# "Reading landscape" in a Mediterranean peri-alpine catchment basin

Christian Depraetere, Benoît Deffontaines, Jiun-Chuan Lin, Jean-Simon Pagès

► To cite this version:

Christian Depraetere, Benoît Deffontaines, Jiun-Chuan Lin, Jean-Simon Pagès. "Reading landscape" in a Mediterranean peri-alpine catchment basin. 2025 International Natural Conservation Conference, Penghu islands, Taiwan, June 8-10 2025, National Taiwan University, Taiwanese Geoparks Association, Jun 2025, Penghu islands, Taiwan. hal-05169749

HAL Id: hal-05169749

https://hal.science/hal-05169749v1

Submitted on 18 Jul 2025

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Hydrogeomorphometry from of dense Lidar/DEM:

« Reading landscape » 讀景 in a Mediterranean peri-alpine catchment basin
(Vallée des Duyes, Geopark UNESCO of Haute Provence, Alpes de Haute Provence, France)

Christian Depraetere¹, Jiun-Chuan Lin², Benoît Deffontaines³, Jean-Simon Pagès⁴

(1) Institut de Recherche pour le Développement, Maison de la Télédétection, 38000, Montpellier, France*

(2) Geography Department, National Taiwan University, Taipei, Taiwan

(3) Université Gustave Eiffel - ENSG, Av. Blaise Pascal, 77420 Champs-sur-Marne, France

(4) Géoprc UNESCO de Haute Provence, Digne-les-Bains, France
# Summary

Part 1: Adaptative resampling from 1 to 10 metres (Why?)

Part 2: Hydrogeomorphology: simulation of sub surface percolation (How?)

Part 3: Patterns of landform and hydrological processes (What?)

## Conclusions

"The waters subjected to follow at first the relief of an uneven ground destroyed little has little, the irregularities of the slope... Here, they gnawed; there they exhaussed... The result of all these actions was to create a new curve, which is better suited than the primitive profile of the ground to the flow of water." (Surell, 1841)
Few words on « Reading landscape » 讀景

As part of their formation and training, high school pupils and students must be able of “reading landscape” during field works apart of other methods (remote sensing, geographical information system, ...).

Observations “in the ground” of geomorphological features reshaping the geological basement offers many tangible insights on past and present processes. It contributes to understand the entanglement of landform formations at various spatial and temporal time scales.

<figure><img src="image_1.png" /></figure>

(Lin, 2022)

<figure><img src="image_2.png" /><figcaption>Fig. 7. A simplified model of detachment of rock mass.</figcaption></figure>
# Geoparks as open air museums of landform

The practice of field works reconnects environmental sciences to a steady observations of nature at large, not "piecewise" through the screen.

As such, the geoparks must be consider as unique scientific platforms to bind together naturalist knowledge shared with local people combined with high tech methods.

<figure><img src="image_3.png" /></figure>
# A glimpse on the UNESCO Geopark of Haute Provence

## HISTORICAL BACKGROUD

The Haute-Provence national geological reserve was created in 1984 in the Southern French Alpes and became the prototype of the future concept of geoparc.

In 1991, the Geological Reserve organised in Digne-les-Bains, with the help and support of UNESCO, the first international symposium on the protection of geological heritage. At the end, they Declaration of the Rights of the Memory of the Earth was adopted, also known as the "Déclaration de Digne".

In 2000, UNESCO recognized the world's very first Geopark: the Haute-Provence Geopark, a unique territory located in the southern Alpes, covering 2000 km², 67 communes and protecting 18 sites in collaboration with many Geopark's partners.

<figure><img src="image_4.png" /></figure>
# Part 1:
Hydroclimatic events
in Provence.
# Hydroclimatic events in Provence during the XVIIIᵉ century

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

The event of May 1774 in the Duyes valley (Pichard et Roucaute, 2014)

<footnote>* Source Pichard, G., & E. Roucaute 2014. Sept siècles d'histoire hydroclimatique du Rhône d'Orange à la mer (1300-2000). Climat, crues, inondations. Revue Méditerranée, HS, PUP, 194 p.</footnote>
The event of April 1774 in the Duyes valley

Local archives record a major hydroclimatic event associated with **ten days of heavy rainfall** that affected the valley in April 1774, with a **landslide on a massive scale** and several damage events: sinking of arable land, catastrophic flooding, destruction of buildings and roads.

The archives describe the events in the terms of the time: “continuous rains”, “abundance of water from the torrents”, “revolution of the earth”, “openings and crevasses”, and so on. The documents also note that these “strange and inconceivable revolutions” were **concomitantly associated with two seismic tremors**, without specifying whether this was a cause or a consequence of the detachment of several million tons of rocky masses and mud.

The archives insist on the extensive damages caused by **flash floods in the torrential basins**, in the village of Thoard by the Riou torrent and downstream in the Duyes valley.

