Stability state of a Mediterranean coastal environment threatened by erosion: the case of the Djerba Island (Gulf of Gabes, SE-Tunisia)

doi:10.21203/rs.3.rs-2437129/v1

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Stability state of a Mediterranean coastal environment threatened by erosion: the case of the Djerba Island (Gulf of Gabes, SE-Tunisia)

https://doi.org/10.21203/rs.3.rs-2437129/v1

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The Djerba Island (Gulf of Gabes, SE-Tunisia) that is part of the South Rive of the Mediterranean is classified lately as an international heritage site. Despite this, it suffers from erosion. So, it's necessary to study and monitor its stability state (erosion/accretion) to predict future erosion.

In the present work, the stability state was determined by studying the coastline evolution using photographs (1991) and satellite images treated by Remote Sensing and GIS tools. Extracted coastlines (schematized by polylines or extracted by the calculation of MNDWI index) have been superimposed. Their evolution (distances and rates) was calculated automatically using the Arc GIS Digital Shoreline Analysis System extension. Results show accelerated erosion over the last period 2016–2020 for all cells on the Island.

The evolution over a period of 45 years, from 1975 to 2020, shows a coastline retreat along the entire Island (3 to 10 m/year), with the exception of the sandy spits of Ras Rmal and Borj Kastel (accretion due to the contributions of sediments by coastal drift currents). The sea level rise and the various hydrodynamic and anthropogenic forcings have led to this state instability and this generalized erosion.

Stability state

Erosion

Island

Sea Level Rise

Coastline

Mediterranean

Tunisia

Gulf of Gabes

Djerba Island

An Island is any segment of a continental landmass characterized by isolation and remoteness (Hay, 2006). Many Islands in the world are considered as attractive places to visit and carry out field work (Michael Hall, 2012). Unfortunately, these coastal environments are threatened by both natural disaster and anthropogenic actions leading essentially to pollution and erosion (Cramer et al., 2018). Coastal erosion is a serious problem with long-term economic and social consequences (Asbury H. Sallenger Jr et al., 1987).

It's the subject of many studies from all over the world (Swift, 1968; Paskoff, 1994; Viles and Spencer, 1995; Gonzalez and Medina, 2001; Anfuso et al., 2011; Pappone et al., 2012; Saïdi et al., 2014; Ahmed et al., 2021...). Many coastlines in Tunisia (the southern shore of the Mediterranean) are affected by erosion (Oueslati, 1994; Amrouni et Abdeljaoued, 2009; Saïdi, 2013; Saïdi et Zargouni, 2019...). But, they still remain important areas for leisure, tourist and economic activities and natural heritage to be preserved.

The Djerba Island (Gulf of Gabès, SE-Tunisia) is listed, since natural 2021, by UNESCO as an international heritage site (Bouabid, 2021). So, it's necessary to study and monitor its state of stability and deduct the principal involved factors (and anthropogenic actions and sea level rise) in order to predict future erosion. The work was carried out within the framework of the Co-Evolve4BG project “Coevolution of coastal human activities and Mediterranean natural systems for sustainable tourism and blue growth in the Mediterranean”. The project is funded by the “ENI CBC Med Programme”. It is piloted by the National Institute of Science and Technology of the Sea (INSTM) in collaboration with national and international partners especially the Faculty of Sciences of Tunis.

The theme has been the subject of several research works such as that of Oueslati (1986) who worked on the geomorphology of a few sectors of the Djerba Island. The stability state of three sectors of the Island (Ajim, Ras Rmal and Borj Kastel), by analyzing the evolution of the coastline, was studied by Bouchahma and Yan in 2012. The detailed study of the stability state of all the coastlines of this international heritage was therefore very necessary. It's the subject of the present work by resorting to an automatic calculation of the evolution of the whole coastlines of the Djerba Island using the Arc-GIS Digital Shoreline Analysis System extension proposed by Thieler and al. (2003).

