Coastal sensitivity index
The coastal sensitivity describe here the integrate information from eight parameters. The theoretical value of each of the selected variables is included in the matrix based on user guidelines. The structural variables are (a) geomorphology; (b) hinterland topography; (c) coastal slope; (d) sediment facies. The physical variables are (e) shoreline exposure (f) shoreline changes (g) tide (h) sea-level trend.
Coastal morphology is the interplay between hydrodynamic processes and landforms. Beaches and sandy spits of Jerba Island are sensitive to the changes in energy and human influences. The common E-W longshore drifts, formed during the NE prevailing sea swell move sediments from the eroding rocky shores of Aghir toward the sandy spits of Kastil and Ras Rmal. Rocky coasts and cliffs are more stable and reflect a decreasing sensitivity to physical variables. The depositional features are observed in the coastal areas that are fronted by deep shoreface. Morphology of Boughrara lagoon is marked by sea cliffs and high plateaus. The average depth in the basin is about 4 m with a maximum of 16 m in the center part. The portrayal of the studied zoneshows that 155 km of the coast (43%) are mapped as marshes and flats(Figure 3a), 29% of the coast as sandy shores (21% beaches, 8% sandy spits and Barrier Island) and 8% as rocky shores.
The hinterland topography is the most important factors that isconsidered to estimate the impact of the sea level rise on the given transects. It expresses the relative erodibility and the degree of resistance of the coast. The low lying coastal areas are exposed to the littoral inundation and flooding. However, the high plateaus are exempt to flooding risk. The northern edge of the studied area is marked by a series of hillocks. The lithological character of these plateaus consists to a conglomerate constructionwhich has been incised by drainage network. The analysis of the hinterland topography variable shows that 25% of the coast mapped as high plateaus, 28% as sebkhas, 20% as smooth shapes, 16% as lagoons and 11% as former outcrops (Figure 3b).
Several transverseprofiles of the coast are analyzed in order to extract the lateral variability of the substrate slope. Coastal slopes values between 0.03° – 0.05° are ranked as lower risk where hydrodynamic processes are mitigated. The beaches of the northern side of Gabes city are more energeticand shows a highly slope angle (0.2° -0.3°). The southern band of Gabes is marked by a gently substrate slope angle (0.15°; Gzam, 2013). Around Jerba Island, bathymetry is irregular; the eastern side is marked by steep substrate where coasts are exposed to waves. The northern side is sheltered from the open sea by a wide regular platform densely occupied by sea grass. In Boughrara lagoon bathymetry is marked by the abundance of sandy shoals which are dissected by deep channels. Figure 3 (c) shows that 66% of the coast were mapped as very low angle slope (0.03°-0.059°), 4% as gently inclined substrate (0.06°-0.1°), 18% as moderately inclined slope (0.11°-0.3°), 10% as steep angle slope (0.31°-0.5°) and 2% as very high angle (0.51°-0.7°).
The nearshore zone is generally formed by homogeneous well-sorted medium- to fine-grained quartzose sand (2.2Φ < Mz<3.3Φ). Most of these sediments in the nearshore contain shells and shell fragments. Mineralogical analysis shows abundance of carbonate fraction in the sediments therein amount of calcite and aragonite attains respectively 30% and 50% (Brahim et al., 2015).The very fine sediment (3Φ < Mz<3.3Φ) is classed as lower risk. This class occurs in sheltered area with low hydrodynamic processes. Sediments are composed also by a great amount of algal and vegetal debris. The sandy shoals are marked by high energy coarser grained sediments where carbonate production is very intensive. At the eastern side of Jerba Island beaches are composed of medium to coarse sand (1.8 Φ < Mz<2.6 Φ). Figure 3 (d) indicates that 54% of the coasts were mapped as fine facies, 26% as medium facies and 20% as coarse facies.
