediment storage area

- Efficiency of the sediment capture countermeasures but higher sediment fluxes in a context of increased landscape sensitivity

Before 2017

<figure><img src="image_44.png" /><figcaption>2017 post-event landscape profile of the Souzu river watershed (寒水川)</figcaption></figure>

2017
post-event

<figure><img src="image_45.png" /><figcaption>Post-2017 landscape profile of the Souzu river watershed (寒水川)</figcaption></figure>

Post 2017
Reconstruction
/ Before 2023

<figure><img src="image_46.png" /></figure>
Thank you !

<figure><img src="image_47.png" /></figure>

# EGU25-12564: Landscape coevolution of human-geomorphic system in northern Kyûshû (Japan) induced by the July 2017 heavy rainfall

**Vincent Siccard**, Université Lyon 3 Jean Moulin, Environnement, Ville, Société (EVS) UMR 5600 CNRS (France). **vincent.siccard@univ-lyon3.fr**

**Mélody Dumont**, Université Paris Cité, PRODIG UMR 8586 // Kobe University, Laboratory of Sediment Hazards And Disaster Risks (France - Japan)

**Candide Lissak**, Université de Rennes, Inserm, EHESP, Irset (Institut de recherche en santé, environnement et travail) - UMR_S 1085, F-35000 Rennes. (France)

**Christopher Gomez**, Kobe University, Laboratory of Sediment Hazards And Disaster Risks (Japan)

**Gilles Arnaud-Fassetta**, Université Paris Cité, PRODIG UMR 8586 (France)

<figure><img src="image_48.png" /></figure>

EVS
Environnement
Ville Société

cnrs

UMR 5600

<figure><img src="image_49.png" /></figure>

UNIVERSITÉ
DE LYON

<figure><img src="image_50.png" /></figure>

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Sediment Hazards And Disaster Risks

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* Dumont, M.; Arnaud-Fassetta, G.; Gomez, C.; Lissak, C.; Viel, V. From the Hydroclimatic Disaster to the Forced (Re)construction: Case Study of the Akatani Watershed in Japan. Proceedings 2023, 87, 42. https://doi.org/10.3390/IECG2022-14820

* Dumont, M.; Gomez, C.; Arnaud-Fassetta, G.; Lissak, C.; Viel, V. The Disaster Protection System of Mountainous Rivers in Japan: The Example of the Akatani Watershed's Reconstruction. Sustainability 2023, 15, 15331. https://doi.org/10.3390/su152115331

* MEXT, JMA (2020) 'Climate Change in Japan: Report on assessment of observed/projected climate change relating to the atmosphere, land and oceans', Ministry of Education, Culture, Sports, Science and Technology (MEXT) ; Japan Meteorological Agency (JMA), Available online : https://www.data.jma.go.jp/cpdinfo/ccj/2020/pdf/cc2020_gaiyo_en.pdf

* MLIT (2020) 'Interim Report of SABO Technical Panel for Climate Change' Ministry of Land, Infrastructure and Transport (MLIT), Available online: https://www.mlit.go.jp/river/sabo/committee_kikohendo/200521/chukan_torimatome.pdf

* Jiban, 2017. 2nd plenary meeting on the disaster caused by torrential rain in July 2017 in northern Kyushu. https://www.jiban.or.jp/wp-content/uploads/2017/09/syamen20170903.pdf

* Geospatial Information Authority of Japan, https://www.gsi.go.jp/ENGLISH/index.html

* Chorley, R.J.; Kennedy, B.A. Physical Geography: A Systems Approach; Prentice-Hall: London, UK, 1971; ISBN 978-0-13-669028-3

* MLIT, 2017 - "Report of the Technical Committee for the Rehabilitation of Erosion Control and River in Chikugo Right Bank Catchment. 2017", Technical Committee for the Rehabilitation of Erosion Control and River in Chikugo Right Bank Catchment. p. 130. Disponible en ligne : http://www.qsr.mlit.go.jp/bousai_joho/H29hokubugouu/taisaku.html

* MLIT, 2017. https://www.mlit.go.jp/river/sabo/jirei/h29dosha/171002_saikansabou_hukuoka1.pdf

* Asakura City, 2018. Restoration and reconstruction efforts (fukkyū fukkō no torikumi nitsuite). Asakura City - Rehabilitation Headquarters (Asakura shi fukkyū fukkō suishin honbu)

* Komuro T., Akamatsu Y., Yamaguchi K., Watanabe Y., Moriya H., Nihei Y., 2018. Investigation of sediment flooding in souzu river, Chikugo river system by the northern Kyûshû heavy rain in July 2017 (heisei 29 nen kyūshū hokubu goū niokeru chikugogawa suikei shōzugawa no dosha hanran no jittai kaimei). 74, I_1189-I_1194. https://doi.org/10.2208/jscejhe.74.I_1189

* Miyase M., Miyagi A., 2024. On-site survey report - Disaster investigations caused by heavy rains in June and July 2023 (genchi chōsa hōkoku - reiwa 5 nen 6 tsuki oyobi 7 tsuki no ōame ni kīn suru saigai chōsa). SABO & Landslide Technical Center (STC). SABO Vol.135 2024 Winter

* Honma T., Ura M., Kosara T., Mizuno H., 2024. Understanding the actual state of sediment movement using LP data in the Souzu River (shōzugawa niokeru LP dēta o mochita dosha idō jittai no hāku). Pasco corporation, Kyushu University. Japan Society of Erosion Control Engineering (JSECE) 2024 conference

* Sato, T., Shuin, Y., 2025. Variation in the frequency and characteristics of landslides in response to changes in forest cover and rainfall in Japan over the last century: A literature review. CATENA 249, 108639. https://doi.org/10.1016/j.catena.2024.108639

* Fukuoka Prefecture, 2018. Regarding the disaster situation caused by the heavy rains in northern Kyushu in 2017 and disaster recovery projects for buried rivers (constant disasters) – (heisei ni kyū nen kyūshū hokubu goū no hisai jōkyō to uma 塞 kasen no saigai fukkyū jigyō (ittei wazawai) nitsuite). Fukuoka Prefecture Land Development Department River Management Division – (fukuoka ken kendo seibibu kasen kanrika).