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

<footnote>* Christian Depraetere, Benoît Deffontaines, Jiun-Chuan Lin, Jean-Simon Pagès, (2025). Évènements hydroclimatiques passés et actuels dans le bassin versant de la vallée des Duyes (Alpes de Haute-Provence). Revue Open-access Méditerranée, n° spécial Hommages à l'historien Georges Pichard, juin 2025.</footnote>
# The geologic context of the April 1774 landslide

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

1. Overthrust front

2. Faults and gypsum slip plane

3. Gypsum layer

4. Shifting tectonic "navette"

5. Molassic conglomeratic basement

6. Composite superficial landslides

7. Quaternary and Holocene alluviums

Synthetic block diagram associating geology and tectonics illustrating the overlapping units at the summit Siron (1) (nappe de la Robine); the tectonic shuttles including the Chapelle Ste Madeleine (4) located in the fault zone (2) of the Alpine overlapping deformation front; the gypsum in yellow (3) located in the brecciated fault zone; and the autochthonous Valensole conglomeratic molasse (5) overlapped and folded into the valley. A series of overlapping pellicular landslides (6) is also shown, whose numerous reactivations will push the bed of the Duyes river towards the right bank, resulting in a westward avulsion.
Part 2:

The landslide of April 1774
in the Rousset and Vaisselet catchments
The landslide of April 1774

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

The hydroclimatic event of April 1774 was a complex combination of several landslides, rockfalls and mudflows staggered from the glacis at the foot of the Siron scrap downto the bed of the main Duyes river.
The landslide of April 1774

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

The collapse of the Pas du Lièvre navette in April 1774 (1) is associated with two slight earth tremors. The rockslide associated with gypsum material led to the formation of a lens shape sliding (2) and several diverging percolations downhill [3].
The landslide of April 1774

<figure><img src="image_10.png" /></figure>

The collapse of about **3 millions m³** of the Pas du Lièvre in April 1774.
Composite material of marly limestones and gypsum
Field observation: upper part

<figure><img src="image_11.png" /><figcaption>Marly limestone outcrop and weathering deposits at the Pas du Lièvre</figcaption></figure>
Field observation: mid-slope

<figure><img src="image_12.png" /></figure>

Rotational sliding at the head of a gully
Field observation: downstream part

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

Colluvial deposit of sand and silt in the downstream section
Field observation: downstream part

<figure><img src="image_14.png" /><figcaption>Left: Toe of a debris flow</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Right : one of the largest tree of the valley (Salix alba*). this tree suggests the presence of a constant inferoflux in the alluvial deposits of the creek</figcaption></figure>

<footnote>* white willow</footnote>
Hydrogeomorphology

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

Sub superficial hydrography of Vaisselet and Riou the catchment bassins
Hydrogeomorphology

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>Comparison of the river long profile (elevation against downstream distance) of the Vaisselet and Riou basin (computed from Lidar DEM)</figcaption></figure>
Did the bassin du Vaisselet exist before 1774?

<figure><img src="image_19.png" /><figcaption>“Cassini” map about 1750</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>IGN map 2020</figcaption></figure>

On the old Cassini map, the river of the Vaisselet is not indicated!?
Part 3:
The flashfloods
of the Riou catchment
in April 1774... and now
The damages due to flooding in the Riou torrent
and downstream in the Duyes Valley

The archives mention also flooding and landslides along the main bed of the Duyes downstream of Le Rousset and the confluence with the Riou noticeably in the Bourres and Bannon districts. The “Chemin du Roi” (the King’s Road) was rendered unusable despite it was on the right bank of the Duyes which means that the valley was cut off from the rest of the region.

Records also show that the walls of the village of Thoard partially collapsed as a result of undercutting of the river banks. Further downstream in the Duyes river bed, several mills were buried under rocks and sediment, rendering them unusable.

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

Archives départementales des Alpes de Haute Provence

Dossier E DEP 2H/DD2, recto du feuillet E209/47

Bourdesant Noutz jumjois thomis Batt hazard de
Gaudrions avoiaren la four, dure Paul Rousset marchand
et Joseph Bine Arnouz aubergiste jumis Mains et
consule de la commanacte de fette ville de Diguez, chefs
The Riou torrent today

<figure><img src="image_22.png" /><figcaption>The arch of the Thoard bridge calibrated to evacuate the floods of the Riou torrent</figcaption></figure>
# The Riou torrent today. What if...

There are no rainfall or hydrographic monitoring at the moment.

