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Evolution of the Unzen Volcano and the Shimabara Peninsula (Japan) during the last 60 years: The role of the 1990-1995 eruption in modifying the landscape

Evolution du volcan Unzen et de la péninsule de Shimabara (Japon) durant les 60 dernières années : le rôle de l’éruption de 1990-1995 dans la modification du paysage
Christopher Gomez et Patrick Wassmer
p. 205-216

Résumés

Le volcan Unzen est situé au cœur de la péninsule de Shimabara, sur l’ile de Kyushu au Sud du Japon. Après près de deux siècles de quiétude, la phase éruptive de la période 1990-1995 a produit de nombreuses coulées pyroclastiques et des lahars qui ont recouvert une grande partie de la structure initiale et détruit une partie de la ville de Shimabara. Vingt ans plus tard, il est intéressant de regarder comment le paysage volcanique a évolué et comment cette phase d’activité a contribué à la modification de ce dernier. Ces deux questions s’inscrivent dans une réflexion sur le rôle des facteurs anthropiques et de leur contribution dans notre entrée dans l’Anthropocène, et comment nous avons modifié à long terme la dynamique de nos paysages. Afin d’atteindre ces objectifs, cette contribution se base sur l’utilisation de la méthode photogrammétrique SfM (Structure from Motion) appliquée a des images historiques, en se concentrant sur différents points clefs du volcan durant la période 1947-2013. Les résultats montrent que le volcan a évolué de manière dissymétrique depuis l’éruption, avec la partie sommitale qui est toujours majoritairement recouverte par les dépôts volcaniques de l’éruption de 1990-1995 (avec des épaisseurs atteignant 50 m localement) et qui a aussi vu un retour rapide de la végétation. Le pied du volcan est, par opposition, beaucoup moins végétalisé et comporte une proportion réduite de dépôts volcaniques. Cette distribution, contre-intuitive, est le résultat de l’activité humaine, surtout autour de la vallée de la Mizunashigawa, où les infrastructures anti-lahars ont limité le retour de la végétation et où les dépôts de coulées pyroclastiques et de lahars ont été largement excavées. Le long de cette même vallée, les matériaux érodés et évidés du pied du volcan ont été déposés en bord de mer, repoussant la ligne de rivage de près de 500 m. Ce changement, une fois encore, est le résultat de l’ingénierie et non de l’éruption. Les interventions humaines ont ici créé un contrôle qui fige le paysage alors que, très clairement, les zones où l’influence humaine a été moindre semblent avoir été beaucoup plus résilientes dans leur capacité à se transformer d’un état « post-éruptif » à un retour à la « normale ».

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Notes de la rédaction

Article soumis le 10 janvier2015, reçu sous sa forme révisée le 10 juillet 2015, définitivement accepté le 20 août 2015.

Texte intégral

The research would not have been possible without the help and the data available at the Shimabara City Council and thanks to the imagery database of the GSI. The authors are also indebt to two anonymous reviewers and the editor for the manuscript improvement and the advices they provided.

1. Introduction

1In an article published in Nature in 1879, H.B. Guppy makes an account of his travel to Shimabara peninsula and “Mount Unzen”: Is Mount Unzen a Volcano? In a recent visit to the Shimabara Peninsula, about twenty miles east of Nagasaki as the crow flies, an opportunity was afforded me of ascending “Unzen”, a mountain which rises about 4,700 feet above the sea (by aneroid). If tradition is to be believed “Unzen” is an active volcano, the subterranean fires of which have been slumbering since the close of the last century, when a disastrous earthquake accompanied by a volcanic eruption, destroyed 53,000 of the inhabitants of the district. But I failed to find any trace of a recent volcano, which, wherever it may be, is certainly not situated in the higher peaks of the mountain, where popular belief has located it. (Guppy, 1879-p.153).

2We have since then learnt that Mount Unzen was indeed a volcano and that the popular belief was most certainly true. Whether Guppy was expecting a lava-lake with Etna-like scenery - more familiar to European – or whether his comments were just motivated by the state of the volcanology in 1879, there is a lesson for the present geomorphologist: the landscape we observe is the resultant of complex evolution, showing as much – deposits – as it hides and takes away from the researcher – erosion, and especially erosion in between phases of sedimentation. It is like a book with missing pages, but which still makes sense regardless of how much pages are missing. Recognizing this well-known issue and the paradigm implications for geomorphology, the present contribution aims: (1) to provide a diachronic analysis of the evolution of Shimabara Peninsula and Unzen Volcano during the last 60 years; (2) to show by localized examples how the 1990-1995 eruption has influenced the modification of the volcanic- geomorphology and landscape – please note that in the present publication the term landscape is used as a term depicting the spatial construct of the physical environment and does not entail into the existing debates on the meaning of the term; and (3) finally the last and maybe most important aim is to show that volcanoes have also moved into the Anthropocene, where it is not their own activities but the interactions with human activities, which are modifying the landscape in the long run.

2. The Unzen Volcano and the 1990-1995 Heisei eruption

3The Unzen is an active volcano located at the heart of Shimabara Peninsula, - Nagasaki Province, Kyushu Island, South Japan - (fig. 1). The back-arc volcano, located in the Beppu-Shimabara graben, occupies a surface of approximately 16 km x 15 km (Yamamoto et al., 1993) and it culminates at the Unzen-Fugendake peak (1359 m a.s.l.). The structure of the volcano above sea-level approximates 100 km3 and it mostly consists of dacite and andesite materials, which take shape in the landscape as series of cones and partly dismantled stratovolcanoes (Tanaka and Nakada, 1988).

Fig. 1Topography of Shimabara peninsula and the Unzen Volcano on Kyushu Island, South Japan.
Fig. 1Carte topographique de la péninsule de Shimabara et du Volcan Unzen.

Fig. 1 – Topography of Shimabara peninsula and the Unzen Volcano on Kyushu Island, South Japan. Fig. 1 – Carte topographique de la péninsule de Shimabara et du Volcan Unzen.

The Digital Elevation Model has an horizontal resolution of 5 m and a vertical accuracy of less than 1 m. The present dataset has been created from a dataset made available in 2013.
La convergence des transects se fait au niveau du sommet de la dernière éruption: l’Unzen Fugendake. Sur le transect C1-C2, on note à l’est du sommet, la masse restante du Mayuyama qui s’est effondré en 1792. Le MNT, qui a une résolution horizontale de 5 m et une précision verticale inférieure à 1 m, est basé sur des données de 2013.