The Djerba Island belongs to the Gulf of Gabes. It's situated in the South-East of Tunisia (Fig. 1) which is located in the Southern rive of the Mediterranean.

It is the largest in North Africa. It has the shape of a quadrilateral with an area of 514 km2. The Island is a shoal area. It is separated from the mainland by the Boughrara lagoon.

It is marked by a low, regular and monotonous topography which is very similar to the tabular relief which marks the southern coast of the Gulf of Gabès (ASSIDJE, 2001).

This relief presents series of slopes; these are the main accidents, to the west and to the south, oriented WNW-ESE and facing south (Paskoff and Sanlaville, 1983).

The topography of the Island is characterized by a succession of low plateaus and very low coastal plains with a highest point at 53 m near the Guellala water tower (Oueslati, 1995).

The surface of the plateaus is broken by a series of slope failures, the most important of which is in the southern part of the Island, the Guellela accident (APAL, 2018).

The coastal fringe is narrow and is distinguished by its flatness and very low altitudes, very often less than 5 m, or even 2 m. The shore of the Island is formed by a ribbon of beaches and a series of moving dunes (ASSIDJE, 2001) and also by rocky coasts (Fig. 2):

Quite varied beaches: the sandy beaches occupy the North-East and East part of the Island. The marshy beaches mainly occupy the southern part. The rocky coasts are found in the western, northwestern and northern parts of the Island.
Dunes, in the middle of which appear sebkhas, often occupying large spaces in coastal areas (Burollet et al., 1979).

The South-East and South-West parts of the shore are less covered with vegetation than with human occupation.

Rocky coastlines: Cliffs are rare on the Island and of varied lithology. the highest cliff barely exceeds 3 m (Oueslati, 2016). They are located around Guellala and occasionally near Rass Rougga, Lella Hadhria, Torbkhana and Sidi Salem. They are formed by clays, sandstones, sand and silts. They promote effective differential erosion (Oueslati et al. 2015). Low rocky coasts are limited to the shores of Terbella and part of the Aghir-Torbkhana coast (Oueslati et al., 2015).
Maritime marshes: They are found mainly in the southern part of the Island, especially in the shelter of the peninsulas (El Kastel peninsula) or the coastal spits (Rass Rmal, Lella Hadhdria (Oueslati et al. ., 2015).

- Geological setting

The Djerba Island forms an emerged eastern part of what remains of a vast collapsed anticline dome. It follows the Djeffara depression and is separated from it by a long fault running NW-SE (CRGM, 2003).

It is affected by a network of faults, oriented NW-SE, parallel to the major collapse fault passing through Mareth, Medenine and Ben Guerdane (Fig. 3) (Trabelsi et al., 2012). It thus presents an important tectonic activity.

The various geological formations that outcrop on the Djerba Island are of Quaternary age. According to Atoui (2009), there are continental formations, marine and coastal formations:

Continental formations include the Villafranchian which is formed at the base by a layer of red to pink silt 1 to 2 m thick which contains numerous limestone nodules multiplying towards the top of the layer to gradually pass to a slab of massive limestone several meters thick (Jédoui, 1980; Bouaziz et al, 2003 and Oueslati, 2004). It corresponds to a crust whose periphery is underlined by Helicidae shells called "Salmon Helicidae crust".
Marine and coastal formations belong to the last three eustatic cycles: the Tyrrhenian, the Neotyrrhenian and the Flandrian.

- Climate

The Djerba region has a Mediterranean type climate with a semi-arid tendency. It presents the balancing medium of the Mediterranean and Saharan air masses.

- Tide

The highest tide in the Mediterranean is in the Gulf of Gabes where the tidal range can reach 2.10 m in spring tides (ASSIDJE, 2001). Around the Djerba Island, the tidal range varies between 0.90 and 1.40 m. At Boughrara, the tidal range is 0.6 m at high tide and 0.1 m at low tide. On the lagoon side, it is 1.2 m at high tide and 0.2 m at low tide. To the east, at El Kantara, it is 0.9 m at high tide and 0.10 at low tide (SEPIA International, 1983).