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

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

Nevertheless, a plausible hypothesis based on Instantaneous (A) and Geomorphological Unit hydrograph with a rainfall scenario (B) gives some hints on possible flash flood. For a rainfall event of 200 mm in hour (P) with 25% of surface runoff (Pe), the time to peak (Tp) would be of about 1 hour with a peak discharge (Qmax) of 15 m³/s.
The Riou torrent today

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

The large alluvial fan down stream from the bridge of Thoard

<footnote>* Computed from the RGEalti Lidar DEM at 1 meter resolution of the Institut Géographique National (download from https://geoservices.ign.fr/rgealti).</footnote>
The Riou torrent today

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

A flash flood could damage many buildings and households

<footnote>* Computed from the RGEalti Lidar DEM at 1 meter resolution of the Institut Géographique National (download from https://geoservices.ign.fr/rgealti).</footnote>
Conclusions:

Within the framework of French UNESCO geoparks, it is possible to envisage thematic stays of several days combining scientific hikes in the morning and an introduction to the use of geographic information systems in the afternoon.
Thank you

<figure><img src="image_27.png" /><figcaption>Hydrogeomorphological field trip in the Duyes valley
with a delegation from the National Taiwan University (18th June 2023)</figcaption></figure>
Dynamic equilibrium between landforms
and hydrological processes:
a quotation from 1841

"The waters subjected to follow at first the relief of an uneven ground destroyed little has little, the irregularities of the slope... Here, they gnawed; there they exhaussed... The result of all these actions was to create a new curve, which is better suited than the primitive profile of the ground to the flow of water." (Surell, 1841)
# Appendices
**References:**

Beven, K. J. and Kirkby, M. J. (1979): "A physically-based variable contributing area model of basin hydrology", Hydrol. Sci. Bull., 24, 43-69, 1979.

Beven, K., and Germann P. (1982): "Macropores and water flow in soils". Water Resour. Res., Volume 18, Issue 5, pages 1311-1325, October 1982.

Chorowicz J., Ichoku C., Riazanoff S., Youn-Jong Kim, Cervelle B., (1992). "A Combined Algorithm for Automated Drainage Network Extraction" in Water Resources Research, vol.28, n°5, pp.1293-1302. American Geophysical Union.

Depraetere C. and Riazanoff S., (2004): « The new Digital Elevation Model data set from the Shuttle Radar Topography Mission : Hydrogeomorphological applications in the Ohrid region (Albania, Greece and Macedonia) ». Conference on Water Observation and Information System for the Balkans Countries BALWOIS, Ohrid, République de Macédoine, 25-29 Mai 2004.

Depraetere, C., (2013). "Introduction à l'hydrogéomorphométrie : études des relations entre le modelé des formes de terrain et les processus hydrologiques dans des contextes insulaires". Document inédit d'Habilitation à Diriger des Recherches (HDR), 8 novembre 2013 à l'Université Paris-Diderot VII, 249 pages.

Christian Depraetere, C., Juin-Chuan Lin, Benoît Deffontaines, Jean-Simon Pagès (2023). Processus hydrogéomorphologiques passés et actuels dans le bassin versant de la vallée des Duyes (Alpes de Haute Provence). Atelier en hommage à Georges Pichard, Aix-en-Provence, 21-22 novembre 2023. https://hal.science/hal-04431637v1/file/PPT_Atelier_Aix-en-Provence_20_11_2023.pdf

Descroix L. (1998). "L'érosion hydrique dans les terrains peu résistants des Préalpes françaises du sud : mesures et variables explicatives sur parcelles et bassins-versants". In : Recherches dans les Alpes françaises du sud : bocages du Champsaur, cartographie des alpages, érosion et mouvements de terrain; tourisme en haute montagne. Revue de Géographie Alpine, (1), p. 43-58.

Dupéret A., Deffontaines B., Passalacqua O., (2003): « Critères géomorphométriques issus des modèles numériques de terrain au service des applications hydrologiques ». Bulletin de la Société française de photogrammétrie et de télédétection 2003, n°172, pp. 107-121.

Fairfield J, Leymarie P. (1991). "Drainage networks from grid digital elevation models". Water Resources Research 27: 709-717. CrossRefWeb of Science.

Merot P, Ezzahar B, Walter C, Aurousseau P. (1995). "Mapping water logging of soils using digital terrain models". Hydrol Process 1995;9:27-34.

**Websites:**

Downloading of RGEAltı DEM (Institut Géographique National, IGN): https://geoservices.ign.fr/rgealti

Information on the EEA-10 Copernicus DEM (in fact DSM): https://spacedata.copernicus.eu/collections/copernicus-digital-elevation-model

HDR Christian Depraetere (2013) :
Hydrogéomorphométrie. Habilitation à Diriger des Recherches, Université René Diderot, novembre 2013 (En français).

Partie 1, Historique : https://www.espace-dev.fr/wp-content/uploads/2022/03/HDR_01_Hydroge%CC%81omorphome%CC%81trie_Partie-1-Historique.pdf

Partie 2, Méthodes : https://www.espace-dev.fr/wp-content/uploads/2022/03/HDR_02_Hydroge%CC%81omorphome%CC%81trie_Partie-2-Methodes.pdf

Partie 3, Applications : https://www.espace-dev.fr/wp-content/uploads/2022/03/HDR_03_Hydroge%CC%81omorphome%CC%81trie_Partie-3-Applications.pdf