2.1. Chronology of historical eruptions

4The Unzen volcano sits on Pliocene formations composed of basaltic andesite lava, andesites, siltstones and sandstones (Otsuka and Furukawa, 1988), with the basalts dated to 4.6 Ma (Yokoyama et al., 1982). As most of the Unzen Volcano is located in a graben, its structure has been recorded to depths of 300 m to 1000 m below sea level, while the volcanic structure that has grown outside the graben is only 100 m deep (Ohta, 1987). The ‘Older Unzen’ formations have been dated to 400 and 500k. BP and present material that is close to the one characterizing the more recent eruptive activity (tab. 1). Indeed, a mix of deposits from flank collapses, pyroclastic flows, lava flows and lava domes has been found (Hoshizumi et al., 1999). The ‘Older Unzen’ eruptive activity then stops around 180k. to 190k. BP to enter a period of dormancy of 100,000 years. Around 100k. BP the ‘Young Unzen’ period then starts with the construction of new volcanic edifices, their collapses and recorded episodes of ash falls, pyroclastic flows, etc. (tab. 1). The historic period is marked by the growth of a dome at Mount Mayuyama, which famously collapsed in 1792, and entering into the Ariake sea, triggered a tsunami. The volcano, then, entered into a period of relative quiescence, before erupting in 1990.

Tab. 1Overview of the eruptive history at Unzen Volcano during the last 500 k. years.
Tab. 1 – Présentation générale de l’activité éruptive du Volcan Unzen durant les 500 000 dernières années.

Date

Classification

Activity

Details

Source

500k. BP

Older Unzen

 

Mix of debris avalanches, pyroclastic flows, lava flows and lava domes.

Hoshizumi et al., 1999

100k. BP

Younger Unzen

Thought start of the Younger Unzen

Mostly lava domes and pyroclastic flows

Hoshizumi et al., 1999

100 - 70k. BP

Younger Unzen

Growth of Nodake edifice

Mostly lava domes and pyroclastic flows

Hoshizumi et al., 1999

30 - 20k. BP

Younger Unzen

Growth of Myokendake

Mostly lava domes and pyroclastic flows

Hoshizumi et al., 1999

> 20k. BP

Younger Unzen

Growth of Unzen Fugendake

Mostly lava domes and pyroclastic flows

Hoshizumi et al., 1999

4,000 BP

Younger Unzen

Magmatic eruption

Formation of a large lava dome at Maruyama; volcanic bulge

Hoshizumi et al., 1999

1663

Younger Unzen

Magmatic eruption

Lava eruption in December, on the northern side of the volcano, producing a flow of of 0.15 km wide and 1 km length

JMA

1792

Younger Unzen

Magmatic eruption

The event may have started in November 1791, with a swarm of earthquakes that triggered landslides in December. The 10 February 1792, the volcano rings and fumeroles and sediments are ejected from the Jigokuato crater. On March 1st, a new lava flow starts, lasting about two months and depositing a lava tongue of 2.7 km length and a width varying from 220 to 360 m. On May 21st a large earthquake create a large flank collapse into the sea generating a tsunami in the Ariake sea. Earthquakes and earth ringing continue until June 1792, with episodic ash falls.

JMA

1798

Younger Unzen

Ash plume

 

JMA

1968

Younger Unzen

Sediment ejection from a hotspring

 

JMA

1975

Younger Unzen

Fumerole activity

Fumerole activity on the Eastern Flank of the Unzen central structure.

JMA

1990

Younger Unzen

Fumerole activity

Series of volcanic earthquakes during July 1990; May 17, beginning of the eruption with ash falls near the summit. The highest plume reaches 400 m. Earthquakes due to fracturation continues to shake the structure until the end of the year.

JMA

1991

Younger Unzen

Magmatic eruption

Growth and appearance of a lava dome on May 20th and on May 24th the pyroclastic flow begins. On June 3rd, 43 people perished and 179 houses were destroyed. On June 8th, 218 buildings were destroyed. The same year, rain starts and trigger lahars. The zone closed extends to its maximum on June 7th and 11,000 people are displaced.

JMA

1992

Younger Unzen

Magmatic eruption

Growth and collapses of several lava domes. The August 8 pyroclastic flow destroyed 17 buildings and 2,000 extra-people are evacuated.

JMA

1993

Younger Unzen

Magmatic eruption

Growth and collapses of lava domes resulting in 187 buildings destroyed and one casualty due to June 23 and 24th pyroclastic flows.

JMA

1994

Younger Unzen

Magmatic eruption

Although the growth of the dome towards the South-East, the domw starts growing towards the North-west in January, creating pyroclastic flows on the western flank, while they only concerned the East and Southeast direction up to 1994.

JMA

1995

Younger Unzen

Magmatic eruption

Athough the dome continue to grow, the eruption intensity reduces and from February 11, there are no more pyroclastic flows, and the area below the dome becomes aseismic. At this date, a total number of 9400 pyroclastic flows have been recorded by seismometers and an equivallent of 200 million cubic meters of lava has been produced.

JMA

JMA: Japanese Meteorological Agency

2.2. Zoom in on the 1990-1995 or Heisei eruption

5The eruption of 1990-1995 interrupted a period of 198 years of slumber. This eruption is sometimes also named in Japanese the ‘Heisei-eruption’ (平成噴火), referring to the period system used in the Japanese calendar to count the years indexed on the emperor’s reign. The eruption started on 17 November 1990, with a small emission of ash from the summit of Unzen Fugendake. Pass this first stage, a series of phreatic and phreatomagmatic eruptions started until the 10th May 1991, to be quickly followed by a bulge of the summit, which was then followed on 20th May 1991 by the emergence of a dacite lava dome, which rose from the Jigoku-ato crater (jigoku-ato meaning the aftermath of hell in Japanese). During the 5 years eruption period, a series of 13 lava domes grew (Nakada and Motomura, 1999) and collapsed by gravity. Some of them went through visco-elastic flow phases before reaching a rupture point, in part commanded by the crystal fraction of the lava (Cordonnier et al., 2009). In turn, the gravitational collapses generated numerous pyroclastic flows - to which volcanologists Harry Glicken, Maurice and Katia Krafft met their death with a group of about 40 journalists (Kerr, 1991) on June 3rd, 1991. The amount of material produced rose from less than 200,104 m3 during the period 23rd May – 10 June, to almost 600,104 m3 by the end of July, creating a large unbalanced mass at the summit of the volcano (Yamamoto et al., 1993). The pyroclastic flows, generated by gravitational collapse of the dome, have been widely monitored by the Japanese authorities and scientists, using a range of ground and airborne instrumentation and cameras, leading to one of the best-documented eruption at the time. The first pyroclastic flow was triggered on 24 May 1991, by the collapse of the first dacite dome that grew from Jigoka-ato crater. It travelled on the eastern flank of the volcano towards the Mizunashigawa valley, becoming the principal zone towards which the Showa eruption spilled pyroclastic material. During the month of May 1991, 4 pyroclastic flows with runouts of 1000 m to 2400 m on slopes of 17 to 26 degrees, reached average velocities of 15 to 24 m/s. It is noteworthy that the fastest velocities were recorded on the gentlest slope (17 degree). Beyond the major pyroclastic-flows that have left significant deposits, numerous smaller scale events have also swept the flanks of the Unzen. In May 1991, slightly less than 150 pyroclastic flows were recorded, but during the month of June and September 1991, around 500 pyroclastic flows were recorded for each month (Nakada and Fujii, 1993; Nakada et al., 1999; Watanabe et al., 1999) .