The tide is semi-diurnal, with two low tides and two high tides for 24 hours (Sarbeji, 2000).

There are two types of tidal currents (Hattour and Ben Mustapha, 2013):

flood currents with a west-northwest sector and a speed equal to 0.40 m/s.
ebb currents in the East-South-East sector and with a speed equal to 0.65m/s.

- Waves

The northeast coast of the Djerba Island is marked by the predominance of waves in the northeast sector which has a significant ten-year height of 4.8 m and a period of about 9.9 s (Fig. 3). The southeast coast is subject to the action of waves from the northeast sector. However, it is exposed to a swell from the southeast sector which has a significant decadal height of 4.6 m and a period of 9.6 s (Hydrotecnica Portuguesa, 1995). The South and South-West coasts are protected by the Boughrara lagoon against the attack of swells coming from the open sea.

When arriving at the coast, the swells generate littoral drift currents in E-NW and E-SW directions (Fig. 4).

- Anthropogenic actions

The Djerba Island is known by tourism. The tourist area extends over the entire coastal strip of Ras Rmal and Aghir over about thirty kilometers. The town centers of Midoun and Houmt-Souk include 118 hotels, 20 restaurants, 41 travel agencies, a casino, a golf course, public administrations specializing in tourism (Dribek, 2012) and the international airport of Djerba -Zarzis which is considered an important tourist center and a generator of economic growth for the region (DDHST, 2010). In 2007, it was able to accommodate almost 4 million passengers (Dribek, 2012). Marinas are a degradation factor for some coastal shores (Bounouh, 2010) although recreational activities are not very developed.

The Djerba Island had about 61,000 inhabitants in 1956 (INS, 1967). From 1975, a significant evolution of the population was observed (Fig. 5). It reached 150,000 inhabitants in 2004 (SDARESE, 2010). This demographic pressure concentrated on the coast represents a permanent danger for the ecological systems (Alaoui, 1986), the quality of the sea water and the cleanliness of the beaches following the discharge of various pollutants by the hotels (Dribek, 2012).

The study of the stability of the Djerba Island was carried out by studying the spatio-temporal evolution of the coastline, over a period of 45 years, from 1975 to 2020, by aerial photographs taken in June 1991 (scale 1/20000), the Office of Topography and Cadastre (OTC) and the National Center for Cartography and Remote Sensing (CNCT) and the satellite images for the years 1975, 2001, 2016 and 2020 obtained by the site https://earthexplorer.usgs.gov (Table 1).

Table 1

Characteristics of satellite images used in this study
	Capteur	Spatial Resolution	Date	Format	Projection System
Satellite Image 1	Landsat_8 OLI_TIRS	30 m	24/08/2020	GeoTIFF	WGS 1984 UTM Zone32N
Satellite Image 2	Sentinel 2A	10 m	15/07/2016
Satellite Image 3	Landsat 7 ETM+	30 m	27/07/2001
Satellite Image 4	Landsat 1 MSS	60 m	07/07/1975

Aerial photographs were georeferenced under Arc GIS (Fig. 6) and the coastline was drawn as polylines (Fig. 7).

Coastlines were extracted from satellite images using the MNDWI index (The Modified Normalized Difference Water Index) proposed by Hanqiu (2006). The general formula of MNDWI (Fig. 8):

Mndwi = (Green – Nir) / (Green + nir)

Selected bands used in the formula are presented in the Table 2. Shorelines were detected from Landsat images after binarization to determine the intersection between land (1) and sea (0).

