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Environmental changes and vulnerability in the Gharbi Island (Kerkennah, Tunisia)
Lucile Etienne, Riadh Bouaziz, Salem Dahech, Abdelkarim Daoud, Gérard Beltrando
To cite this version:
Lucile Etienne, Riadh Bouaziz, Salem Dahech, Abdelkarim Daoud, Gérard Beltrando. Environmental changes and vulnerability in the Gharbi Island (Kerkennah, Tunisia). EGU General Assembly 2012, Apr 2012, Vienna, Austria. �hal-01108654�
- 9 aerial photographs from 1963. Scale 1:25000
- 1 Spot 5 image from 2010. Scale 1:25000. Level of
treatment: 3 (orthorectified).
Identification of
correspon-dance points between both
images: buildings, roads and
intersections and tree
pat-terns.
Verification of image
ca-libration. Improvement
with integration of new
points when necessary.
Cutting of distorted borders and
images mosaicking.
In cliff areas, the coastline
is easily detectable using
the boundary between
wetland and upland.
Mo-zaicking and digitazing
result in a 5m margin
error.
The limits of sebkhas are
defined by the limits of
the trees and the
coast-line. In 2010, this limit is
disturbed by the creation
of agricultural parcels in
the sebkhas.
Environmental changes and vulnerability in the Gharbi Island (Kerkennah, Tunisia)
Calibration of images with an average of 130 points by
images and a RMS error of 1.28.
L. Etienne (1), R. Bouaziz (2), S. Dahech (3), A. Daoud (4), and G. Beltrando (1)
(1) Université Diderot Sorbonne Paris Cité, UMR PRODIG du CNRS, (c.c. 7001) 75205 Paris France (lucileetienne1985@gmail.com ; beltrando@paris7.jussieu.fr),
(2) Laboratoire de Cartographie Géomorphologique des Milieux des Environnements et des Dynamiques (CGMEG), Faculté des Sciences Humaines et Sociales de Tunis, riadhbouaziz@yahoo.fr (3) Université de Sfax, Faculté des lettres et Sciences Humaines, Département de géographie. Laboratoire SYFACTE Sfax 3000. Tunisie. salem.dahech@gmail.com
(4) Université de Sfax, Faculté des lettres et Sciences Humaines, Département de géographie. Laboratoire Eau, Energie, Environnement .B.P.553 Sfax 3000. Tunisie. daoudabdelkarim@yahoo.fr
A
B
125 120 115 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 4 2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 -24 -26A
B
Ev olution in met ers 10 20 30 40 50 60 70 80 90 100 110A
B
0 500 1 000Meters 120 Agricultural parcels Bare soil Mediterranean Sea Habitat In 1963 In 2010 Ways Baseline TransectsGeological sections A and B
Shoreline DSAS analysis Landuse Erosion Progradation Graphic N EGU 2012 Vienna
1. Beige silty clay deposit rich in marine malacofauna and containing rare fragments of calcareous crust. Shaped into not-ches at the base (1a), which is sometimes 1 to 2 meters depth and favoring collapses and coast retreat.
2. Coarse and consolidated material, very rich in marine malaco-fauna and containing fragments of reworked limestone and sandstone crust. It experiences a severe erosion as evidenced by the numerous collapsed blocks (1b).
1. Mio-Pliocene Gypsiferous clays forming the bedrock of the ar-chipelago. Rapid retreat due to mechanical and chemical action of seawater. Manifestation of erosion: deep notch of 1 to 2 metres depth (1a) and rockfall (1b).
2. Sandy-silty deposits rich in calcareous nodules capped by a calcareous crust of Lower Pleistocene age. This crust often cracks in relation to infiltration of stormwater. It will be quickly blocked by contact with Mio-Pliocene clays. The formed aquifer was ex-ploited since antiquity by wells (3).
4. Reddish silty colluvium and containing pebbles and potsherds.
The comparision of the two dates shows a global retreat of the shoreline on the cliff area. Erosion areas are predominant and display important values (maximum -26m ±5 m). Very few progradations areas are observed, and their values are all inferior to the error margin (±5 m). These are likely due to the confusion between
the inland and some rockfall and cannot be considered as a progress of the coastline.
The erosion appears more or less rapid depending on the height and the geological composition of the cliffs (see the geological sections A and B).
A
B
Road Agricultural parcels Other fields In 1963 In 2010 Habitat WaysSebkha Marsett Essendouk Landuse 0 625 1 250 2 500Meters
A
B
N 0 625 1 250 2 500Meters NStep 1:
Search for common points
and georeferencing
Step 2:
Superposition and
quality verification
Step 3:
Mosaicking
N2010
1963
Step 4:
Digitizing of studied elements
Data and methods
Data
The Marsett Essendouk sebkha was 177,65 ha large in 1963 and 270,06 ha large in 2010. This represents an increase of 92,41 ha (+52%) between both dates.
The development of the Marsett Essendouk sebkha takes a northward direction and is very important to the north of the main road. We believe that the extension of the sebkha is led by the micro-topography visible in the field. This results in an increase in soil salinity and eventually the death of palm trees.
Visible gradient of tree health from the heart of the Sebkha (A, south-west) to the outside (B, North-east).
In 2010, we observed that some recent agricultural parcels are located on the edge of the sebkha Marsett Essendouk.
These parcels, located outside the irrigation areas are intended to tree crops (olive, almond...). They are often plowed in the slope direction, which promo-tes a faster expansion of salini-zation.
Mediterranean
Sea Mediterranean Sea
Context
Gharbi is one of the two biggest islands in the Kerkennah
archi-pelago located in the Gabes Gulf (Tunisia). It shows several
speci-ficities including:
- Very low topography (13 meters max)
- Numerous sebkhas areas that represent 39.2% of the
total surface of the island (i.e. 1884 ha)
- Loose and erodable materials
- Semi-arid climate
- Numerous agricultural parcels
(especially on the borders of the sebkhas)
Since the 1950’s, climate change in the Mediterranean Sea has
been highlighted by several observations :
- Changes in the variability of precipitations
- Sea level rise (absolute and relative, with evidences of
subsidence since the end of antiquity)
- Increase of temperatures and relativ evapotranspiration
especially in summer
- Changes in the direction of the strongest winds
The fragility of the Gharbi Island therefore appears through several clues including marine erosion and
soil salinization. It is due to multiple factors of geologic, geomorphologic, climatic and anthropic nature.
In this context, we attempt to quantify the evolution of a cliff portion and the surface of the Marsett
Es-sendouk sebkha between 1963 and 2010, using a photo-interpretation approach.
Studied areas
Toponymy Spaces other than sebkha
Sebkah areas Urban areas
Sebkha evolution
Shoreline evolution
Conclusion
The Gharbi Island is vulnerable, especially regarding the extension of the sebkha and the retreat of the shoreline. Studied areas show
an important erosion of the shoreline (with a maximum of -26 m ± 5 m) and a major extension of the sebkha Marsett Essendouk (+52%
in 47 years).
These two phenomena are related to both climate change and men who move into areas that are already vulnerable or will be in the
near future.
The extension of the sebkhas and the sea advancing in the land involves a reduction of arable land on the island and thus increases
vul-nerability to climate change and farming operations.
Oueslati, A. Les côtes de la Tunisie géomorphologie et environnement et aptitude à l'aménagement, Université de Tunis, 1993, 387p. Rocchini, D. & Di Rita, A. Relief effects on aerial photos geometric correction Applied Geography, 2005, 25, 159-168