3. Methodology

6Although the 1990-1995 eruption has been very well recorded, with several measurement, models, maps and aerial photographs, we will draw from the fact that this period did not see the development of a detailed topographic record yet, nowadays accessible thanks to airborne and terrestrial laser or digital photogrammetry. For the present contribution, we therefore use historical archives and derive new datasets by using the latest developments in photogrammetry.

3.1. Automated uncalibrated photogrammetric method

7In order to create digital elevation models and orthophotographs from aerial photographs, for which (a) the elevation data are missing or are too sparse, (b) and for which the camera calibration is absent or insufficient for traditional digital photogrammetry, the authors have used the structure-from-motion (SfM) and multiple-view stereophotogrammetry (MVS) using the easy-to-use software developed by Agisoft: Photoscanpro (www.agisoft.com). This method has seen a real boom in the field of geosciences during the last 5 years (Gomez et al., 2015a).

8SfM-MVS is a method that emerged in the 1970s’ in computer vision and has seen a real boom during the last 5 years in the fields of geosciences and archaeology. Although SfM-MVS has been mostly used in geosciences using dedicated photographs, taken from the ground (Westoby et al., 2012, Gomez and Kataoka, 2015) or UAVs (Kaiser et al., 2014; Mancini et al., 2013; Obanawa et al., 2014a, Tonkin et al., 2014), similar work can be realized using crowd-sourced images (Gomez, 2013) and historical photographs (Gomez, 2013, 2014; Gomez et al., 2015b) – it is noteworthy that old aerial photographs archives through Europe are a gold mine for anybody interested in chronological evolution of geomorphic landscape during the last 100 years, allowing the creation of DEMs of areas that have changed or disappeared, allowing now the reconstruction of historical 3D evolution of geomorphic landscapes. Comparisons between pointclouds collected with terrestrial laser and SfM shows that - if employed properly - SfM can produce pointcloud of comparable accuracy - with millimeter to a few centimeter differences for a horizontal area of several hundred square meters – (Obanawa et al, 2014b).

3.2. GIS data extraction and analysis

9Using the dataset produced by SfM-MVS, indicators were created using GIS data extraction and analysis. First, a river density analysis was performed using the software ArcGIS and the existing ‘density’ function. This was performed as river density can be used as one of the indicator to measure the geomorphological modification in the landscape of Unzen Volcano. The density of rills and waterways provide a proxy on the nature of the material and how the eruption has modified the surface water dynamics. The second indicator that was extracted using GIS concerns the thickness of material produced during the eruption. Based on the differential of DEM, the 3D analyst toolbox of ArcGIS was used.

3.3. Field work

10The remote-sensing dataset was completed by field-work, which was conducted in March, May and June 2005, in order to survey the SABO countermeasure and warning system installed on the Unzen Volcano, and particularly the Mizunashigawa as well as recording lahar deposit characteristics in term of material transported, vegetation return and also how in a semi-engineered environment the lahars were evolving downstream. A series of outcrops were also drawn, in order to assess the impacts of the SABO network on the Mizunashigawa deposit. This complementary dataset is used in the present contribution to complete the data mostly acquired by remote sensing techniques. The combination of the methods depicted above following a spatial-and-temporal logic using GIS have resulted into the following findings.

4. Results

4.1. The geomorphologic changes at the volcano’s summit

11The 1990-1995 eruption has modified most of the geomorphological landscape in the summit area of the Unzen Fugendake (fig. 2). The area directly affected by the pyroclastic-flow deposits (fig. 2, No. 1) covers an area of 10 km2. The large amount of loose material escaped the summit in the form of pyroclastic-flows that were partly remobilized into lahars by rainfall. The combination of the two processes generated the fans around the Unzen Fugendake (fig. 2). The four main fans are in clockwise direction and beginning with the smallest to the south: 0.17 km2, 1.2 km2, 0.96 km2 and 1.7 km2. The third fan eventually developed further downstream in the Mizunashigawa valley, but the construction of the SABO dams system saw the removal and the transformation of vast area, which wasn’t taken into account for the present measurement. These fans have an interesting morphology, as they have been constrained by the previous volcanic topography. They either filled in depressions at the back of older volcanic rims or tool elongated shapes in between topographic highs (fig. 2). The pre-1990-1995 eruption volcanic structure has therefore acted as a belt for the pyroclastic-flows and lahars that have been concentrating their escape path into the Mizunashigawa valley towards the East.

Fig . 2Geomorphological sketch of the summit of the Unzen Fugendake after the eruption from aerial photographs of 1998.
Fig. 2Schéma géomorphologique du sommet de l’Unzen à la suite de l’éruption ‘Heisei’, à partir des photos de 1998.

Fig . 2 – Geomorphological sketch of the summit of the Unzen Fugendake after the eruption from aerial photographs of 1998. Fig. 2 – Schéma géomorphologique du sommet de l’Unzen à la suite de l’éruption ‘Heisei’, à partir des photos de 1998.

1: Zone recovered by volcanic material, and originally mostly by pyroclastic-flow deposits of the last eruption that were partly remobilized; 2: the lava dome in 1998; 3: Fan deposit of lahar and pyroclastic material; 4: slide direction of the dome; 5: secondary rims and steep walls; 6: main volcanic walls and rims; 7: rills and gullies in volcanic material (1990-1995 and previous eruptions).
1 : zone recouverte de dépôts pyroclastiques liés à la phase éruptive de 1990-1995 ; 2 : le dôme en 1998 ; 3 : tablier de dépôts pyroclastiques et lahars ; 4 : orientation du dôme coulée ; 5 : lignes de crêtes et escarpements principaux ; 6 : rebord et escarpement de la couronne volcanique principale ; 7 : barranco incisés dans le matériel volcanique.

12Contrasting with the loose material, which covered most of the slope, the latest volcanic lobe of the dome is at the top of the Unzen-ugendake and its gravitational movement have created the overhanging feature oriented towards the East.

13When comparing the DEMs before the eruption and in the aftermath of the eruption, topographic outcrop analysis show that the volcanic material has been infilling the existing topographic depression and by then smoothed the landscape (fig. 3).

Fig. 3Topographic change between 1965 and 2013, calculated from SfM-MVS based topography along a 2 km long transect.
Fig. 3Evolution morphologique entre 1965 et 2013, établie à partir d’une topographie réalisée par SfM-MVS sur un transect de 2 km de long.