Table 2

Corresponding bands of satellite images for the calculation of MNDWI
	Landsat8	Sentinel 2A	Landsat 7	Landsat 1
Selected bands	B3 = Green B6 = MIR (Mid-Infrared)	B3 = Green B11 = MIR (Mid-Infrared)	B2 = Green B5 = MIR (Mid-Infrared)	B4 = Green B7 = MIR (Mid-Infrared)
Applied MNDWI formula	(B3-B6) / (B3 + B6)	(B3-B11) / (B3 + B11)	(B2-B5) / (B2 + B5)	(B4-B7) / (B4 + B7)

After calculating the MNDWI, the values were reclassified in a raster to obtain 2 classes marking the boundary between land and water (Fig. 9).

The resulting raster file was converted to polygons and then to poly-lines (Fig. 10a and b).

The coastlines were then superimposed (Fig. 11).

The diachronic evolution of the coastline using the Digital Shoreline Analysis System (DSAS) extension of ArcGIS by following 2241 profiles perpendicular to the coast and equidistant by 50 m (Fig. 12) and for each period, the measurements were made in relation to a reference line called the “baseline” (Fig. 13). Distances and rates of shoreline advance and retreat were calculated automatically in the form of statistics (Thieler et al., 2003). The statistics calculated in this work are:

NSM (Net Shoreline Movement): it is the distance of evolution of the coastline (advance and retreat (m).
EPR (End Point Rate): it is the rate of the coastline evolution in m/year.

The principle of dividing into littoral cells becomes necessary to better study the coastline evolution: littoral cells are defined as basic units and independent segments (Inman and Frautschy, 1966; Carter, 1988) bordered by natural barriers or artificial (Lowry and Carter, 1982).

The Djerba Island coastline has been subdivided into seven coastal cells:

cell 1, between Borj kastel and the Ras Terbella peninsula,
cell 2, between the Ras Terbella peninsula and the Ajim wastewater treatment plant,
cell 3, between the Ajim wastewater treatment plant and Sebkhet Mellita,
cell 4, between Sebkhet Mellita and the sandy spit of Ras Rmal,
cell 5, between the coastal spit of Ras Rmal and the spit of Lella Hadhria,
cell 6, between the coastal spit of Lella Hadhria and the port of Aghir,
cell 7, between the Aghir harbour and the coastal spit of Borj Kastel.

Coastline evolution

- Cell 1, from Borj kastel Snd Spit to Ras Terbella Peninsular (Fig. 15)

- Between 1975 and 1991

During this period, the coastline evolution shows an erosion of 3.91 to 7.67 m/year ± 3.86 m/year.

- Between 1991 and 2001

The coastline generally advanced downstream of the Borj kastel Spit. Accretion was from 1.61 to 6.09 m/year ± 1.59 m/year.

- Between 2001 and 2016

During this period, the coastline generally appears stable.

- Between 2016 and 2020

The recent coastline evolution, between 2016 and 2020, shows erosion over the entire cell, reaching − 10.68 m/year ± 10 m/year at Ras Terbella.

- Cell 2, b between Ras Terbella Peninsular and Ajim wastewater treatment plant (Fig. 16)

- Between 1975 and 1991

During this period, the area experienced general erosion. The coastline has receded by 10.67 m/year ± 3.86 m/year.

- Between 1991 and 2001

The coastline has receded by 7.88 m/year ± 1.59 m/year almost all along the cell with an increase of 1.61 to 4.66 m/year ± 1.59 m/year very close to the Ajim wastewater treatment plant.

- Between 2001 and 2016

The coastline appears stable along the hole coastline.

- Between 2016 and 2020

During this period, the coastline receded from Ras Terbella towards the Ajim wastewater treatment plant.

Cell 3, between Ajim wastewater treatment plant and Sebkhet Mellita (Fig. 17)
Between 1975 and 1991

The area suffered from erosion of 3.87 to 10.79 m/year ± 3.86 m/year located mainly between the port of Ajim and Sebkhet Mellita.

- Between 1991 and 2001

During this period, erosion is detected all along the cell with a rate of 1.61 to 9.66 m/year ± 1.59 m/year.

