Understanding the coastline evolution in the Kerkennah archipelago

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Understanding the coastline evolution in the Kerkennah archipelago

Lucile Etienne, Abdelkarim Daoud, Gérard Beltrando

To cite this version:

Lucile Etienne, Abdelkarim Daoud, Gérard Beltrando. Understanding the coastline evolution in the Kerkennah archipelago. COCORISCO, Jul 2014, Brest, France. �hal-01108632�

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Understanding the coastline evolution in the Kerkennah archipelago

L. Etienne (1), A. Daoud (2), 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) 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)

Cocorisco, Brest, 2014

Supported by:

Conclusion

- The rapid alternation between protected and unprotected areas generates (i) the perturbation of

the coastal dynamics and (ii) the acceleration of the erosion.

- The evidences are obvious and recent and may be (i) geomorphological, (ii) archelological, (iii)

bo-tanical and (iii) due to the destruction of recent buildings espacially the individuals protections.

- For the moment and despite some efforts, there is no global and integrated management to

coun-teract the erosion of the coast the small islands of the Kerkennah archipelago

Bibliography

- Kebaïli Tarchouna, M. 2013. L'archipel de Kerkena: organisation de l'espace et aménagement. Thèse de l'Université de Tunis. Faculté des Sciences Humaines et Sociales de Tunis. Ecole Doctorale : Structures, Systèmes et Modèles. pp: 301.

- Oueslati, A. 2004. Littoral et aménagement en Tunisie. Publication de la Fa-culté des Sciences Humaines et Sociales, Tunis, pp: 526.

- Thieler, E. R., Himmelstoss, E. A., Zichichi, J. L. et Ergul, A. 2009. Digital Shoreline Analysis System (DSAS) version 4.0 (updated for version 4.3.): An ArcGIS extension for calculating shoreline change. U.S. Geological Survey Open-File Report 2008 - 1278 pp: 79.

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18 "Grand Hôtel"

"Cercina" hotel

500m

0 L. E

TIENNE

N

Evolution in meters

30

20

10

0 _-10

_-20

_-30

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Baseline Cross sections (every 200 m) DSAS Agricultural parcels Buildings Way Road Land use Erosion Progradation

Graphic (space between each stick = 10 m) Error margin Coastline In 1963 In 2010 Longshore drift

Roman period remains Recent rockfill

Littoral dynamic

Dyke and accumulation of sand

Results

A

Evolution of the coast (in %)

+

-41

55

57

46

42

59

45

40

50

52

3 ₅

₆

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Gharbi South

Sidi Fonkhal Gharbi North Chergui South

Sidi Frej

% Stability

% Erosion

% Advance

Few or no human

infrastructures

Many human

infrastructures

- No advance measured in the areas free of human

infrastruc-tures

- Erosion affects more the coastline south of the archipelago

- In the areas where the human intervention is important, the

erosion seems to be more important and stability less important

The coasts of the archipelago are susceptible to

erosion (or stability). Human intervention

desta-bilize the natural dynamics of the coastline.

Example of human perturbation: The touristic area of Sidi Frej

A

13 B

B

C

D

0 50 100 200m 1963 2010 0 50 100 200m

A

_{Longshore drift}

pertur-bation with the

touris-tic development of the

beach

May 2011 L. ETIENNE; coastline erosion and corrosion

M ay 2012 L. E TIENNE No vember 2011 L. E TIENNE

D

_{Unprotected or low protected area = High erosion rates or very obvious actual}

erosion evidences : Archeological, geomorphological...

C

_{Recent rockfill sometimes built by moving}

forward the coastline (artificial

prograda-tion) = strengthening of the perturbation

March 2010 L. ETIENNE; Front of the Cercina hotel

December 2012 L. ETIENNE; individual wall made on the beach

B

Erosion more important =

individual protections =

strengthening

of

the

pertubation

May 2011 L. ETIENNE; Wall destroyed by erosion sea

Data and methods

Data used

- 59 aerial photographs

from 1963 combined in a

mosaïc. Scale 1:25000

- 1 Spot 5 image from 2010.

Scale 1:25000. Level of

treat-ment: 3 (orthorectified)

DSAS analysis

2010

1963

Data

- Shoreline identification at both

dates by using the top of cliffs

and vegetation limits

- Shoreline evolution rates

identi-fied in the areas where the error

margin is lowest than ±6m:

iden-tification of the 6 studied areas

0 250 m

N

DSAS 4.3 (Digital Shoreline

Ana-lysis System) anaAna-lysis:

- Transect every 10 meters

- Net Shoreline Movment

- Identification of 6 studied areas

Shoreline detection

+

Field observations

and verifications

2010 - 2013

0 -4 -8 -12 -16 -20 -24 E v olution in met ers 0 4 km N N 0 4 km N

G o l f e d e

G a b è s

R a m l a Î l e C h e r g u i Î l e G h a r b i 0-2.5 2.5-5 5-7.5 12.5-13 7.5-10 10-12.5 A l t i t u d e s ( i n m e t e r s ) N a m e s L o c a l i t y Studied areas V e r y s m a l l i s l a n d s I s l a n d s Fi e l d s m a r ke r s B u i l d i n g s R o a d s 0 2 4 Km G r e m d i L a z d a d R o u m a d i a R e m a d i a S e f n o u M e l l i t a G harb i Sou th Sid i Fon kh al G harb i N orth Ch e rgu i Sou th Sid i Fr ej

Kerkennah archipelago:

Local context:

- 10 small islands

- Low altitude: max 13 m

- Different type of coasts

and numerous marks of

erosion

- Lithology mostly

sensi-tive to erosion

- Urban development

Regional context:

- Sea level rise due to

cli-mate change and

subsi-dence of the north of the

Gabes gulf

The consequences of the fragility of the Kerkennah archipelago and of the changes in

re-gional and local scales are (i) the coastline erosion and (ii) the increase of the coastal

vulne-rability.

To better understand that vulnerability we need to measure the evolution of the coastline

position and to identify the factors that influence this evolution.

The aim of this presentation is to understand the human

in-fluence on the erosion rates