Fig. 3 – Topographic change between 1965 and 2013, calculated from SfM-MVS based topography along a 2 km long transect. Fig. 3 – Evolution morphologique entre 1965 et 2013, établie à partir d’une topographie réalisée par SfM-MVS sur un transect de 2 km de long.

The data presents topographic infilling in the low points reaching 50 m +/- 5 m (error estimates based on observed error along the transect).
Les données mettent en évidence un remplissage des vallées et des dépressions qui peut atteindre 50 m +/- 5 m (l’incertitude est estimée sur la base des erreurs observées sur le terrain le long du transect).

4.2. The impacts of the eruption on the river channel network and the shoreline

14In 2005, the landscape of the volcanic apron in the Mizunashigawa valley was still dominated by the Heisei eruption (fig. 4) with lahars channels marked by coarse material supporting blocks in excess of 5 meters, that originated from the dome collapse. This visual change can be quantified using diachronic photogrammetric data (fig. 5), providing proxies of the modification of the river network originating and flowing from Unzen Volcano. In 1947 – prior to the 1990-1995 Heisei eruption -, the stream network density (fig. 5A) did not exceed 6 to 8 different stream segments per circle radius of 500 m, with the majority of the topography being dominated by 0 to 4 streams per 500 m radius. The distribution of the streams and their density then change radically in the aftermath of the eruption. In 1998, the stream density increases its maximum values to reach 12 to 14 streams per radius of 500 m, with a hotspot on the South-Eastern side of the volcano. The proportion of the landscape with 6 to 12 streams per radius of 500 m also increases and reaches areas near the summit, which was dominated before the eruption by 0-2 streams per 500 m radius. This transformation of the waterways’ landscape does not occur symmetrically however. This change only concerns the Eastern side of the volcano where the eruption impacted the landscape. The Western part remains mostly unchanged.

Fig. 4The volcanic landscape of the upper apron of the Mizunashigawa River in May 2005.
Fig. 4La morphologie volcanique de la partie supérieure du cône dans le secteur de la rivière Mizunashigaya en mai 2005.

Fig. 4 – The volcanic landscape of the upper apron of the Mizunashigawa River in May 2005. Fig. 4 – La morphologie volcanique de la partie supérieure du cône dans le secteur de la rivière Mizunashigaya en mai 2005.

A: panorama of the mid-Mizunashigawa showing lahar deposits and andesite blocks from the collapsed domes. This photograph also aims to show the very short distance between the volcano and the sea, explaining the sediment issues at Unzen; B: block from one of the dome on a lahar deposit with the Unzen Fugendake peak in the background; C: plurimetric andesite block transported in the Mizunashigawa valley; d: outcrop cut in lahar deposit.
A : panorama de la moyenne Mizunashigaya montrant des dépôts de lahars et des blocs andésitiques provenant de l’effondrement d’un dôme. Cette image montre également la très courte distance existant entre le sommet du volcan et la mer, expliquant les problèmes relatifs à la sédimentation dans ce secteur de l’Unzen ; B : Bloc de dôme sur un dépôt de lahar avec le sommet de l’Unzen-Fugendake dans le fond ; C : bloc andésitique plurimétrique transporté dans la vallée de la Mizunashigaya ; D : coupe naturelle dans un lahar

Photos C. Gomez, 2005.
Photos C. Gomez, 2005.

Fig. 5Waterways’ network density in 1947 and 1998, with the 1998 digitized network in overlay.
Fig. 5Evolution de la densité du réseau de drainage en 1947 et 1998.

Fig. 5 – Waterways’ network density in 1947 and 1998, with the 1998 digitized network in overlay. Fig. 5 – Evolution de la densité du réseau de drainage en 1947 et 1998.

Both dataset were extracted from the orthorectified aerial photographs mounted using SfM-MVS. a: density map for 1947; b: density map for 1998.
Le réseau hydrographique de 1998 apparaît en surface sur les deux images. Les données nécessaires à cette cartographie ont été extraites de photographies aériennes ortho-rectifiées montées en utilisant la technique SfM-MVS. a : carte de densité de drainage pour 1947 ; b : carte pour 1998.

15The 1990-1995 eruption also impacted the shoreline, which moved seaward to a maximum of 450 m on the west side of the mouth of the Mizunashigawa River. The total gain around the Mizunashigawa, on the East side of the Shimabara peninsula (fig. 6) is 0.35 km2. This extension is not natural however and it is the result of the digging of material on the volcanic apron rather than the natural growth of the river delta. The opposite trend was also observe at the mouth of one of the bypass of the Mizunashigawa, where the sediment fan prograded after the eruption in 1998, and then retracted as seen from the 2013 imagery (fig. 6).

Fig. 6The advance of the shoreline between 1965 and 2013 at the mouth of the Mizunashigawa River, East of Unzen-Fugendake.
Fig. 6L’avancée du trait de côte entre 1965 et 2013 à l’embouchure de la Mizunashigawa sur la baie d’Ariake, à l’Est du complexe Unzen-Fugendake.

Fig. 6 – The advance of the shoreline between 1965 and 2013 at the mouth of the Mizunashigawa River, East of Unzen-Fugendake. Fig. 6 – L’avancée du trait de côte entre 1965 et 2013 à l’embouchure de la Mizunashigawa sur la baie d’Ariake, à l’Est du complexe Unzen-Fugendake.

4.3. The role of the countermeasures in fixing the landscape ‘in concrete’

16Beyond the geomorphic activity of the eruption of 1990-1995, the engineered structure have had a profound effect on the geomorphological landscape of Unzen Volcano, including large portions of the shore (fig. 6).

17Following the Heisei-eruption, the volcano was belted by a series of SABO structures - in Japanese SABO stands for 砂防 or “erosion control” – mostly along the 15.9 km long Mizunashigawa valley, the 9.93 km long Nakaogawa valley and the 13.5 km long Yuegawa valley. The heaviest series of structure was planned for the Mizunashigawa with a total capacity of 2,400,000 m3, because of the funnel shape of the valley, which can potentially concentrate flows in areas where the population is at risk. Statistics of the Japanese Ministry of Land, Infrastructure and Transport (MLIT, 2007) show that 4198 people or 1640 households are living in area at risk on the 739 ha of the Mizunashigawa floodplain. Even more importantly, main communication roads and lifelines (National Road 57 and 251 and the Shimabara railway) and 5 hospitals are also located in this area at risk. The choice of deploy a series of SABO dam was fueled by the large amount of material remobilizable on the flanc of the volcano that generated 12 lahars in 1991, 21 in 1992, 34 in 1993, 8 in 1994, and 10 in both 1995 and 1996, to reach a total of 108 since the eruption.