- Between 2001 and 2016

The coastline remained stable during this period.

- Between 2016 and 2020

This period is characterized by weight loss.

The coastline has receded all along the study cell. Erosion was 10.04 m/year ± 10 m/year between Sebkhet Mellita and the port of Houmet Essouk and 10.06 to 16.02 m/year ± 10 m/year between this port and the Ras Rmal spit.

Cell 4, between Sebkhet Mellita and the Ras Rmal Sand Spit (Fig. 18)
Between 1975 and 1991

This period was marked, generally, by an accretion of 3.89 to 7.16 m/year ± 3.86 m/year.

- Between 1991 and 2001

The coastline evolution shows an accretion of 1.84 to 2.6 m / year ± 1.59 m / year on both sides of the port of Houmt Essouk with an erosion of 1.6 to 10.51 m /year ± 1.59 m/year between this port and Sebkhet Mellita. Erosion was also detected going from the port towards the Ras Rmal spit. The coastline has receded from 2 to 32.66 m/year ± 1.59 m/year.

- Between 2001 and 2016

This period was marked by stability with accretion in a few places. The advance of the coastline was significant near the Ras Rmal spit where it reached 15.69 m/year ± 2.66 m/year.

- Between 2016 and 2020

The coastline has receded all along the study cell. Erosion was 10.06 to 16.02 m/year ± 10 m/year, between the port of Houmet Essouk and the Ras Rmal spit.

Cell 5, between the Ras Rmal Sand Spit and the Lella Hadhria spit (Fig. 19)
Between 1975 and 1991

This cell was touched by significant accretion along the Ras Rmal spit. The coastline advance was from 4.04 to 74.88 m/year ± 3.86 m/year. Erosion is however detected in the direction of the arrow of Lella Hadhria. The coastline retreat was from 3.88 to 4.68 m/year ± 3.86 m/year.

- Between 1991 and 2001

During this period, the coastline receded all along the Ras Rmal spit and the surrounding coast. Erosion was 2.14 to 29.04 m/year ± 1.59 m/year. The coastline has advanced towards the spit of Lella Hadhria via Midoun. The fattening was from 1.63 to 7.91 m/year ± 1.59 m/year.

- Between 2001 and 2016

During this period, the coastline advanced from 2.76 to 20.99 m/year ± 2.66 m/year all along the sandy spit of Ras Rmal to retreat on the rest of the coastal line towards the spit of Lella Hadhria. Erosion was of 2.67 to 5.04 m/year ± 2.66 m/year.

- Between 2016 and 2020

The coastline has receded from 15.53 to 119 m/year ± 10 m/year all along the Ras Rmal spit to be stable heading towards the Lella Hadhria spit.

- Cell 6, between the Lella Hadhria spit and the Aghir harbour (Fig. 20)

- Between 1975 and 1991

The coastline has receded from 3.96 to 4.99 m/year ± 3.86 m/year downstream of the Lella Hadhria spit and from 3.94 to 5.36 m/year ± 3.86 m/year up to the upstream port Aghir.

- Between 1991 and 2001

There is a fattening of 1.89 to 3.98 m/year ± 1.59 m/year downstream of the Lella Hadhria spit then the coastline marks an accretion towards the end of the cell at 5, 88 m/year ± 1.59 m/year upstream of Aghir port.

- Between 2001 and 2016

The coastline remained stable along cell 6.

- Between 2016 and 2020

Downstream of the Lella Hadhria spit, the coastline seems stable to advance upstream of the Aghir harbour.

Cell 7, between the Aghir harbour harbour and the Borj Kastel sand spit (Fig. 21)
Between 1975 and 1991

Downstream of the port of Aghir, the coastline has receded from 4.47 to 12.54 m/year ± 3.86 m/year. It then advanced on a long stretch to Borj kastel (8.03 to 159.96 ± 3.86 m/year) to retreat another time from 4.18 to 24.99 m/year ± 3.86 m/year.

