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Poster De Conférence Année : 2014

Understanding the coastline evolution in the Kerkennah archipelago

Résumé

The Kerkennah archipelago in Tunisia is vulnerable to sea erosion and different marks of coastline retreat indicate that this process is currently active. Our objective is to quantify coastal erosion and to discuss the management of risks along the coastline. For that purpose, we used both photointerpretation and field approaches, and integrated all the collected information into a GIS. The quantification of coastline evolution was done using the Digital Shoreline Analysis System (DSAS) module in the ArcGis 10 software. We focused on 2 dates for which comparable data with a spatial resolution of 2.5m were available: 1963 with the use of 33 aerial photographs and 2010 with the use of a SPOT 5 panchromatic image. Aerial photographs have been georeferenced on the SPOT 5 image, and the coastline has been determined for the two dates following the cliff tops or the limit of the vegetation when there was no cliff. The 6 studied areas correspond to the coasts where the error margin is of 6 meters (minimal error). The comparison of coastline between 1963 and 2010 shows that for a large majority of the studied areas, the coastline is either stable or retreating. The stable coasts concern 46.9% of the studied areas when eroded coasts concern 50.4%. Erosion reaches 42m ± 6m in the touristic area of SidiFrej (or 0,9m/yr in 47yrs) and depends on different factors of vulnerability including lithology, sea level rise and anthropic activities. On the opposite, on some very limited areas, the coastline has advanced between the two dates (until 34m ± 6m) and those advancing patterns are strongly correlated to human infrastructures. The management of the coastline is of 3 types: (i) no intervention; (ii) intervention by the state or (iii) intervention by the local population, more specifically individual actions taken by residents of the coastline. In the areas where the coastline is free of human infrastructures, the coast is either eroded or stable, but never in progradation. In the area where the coastline is protected the evolution rates alternate between progradation, stability and erosion. The states infrastructures are localised in the different harbours, along the coast of Ramla (the biggest locality of Kerkennah) and along a part of the SidiFrej touristic area. They are continuous along the coasts and consist in majority of rockfill (along the coast or dikes) to protect the coasts from erosion, and in minority of embankments to raise the roads along the coasts or to gain land surface especially on the back of certain rockfill. These infrastructures are recent and often correlated with either a stagnation of the coastline or an artificial progradation of the land. They thus appear to be effective, although only at a very local scale. Besides, unprotected areas adjacent to rockfill infrastructures show signs of recent erosion, which might indicate the existence of negative side effects. The areas protected locally by individual actions are not continuous and constitute small fractions of the coastline. The rapid alternation between protected and unprotected areas show that for the moment and despite some efforts, there is no global and integrated management to counteract the erosion of the coast the small islands of the Kerkennah archipelago.

Domaines

Géographie
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Commentaire : Poster Présenté lors du colloque COCORISCO à Brest, 2014

Dates et versions

hal-01108632 , version 1 (23-01-2015)

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  • HAL Id : hal-01108632 , version 1

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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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