18The complex of SABO, which has been established in the Mizunashigawa comprises a set of structures and systems with specific roles, in order to limit the impacts of lahars (fig. 7). At the topographical rupture between the volcanic apron of the Unzen Fugendake and the lower fan of the Mizunashigawa, a series of ‘slit dams’ with pools (fig. 7A) have the role to stop the large blocks thanks to 6 m high steel pillars filled in concrete. Located below the pillars, a concrete pool stops and concentrates the smaller size material. As the amount of material expected to be trapped upstream the ‘slit’ structures was expected to quickly exceed the capacity of the dam, a system of radiocontrolled digger was put in place, in order to quickly and safely evacuate the material trapped upstream (fig. 7B). In order to complete this setting, a series of sediment traps and steps have been put in place in the downstream fan area, in such a way that the finer fraction would not spread into the Ariake sea. Upstream, on the Northern flank of the Unzen Fugendake, a series of check dams (fig. 7C) was emplaced, for the material in the valley looses momentum. Consequently it limits the transport of very large boulders, by ‘breaking’ the flow.

Fig. 7Lahar countermeasures in the Mizunashigawa Valley.
Fig. 7Systèmes de protection contre les lahars dans la vallée de la Mizunashigawa.

Fig. 7 – Lahar countermeasures in the Mizunashigawa Valley. Fig. 7 – Systèmes de protection contre les lahars dans la vallée de la Mizunashigawa.

A: ‘Slit dam’ meant to stop the large block. Each pillar is 6 m high and made of steel filled with concrete; B: quarrying marks in the Mizunashigawa Valley, where manned and unmanned diggers evacuate the material accumulated upstream the SABO dams; C: series of Check-dams, that have the effect of a staircase in dissipating the energy that allow the transport of the largest fraction of the material.
A : les barrages filtrants ont pour objectif de retenir les gros blocs. Chaque pilier est constitué d’un tube d’acier de 6 m de long rempli de béton ; B : traces d'exploitation des matériaux volcaniclastiques dans la vallée de la Mizunashigawa où des excavatrices, contrôlées ou non, évacuent le matériel accumulé en arrière des barrages SABO ; C : succession de barrages. Ce dispositif en escalier dissipe l’énergie des coulées, favorisant le dépôt du matériel grossier responsable des dégâts en aval.

4.4. The destruction and return of the vegetation

19Beyond the geomorphologic and sedimentary aspects of the eruption, one of the main drivers of change in the landscape has been the loss and then the return of the vegetation. At the summit of the volcano, the vegetation almost totally disappeared during the eruption (fig. 8). From the survey areas on the Eastern flank of the Unzen Fugendake, only 58.6 x 1000 m2 of forested surface remained on local high topographic, when the forested surface more than 10 folds in 1965: 863.2 x 1000 m2 (fig. 8). The eruption appears to however be only one of the drivers that control the distribution of forested areas. Indeed, the images of 1947 show that there were less forested areas at both the summit and on the foot than in 1965 (fig. 8D). At the foot of the volcano, the dynamic is different (fig. 8). Deforestation made space for rice fields and other cultures, with forested areas being confined to the surrounding of irrigation waterways and rivers. The 1990 eruption has modified this dynamic. The eruption destroyed and burnt vegetation, mainly through lahars and pyroclastic activity (fig. 8). After the eruption, although vegetation has returned on the upper slopes, one can observe that it has not been the case for the lower slopes, mainly because of the SABO dams, which command a vast stock of sediments.

Fig. 8Forested vegetation dynamics from two study zones for the period 1947-2013.
Fig. 8Dynamique de la végétation forestière durant la période 1947-2013 sur base de deux zones d’étude.

Fig. 8 – Forested vegetation dynamics from two study zones for the period 1947-2013. Fig. 8 – Dynamique de la végétation forestière durant la période 1947-2013 sur base de deux zones d’étude.

A: location map of the two study zones depicted in the figure. B: zone 1 located at the peak of Unzen Fugendake with the faded 1998 aerial photograph as a background. The background gives a visual idea of the level of total destruction of vegetation by the eruptive material. C: zone 2, in the lower Mizunashigawa valley displaying the extent of forested areas in 1947, 1965 and 2013 (no forested area in 1998). The backdround shows the state of the valley in 2013 with the SABO dams. D: barplot of the vegetation cover from 1947 to 2013 for both zones 1 and 2.
A : carte de localisation des deux secteurs d’étude ; B : zone 1 sur les hauts de pentes du Unzen-Fugendake sur fond d’image aérienne de 1998. Le fond donne une bonne idée du niveau de destruction totale de la végétation par les matériaux volcaniques ; C : zone 2 dans la basse vallée de la Mizunashigawa montrant l’extension des étendues forestières en 1947, 1965 et 2013 (absence de surface forestière en 1998). L’arrière-plan montre l’état de la vallée en 2013 avec les barrages SABO ; D : histogrammes des superficies forestières de 1947 à 2013 pour les deux zones étudiées.

5. Discussion and Conclusions

20The present contribution on the evolution of the Shimabara peninsula and the landscape around the Unzen Fugendake volcano concur most certainly with the idea that we have entered the era of the Anthropocene. Indeed, it is not so much the eruption that has shaped the landscape, rather than it is the human structures, and the usage of immutable – or at least not at the decadal scale – constructs, which, instead, govern the present evolution of the landscape of Shimabara peninsula.

21It is nevertheless inevitable in a country of volcanoes, with the second largest number of active volcanoes, just behind Indonesia and a population of 127 millions. This reality has been recently reminded to the Japanese public as the March 2011 megathrust earthquake has brought the alert levels up on numerous volcanoes, including the touristic spot of Hakone to the West of Tokyo, eventually threatening more than 30 millions inhabitants in the metropolis (Japan Meteorological Agency, 2015). In this context of complex interactions between volcanoes and population, the role of the engineered construct on present and future events can’t be omitted. As presented in this contribution, the human influence and the construction of the SABO dams and the sediment reservoirs have created an unbalance. Indeed, the slopes of the Unzen volcanoes are trying to reach equilibrium situations by filling the lower part of the volcanic apron and discharging upstream material, but the excavation of pyroclastic deposits at the base of the volcano upsets this equilibrium. Although numerous lahars have occurred in the years following the 1990-1995 eruptions (Miyabuchi, 1999), those have grown sparse despite the large amount of material remaining on the upper slopes. Consequently, one should expect that future pyroclastic density currents and lahars will behave in a very different manner, as the effects of the slopes on the mechanics of the flowing pattern should rather differ from “natural” volcanic slopes, that see a gentle reduction towards the lower part of the apron. This issue might be encountered at other volcanoes in Japan and South East Asia, where similar protection systems have been put in place, such as the Sakurajima Volcano, or the Merapi Volcano, place where the influence of SABO dams on helping pyroclastic flow to “jump out” of the valleys have been unveiled as well (Thouret et al., 2010) extending the range of unforeseen consequences of human constructs on volcanic flow behavior and subsequent morphological changes. Those changes do not however stop on the volcano, but extend to the coast, where the coast of Unzen has moved by 450 m at the mouth of the Mizunashigawa river, and where the sediments trapped on the peninsula contribute to sediment loss, eventually accentuating coastal erosion as Hart and Gomez (2013) explained.