- Between 1991 and 2001

Erosion continued downstream of the port of Aghir (one notes an erosion of 4.57 to 23.34 m/year ± 1.59 m/year then an accretion of 121.83 ± 1.59 m/year. It seems an erosion which affects the coastline of 30.87 m/year ± 1.59 m/year on the rest of the cell.

- Between 2001 and 2016

Downstream of the port of Aghir, the coastline has receded from 3.11 to 12.99 m/year ± 2.66 m/year. The coast has undergone accretion ranging from 9.2 to 57.06 m/year ± 2.66 m/year, then erosion 4.29 to 11.25 m/year ± 2.66 m/year.

- Between 2016 and 2020

The coastline has undergone an accretion from 10.88 to 128.91 m/year ± 10 m/year then erosion from 13.37 to 34.59 ± 10 m/year.

The recent evolution of the coastline (Fig. 22), between 2016 and 2020, shows that all the coasts of the Djerba Island are in a state of erosion with the exception of cell 6 (arrow of Lella Hadhria- Port d'Aghir) and cell 7 (Port d'Aghir-sand spit of Borj Kastel) which are in a state of accretion. The accretion is due, respectively, to the interception of sediments in transit by the north pier of the port of Aghir and to the lengthening of the Borj Kastel spit following its enrichment by the sediments brought by the littoral drift currents.

The recent erosion may be due to the effect of the rise in sea level which has accelerated since the year 2000 (Church et al., 2013) and also to the concentration of hotel units and seaside resorts in the immediate vicinity of the littoral (Dribek, 2012) which destroyed the border dunes causing a shortage of sediments. Also, the extraction of sand, for the construction of hotels, has seriously affected the balance of the dune cord (ASSIDJE, 2000). This bad confrontation of the coasts has led to widespread erosion.

General behavior of Djerba Island coastlines over a period of 45 years (1975-2020)

Over a period of 45 years, between 1975 and 2020, all the Djerba Island coastlines are in erosion state (Fig. 23). This confirms the effect of the extension of urbanizations and especially hotels in the extraction of sediments and the destruction of the border dunes (especially at the level of the eastern side of the Island) which represent one of the main coastal components entering the transverse exchanges of sediments. In addition, the absence of watercourses, the main sediment feeders of the coasts, at the level of the Island, which is an isolated system, has contributed to the lack of solid inputs and the retreat of the coastline.

Accretion is localized at the level of the sandy spits of Ras Rmal and Borj Kastel which are elongating (Fig. 24) and which are fed by littoral drift currents.

This work is interested in the study of the stability state of the Djerba Island from aerial photographs and satellite images by studying the coastline evolution. Coastlines were digitized and automatically extracted by calculating the water index with the Arc GIS. The coastline evolution was automatically calculated using the Digital Shoreline Analysis System extension.

These tools have made it possible to have precise and rapid statistics of coastline change rates. Obtained results analysis showed that apart from the arrows of the eastern side of the Island which are in the process of lengthening and fattening up, from 1975 to 2020, all of the coasts of the Island are in erosion state. Erosion is more accentuated on the eastern side which is in direct contact with the dominant waves and coastal currents.

The recent coastline evolution testifies the effect of the increase in sea level rise which is added to the effects of hydrodynamic forcing and anthropogenic interventions (extension of urbanizations and hotels on the border dunes, use of sand beaches as building materials...) and the absence of waterways carrying the continental solid load given the isolated insular system that the Island represents.

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No competing interests reported.

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Stability state of a Mediterranean coastal environment threatened by erosion: the case of the Djerba Island (Gulf of Gabes, SE-Tunisia)

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Abstract

Figures

Introduction

Presentation Of The Studied Area

- Geological setting

- Climate

- Tide

- Waves

Materials And Methods

Discussion

General behavior of Djerba Island coastlines over a period of 45 years (1975-2020)

Conclusion

References

Additional Declarations

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