22From a technical perspective, the present manuscript contributes to the recent development of SfM-MVS in the field of Earth Sciences and how it can be applied to various media to derive DSMs and eventually DEMs from crowd sourcing and existing archives. Although the existing publications - except for Gomez (2014), and Gomez et al. (2014, 2015b) - are only working with SfM-MVS using data that have been taken for the sole purpose of SfM-MVS, one of the other appeal of the technique is its ability to be used with other sources, such as the historical aerial photograhs. This realization opens new doors in diachronic geomorphology, because the method created presently allows to rework existing dataset to create DEMs and DSMs that would not be available in any other way. Indeed, we can’t decide today to go and fly a LiDAR over Mt Unzen of 1947, but we can use existing archives to recreate a pointcloud as close as possible to the past-topography and landscape. The other possibility that needs to be explored, is the usage of SfM-MVS from crowd-sourcing. Indeed, the data acquisition technique is relatively simple and does not necessitate any expensive equipment. A simple digital camera is sufficient to generate series of DEMs. In New Zealand for instance, C. Gomez and H. Purdie are surveying the evolution of the valley walls of the Fox Valley where the retreat of the glacier is putting visitors at risk. As the valley is remote, the authors are working in partnership with the glacier guides, who take data on a regular basis to monitor the valley and the risk of slides and rockfalls. Such methodological framework can be adapted to any remote area, where data transfer is possible. Lastly, it is important to note that further work needs to be performed, to make full use of the more precise - if not more accurate - dataset that technological advances give us access to, in order to renew the paradigms and find elements that could not be done without the developed techniques, rather than simply increasing the precision of what has already been researched using older methods (Gomez and Kato, 2014).

23Despite numerous unexplored possibilities, this method isn’t a silver bullet and it includes limitations, due to the method itself and the resolution of the imagery. Indeed, SfM algorithms can accept an array of results to a set of equations, and therefore can generate a set of “false results” that are difficult to detect without ground truthing, and even when adding existing control points to the model by “burning” them back on the generated 3D surface, errors can remains. This issue has been tackled for braided river active channels DSM extraction (Javernick et al., 2014), as the micro-variations and errors can have important impacts on hydrologic work. The authors have divided the over 10,000 RTK-GPS points collected in the field into a series of 95 Ground Control Points (GCPs), which they used to constrain the DSM reconstructed by SfM. Before burning the GCPs back on the DSM, the errors were ranging -2.3 and more than 3 m vertical residual error, with a clear spatial distribution of the error, the central part of the model being lower than the RTK-GPS derived topography and the edge, higher. After optimizing the SfM model with the GCPs, the error was then reduced to a maximum of 0.9 cm, which, over a 1.5 km long floodplain model is a good result. Similar issues have been also observed for mountains and volcanoes, with the highest points tending to be exaggerated by SfM, creating an artificial acceleration of slopes (Gomez et al., 2015b). Further limitations of the method concerning the mechanics of SfM have been emphasized by Westoby et al. (2012). In a seminal paper that presents the possibilities of the technology to geomorphologists, they have also emphasized the excessive need of computer time and computer processing capacity, and large pointclouds can become rapidly an issue, especially because the SfM algorithms are iterative and process each image one after another.

24In conclusion, new methods and techniques bring – with some acknowledge limitations – new datasets for the geomorphologists to work with and tackle the rapidly changing environments that anthropic activity is contributing to, if not dominating. We have certainly reached a time when Geomorphology should also look at the morphology of concrete and all other human-built elements as they contribute as much as natural factors to the environment we evolve through.

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Bibliographie

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Annexe

Version française abrégée

Le volcan Unzen se trouve dans le département de Nagasaki, sur l’ile de Kyūshū au Sud de l’archipel Japonais. L’édifice est situé au cœur de la péninsule de Shimabara et domine la principale ville de la côte Nord qui porte le même nom. Ce complexe volcanique est un stratovolcan dont l’activité la plus récente a créé la structure qui porte le nom de « Unzen Fugendake », culminant au Heisei Shinzan (« le nouveau pic/la nouvelle montagne » de la période Heisei en Japonais) à 1 483 m au-dessus du niveau de la mer (N32o45,41- E130 o 17,56).

L’histoire connue de ce complexe volcanique remonte au « Vieux Volcan Unzen » qui se serait mis en place autour de 400 000 à 500 000 ans. La période qui s’étend des origines jusque 100 000 BP a vu se succéder des phases d’activités éruptives et un déplacement des structures actives au sein de la péninsule, expliquant la complexité du paysage actuel. Cette activité volcanique est caractérisée par des coulées de lave, la formation de dômes andésitiques, une activité phréatomagmatique et des coulées pyroclastiques. Il semble que les manifestations éruptives de cette période soient peu différenciées de la période suivante (100 000 BP – présent). Durant la période historique contemporaine, deux évènements ponctuent l’histoire éruptive avec tout d’abord le 21 mai 1792, l’émission d’une importante coulée dacitique par le Fugendake, suivie par une forte secousse sismique post-éruptive générant le glissement soudain de tout le flanc Est du Mayuyama dans la baie d’Ariake sur le piémont oriental de l’Unzen. Le gonflement d’un dôme sommital avait été reporté sur cet édifice du complexe de l’Unzen en 1791. Les conséquences furent désastreuses. La masse rocheuse rasa une partie de la ville de Shimabara et généra un tsunami. Le nombre de victimes, à Shimabara et sur les côtes environnantes, a été estimé à 15 000.

Le second évènement, et le dernier en date, est la phase éruptive du Unzen Fugendake durant la période 1990-1995, et qui fit le 3 juin 1991 quarante-trois victimes dont les volcanologues Maurice et Katia Krafft ainsi que Harry Glicken. Cet intervalle a été caractérisé par le déclenchement de plus de 100 coulées pyroclastiques sur le flanc est du volcan Unzen Fugendake, suite à la poussée de dômes andésitiques qui se sont écroulés successivement sous l’effet de la gravité. Bien que la majorité des coulées pyroclastiques se soient dirigées dans la Mizunashigawa (« rivière sans eau »), sur le flanc Est, la fin de l’éruption fut marquée par une série d’évènements pyroclastiques qui dévalèrent les vallées du Nord et Nord-Ouest (à partir de 1994).

Dès 1991, les dépôts pyroclastiques furent remobilisés par des lahars – principalement dans la Mizunashigawa – épaississant la zone basse du tablier volcanique. En réponse à la menace représentée par ces coulées de débris, un large système de barrages Sabô (ou protection contre les sédiments en Japonais) a été construit. Il s’agit du plus vaste système de contrôle des matériaux d’érosion volcanique conçu. La structure en marche d’escalier permet de disperser l’énergie ; des barrages filtrants ne stoppent que les matériaux les plus larges et des casiers de rétention/stockage dans la zone basse limitent la part de sédiments arrivant à la mer. Cet ambitieux projet fut également complété par la mobilisation intensive de pelleteuses et autre engins de chantiers radioguidés, afin de vider au fur et à mesure les barrages Sabô remplis par les sédiments pour pérenniser leur efficacité. On notera que la création d’un déséquilibre de pente « appelant » les futurs lahars constitue une rétroaction négative engendrée par le système mis en place.

Cet évènement a donc profondément marqué le paysage de la péninsule de Shimabara, et la présente contribution s’est fixée pour but l’examen diachronique de la zone affectée du sommet de l’Unzen Fugendake jusqu'à la mer. Au-delà de cet objectif de fond, cet article souhaite également démontrer les possibilités de la technique Structure from Motion (SfM) pour la reconstruction des évolutions morphologiques durant les 50 dernières années à partir de photos aériennes d’archives.

La méthode utilisée ici est donc fondée sur une variété de mesures faites sous SIG, à partir de données créées par la méthode SfM. Cette méthode recrée la troisième dimension d’un objet étudié en utilisant les variations de géométries projetées sur une surface 2D (la caméra). Bien que les méthodes de photogrammétrie classiques à n-photos utilisent le même type d’algorithme, la nouveauté de SfM est la capacité à estimer la position de la caméra conjointement avec la reconstruction en 3D de la cible. Pour cette contribution, nous avons utilisé cette technique sur des photographies aériennes datant de la période 1947 jusque 2013 afin de travailler sur les trajectoires paysagères et notamment les trajectoires géomorphologiques. Comme la qualité des données produites à partir de SfM est intimement liée à la qualité des photos, pour les comparaisons entre différentes dates, les auteurs ont toujours travaillé sur la qualité inférieure des deux images.

Les résultats montrent que les éruptions de la période 1990-1995 ont généré de nombreuses modifications dans le paysage. Premièrement, une densification du réseau hydrographique a été observée et mesurée au niveau du sommet de l’Unzen Fugendake. En revanche, dans le même temps, on note une simplification du réseau, canalisé et guidé à travers les séries de barrages dès 1998. Cette modification d’origine anthropique a aussi perturbé la dynamique de disparition et de reconquête végétale. En effet, le sommet de l’Unzen qui s’est – et a été – largement re-végétalisé n’est pas en phase avec la dynamique de la base du volcan, puisque les zones boisées autour des cours d’eau n’ont pu se rétablir en raison des systèmes de barrages et des mécanismes de sédimentation/évacuation des sédiments liés à leur fonctionnement. De la même manière, l’emprise spatiale du système de contrôle des sédiments empêche la réinstallation de la végétation ligneuse sur la base du volcan. L’occupation du sol présente ainsi un aspect original dans sa dynamique sur ce flanc Est du volcan Unzen. En effet, l’empreinte persistante n’est pas tant celle de l’activité volcanique que celle de l’activité humaine qui a figé les conséquences d’un évènement dans un tombeau de béton et d’acier, duquel il est aujourd’hui, 20 ans après, impossible de se défaire.

Au-delà de ces impacts à la surface du volcan, nous avons complété cette étude par une campagne de mesures sur l’épaisseur des dépôts afin d’avoir une idée du volume de matériel clastique encore mobilisable par les pluies sur l’édifice, mais aussi pour démontrer l’utilité de cette méthode SfM pour la surveillance et l’évolution morphologique volcanique. Les résultats ont montré que même sur le flanc Nord, moins touché par l’éruption, l’Unzen Fugendake accumulait toujours en 1998 un surplus de 50 m d’épaisseur dans les vallées, en comparaison avec la période pré-éruptive. Cette couverture, qui drape encore plus de la moitié de la partie sommitale du cône, a créé un relief plus « lisse » malgré les entailles faites par le réseau hydrographique. Sa présence explique pourquoi la végétation ne pousse que très lentement dans les zones sommitales : l’accès à l’eau y est très difficile.

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Table des illustrations

Titre Fig. 1 – Topography of Shimabara peninsula and the Unzen Volcano on Kyushu Island, South Japan. Fig. 1Carte topographique de la péninsule de Shimabara et du Volcan Unzen.
Légende The Digital Elevation Model has an horizontal resolution of 5 m and a vertical accuracy of less than 1 m. The present dataset has been created from a dataset made available in 2013. La convergence des transects se fait au niveau du sommet de la dernière éruption: l’Unzen Fugendake. Sur le transect C1-C2, on note à l’est du sommet, la masse restante du Mayuyama qui s’est effondré en 1792. Le MNT, qui a une résolution horizontale de 5 m et une précision verticale inférieure à 1 m, est basé sur des données de 2013.
URL http://geomorphologie.revues.org/docannexe/image/11048/img-1.jpg
Fichier image/jpeg, 2,7M
Titre Fig . 2 – Geomorphological sketch of the summit of the Unzen Fugendake after the eruption from aerial photographs of 1998. Fig. 2Schéma géomorphologique du sommet de l’Unzen à la suite de l’éruption ‘Heisei’, à partir des photos de 1998.
Légende 1: Zone recovered by volcanic material, and originally mostly by pyroclastic-flow deposits of the last eruption that were partly remobilized; 2: the lava dome in 1998; 3: Fan deposit of lahar and pyroclastic material; 4: slide direction of the dome; 5: secondary rims and steep walls; 6: main volcanic walls and rims; 7: rills and gullies in volcanic material (1990-1995 and previous eruptions). 1 : zone recouverte de dépôts pyroclastiques liés à la phase éruptive de 1990-1995 ; 2 : le dôme en 1998 ; 3 : tablier de dépôts pyroclastiques et lahars ; 4 : orientation du dôme coulée ; 5 : lignes de crêtes et escarpements principaux ; 6 : rebord et escarpement de la couronne volcanique principale ; 7 : barranco incisés dans le matériel volcanique.
URL http://geomorphologie.revues.org/docannexe/image/11048/img-2.jpg
Fichier image/jpeg, 2,6M
Titre Fig. 3 – Topographic change between 1965 and 2013, calculated from SfM-MVS based topography along a 2 km long transect. Fig. 3Evolution morphologique entre 1965 et 2013, établie à partir d’une topographie réalisée par SfM-MVS sur un transect de 2 km de long.
Légende The data presents topographic infilling in the low points reaching 50 m +/- 5 m (error estimates based on observed error along the transect). Les données mettent en évidence un remplissage des vallées et des dépressions qui peut atteindre 50 m +/- 5 m (l’incertitude est estimée sur la base des erreurs observées sur le terrain le long du transect).
URL http://geomorphologie.revues.org/docannexe/image/11048/img-3.jpg
Fichier image/jpeg, 980k
Titre Fig. 4 – The volcanic landscape of the upper apron of the Mizunashigawa River in May 2005. Fig. 4La morphologie volcanique de la partie supérieure du cône dans le secteur de la rivière Mizunashigaya en mai 2005.
Légende A: panorama of the mid-Mizunashigawa showing lahar deposits and andesite blocks from the collapsed domes. This photograph also aims to show the very short distance between the volcano and the sea, explaining the sediment issues at Unzen; B: block from one of the dome on a lahar deposit with the Unzen Fugendake peak in the background; C: plurimetric andesite block transported in the Mizunashigawa valley; d: outcrop cut in lahar deposit. A : panorama de la moyenne Mizunashigaya montrant des dépôts de lahars et des blocs andésitiques provenant de l’effondrement d’un dôme. Cette image montre également la très courte distance existant entre le sommet du volcan et la mer, expliquant les problèmes relatifs à la sédimentation dans ce secteur de l’Unzen ; B : Bloc de dôme sur un dépôt de lahar avec le sommet de l’Unzen-Fugendake dans le fond ; C : bloc andésitique plurimétrique transporté dans la vallée de la Mizunashigaya ; D : coupe naturelle dans un lahar
URL http://geomorphologie.revues.org/docannexe/image/11048/img-4.jpg
Fichier image/jpeg, 3,1M
Titre Fig. 5 – Waterways’ network density in 1947 and 1998, with the 1998 digitized network in overlay. Fig. 5Evolution de la densité du réseau de drainage en 1947 et 1998.
Légende Both dataset were extracted from the orthorectified aerial photographs mounted using SfM-MVS. a: density map for 1947; b: density map for 1998. Le réseau hydrographique de 1998 apparaît en surface sur les deux images. Les données nécessaires à cette cartographie ont été extraites de photographies aériennes ortho-rectifiées montées en utilisant la technique SfM-MVS. a : carte de densité de drainage pour 1947 ; b : carte pour 1998.
URL http://geomorphologie.revues.org/docannexe/image/11048/img-5.jpg
Fichier image/jpeg, 1,7M
Titre Fig. 6 – The advance of the shoreline between 1965 and 2013 at the mouth of the Mizunashigawa River, East of Unzen-Fugendake. Fig. 6L’avancée du trait de côte entre 1965 et 2013 à l’embouchure de la Mizunashigawa sur la baie d’Ariake, à l’Est du complexe Unzen-Fugendake.
URL http://geomorphologie.revues.org/docannexe/image/11048/img-6.jpg
Fichier image/jpeg, 1,3M
Titre Fig. 7 – Lahar countermeasures in the Mizunashigawa Valley. Fig. 7Systèmes de protection contre les lahars dans la vallée de la Mizunashigawa.
Légende A: ‘Slit dam’ meant to stop the large block. Each pillar is 6 m high and made of steel filled with concrete; B: quarrying marks in the Mizunashigawa Valley, where manned and unmanned diggers evacuate the material accumulated upstream the SABO dams; C: series of Check-dams, that have the effect of a staircase in dissipating the energy that allow the transport of the largest fraction of the material.A : les barrages filtrants ont pour objectif de retenir les gros blocs. Chaque pilier est constitué d’un tube d’acier de 6 m de long rempli de béton ; B : traces d'exploitation des matériaux volcaniclastiques dans la vallée de la Mizunashigawa où des excavatrices, contrôlées ou non, évacuent le matériel accumulé en arrière des barrages SABO ; C : succession de barrages. Ce dispositif en escalier dissipe l’énergie des coulées, favorisant le dépôt du matériel grossier responsable des dégâts en aval.
URL http://geomorphologie.revues.org/docannexe/image/11048/img-7.jpg
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Titre Fig. 8 – Forested vegetation dynamics from two study zones for the period 1947-2013. Fig. 8Dynamique de la végétation forestière durant la période 1947-2013 sur base de deux zones d’étude.
Légende A: location map of the two study zones depicted in the figure. B: zone 1 located at the peak of Unzen Fugendake with the faded 1998 aerial photograph as a background. The background gives a visual idea of the level of total destruction of vegetation by the eruptive material. C: zone 2, in the lower Mizunashigawa valley displaying the extent of forested areas in 1947, 1965 and 2013 (no forested area in 1998). The backdround shows the state of the valley in 2013 with the SABO dams. D: barplot of the vegetation cover from 1947 to 2013 for both zones 1 and 2. A : carte de localisation des deux secteurs d’étude ; B : zone 1 sur les hauts de pentes du Unzen-Fugendake sur fond d’image aérienne de 1998. Le fond donne une bonne idée du niveau de destruction totale de la végétation par les matériaux volcaniques ; C : zone 2 dans la basse vallée de la Mizunashigawa montrant l’extension des étendues forestières en 1947, 1965 et 2013 (absence de surface forestière en 1998). L’arrière-plan montre l’état de la vallée en 2013 avec les barrages SABO ; D : histogrammes des superficies forestières de 1947 à 2013 pour les deux zones étudiées.
URL http://geomorphologie.revues.org/docannexe/image/11048/img-8.jpg
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Référence papier

Christopher Gomez et Patrick Wassmer, « Evolution of the Unzen Volcano and the Shimabara Peninsula (Japan) during the last 60 years: The role of the 1990-1995 eruption in modifying the landscape », Géomorphologie : relief, processus, environnement, vol. 21 – n° 3 | 2015, 205-216.

Référence électronique

Christopher Gomez et Patrick Wassmer, « Evolution of the Unzen Volcano and the Shimabara Peninsula (Japan) during the last 60 years: The role of the 1990-1995 eruption in modifying the landscape », Géomorphologie : relief, processus, environnement [En ligne], vol. 21 – n° 3 | 2015, mis en ligne le 01 janvier 2016, consulté le 20 octobre 2017. URL : http://geomorphologie.revues.org/11048 ; DOI : 10.4000/geomorphologie.11048

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Auteurs

Christopher Gomez

University of Canterbury – College of Sciences – Department of Geography – BUG: Biogeomorphometry from Uav to Geocomputing Laboratory, Private Bag 4800 – Christchurch 8140 – New Zealand (christopher2501@gmail.com).

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Patrick Wassmer

LGP Laboratory, UMR-CNRS 8591 – Meudon, France – University of Strasbourg – Department of Geography and Land Use Planning & University of Canterbury, College of Sciences, Dept. of Geography – New Zealand (wassmerpat@aol.com).

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