Shoreline exposure describes the end effects of the wave actions with littoral configuration. Shoreline exposition to the dominant wave influences its susceptibility to the submersion risk. Waves have a lesser effect on sheltered coasts. This variable is ranked as very sheltered, sheltered, semi-exposed, exposed and fully exposed. In the Gulf of Gabes, large wave amplitudes mainly correlate with high wind speed from NE direction (Amari 1984, Abdennadher and Boukthir, 2006). Sea levels appear to rise in response to wind pile-up (Sammari et al., 2006). Waveheight in the NE sector of the Gulf reaches 2.5 m which decrease near the shoreline 0.9 m. Wave is strongly attenuated during its propagation by sea grass beds and shoals. The SE wave sector, with the lowest height 1.1 m, induces a seasonal accretion along the coast. Waves originate from E sector approach the coast without energy dissipation (h= 2.2 m, T= 5.5 s), inducing cross-shore sediment transport. Energy dissipation from oblique waves is resulted from the longshore sediment transitand the development of sandy spits. Using a rose diagram of wave intensity, each transect was assigned a rank value according on its configuration. Figure 4 (e) shows that 70% of the coasts were mapped as sheltered to very sheltered, 11% as semi exposed, 13% as exposed and 6% as fully exposed.
Recent morphologic trend in the Gulf of Gabes is marked by shoreline progradation. Morphologic survey combined to images satellite data show that the Gulf of Gabes is marked by the genesis of a linear ridge formed since 1980’s. The sheltered area is shifted to marsh domain with a branched network of tidal channels. Beaches are dissipative and tend to be the steeper with a gentle slope and wide low tide terrace which may be up to 200 m. Rocky shores exists mainly at the western side of Jerba Island. It consists to a high former dune developed during the Holocene transgression (Frébourg et al., 2010). In term of shoreline changes, Figure 4 (f) indicates that 71% of the coast was marked by progradation, 22% of the coast was mapped as stable area, and 7% of the coast was mapped as eroding.
The tidal regime modulates the hydrosedimentary processes along the transverse profile of beaches. The lower tide range represents a higher sensitivity; with low tide range shoreline is exposed frequently to hydrodynamic processes. Thus a storm surge event has a greater probability of coinciding with a high tide that would cause shoreline instability and erosion. In the Mediterranean, the highest tides are observed in the Gulf of Gabes (Abdennadher and Boukthir, 2006), where tides are principally semi-diurnal in nature. The tidal range is not everywhere the same in the study area. The tidal amplitude is highest (2 m) in the central part of the Gulf (Gabes) and smaller in the south: 1.3 at Jerba Island and 0.6 m at Boughrara lagoon (Sammari et al., 2006). Such amplitude increases sediment mobilization across the beaches. In terms of tidal regime variable, Figure 4g shows that 6% of the coast were marked by a high tides of 2 m, 12% mapped with tide of between 1.9 m- 1.5 m, 43% of the coast mapped with tide of between 1.4 m - 1.1 m, 11% of coast mapped with tide of between 1 m - 0.7 m and 28% of the coast mapped with tide of 0.6 m.
Sea level trend
Based on tide gauge data the rate of global average sea level rise, during the 20th century, is in the range of 1.8 mm/yr between 1961 and 2003 and has accelerated between 1993 and 2003 with a rate of 3.1 mm/year (IPCC, 2013; Oueslati, 2004). Otherwise, Masmoudi, 2010 affirmed that barrier coasts system of Jerba Island are governed by relative sea level change where recent sandy accumulations are contemporaneous with the actual tendency of risen sea level. Coastal marshes are abundant with 43% of the coast. It accretes vertically in response to sea-level rise (Orson et al., 1998; French, 2006). All climatic scenarios indicate a sea level rise, along the Mediterranean coast, of 3 to 14 cm from 1990 to 2025 (IPCC 2013). A higher value (Figure 4h); 3.1 mm/ yr) was considered in the current study.
Coastal state indicators
Statistical quartiles were used to slice the data on the basis of the magnitude of calculated indices for all transects to highlight different relative sensitivities along the coast. The CSI scores range from 14 to 34, with a mean of 22.7 (Figure 5; median is 22). Thus the mid-range (21-23) was considered to be moderate sensitivity zones. High sensitivity values extend from 24 to 26. Values above 26 are assigned to the very high sensitivity category. Values below 21 are assigned to the low sensitivity. A total of 358 km of shoreline is ranked in four categories (Figure 5). A 12.7 % of the mapped shoreline shows very high risk. An 8.6 % of the coastsare classified as high sensitivity and 78.6 % as moderate to low risk. The lowest sensitivity areas include the sheltered tidal flats of the studied zone. These tidal flats are backed by high-plateau and governed by low hydrodynamic conditions. A significant exception to this domain (tidal flats) is recorded along the western edge of Jerba Island where marshes are scored with high sensitivity. Here morphology consists to the rocky shores densely covered by marshes. This domain is backed by low-lying inland and advanced by a relative coastal steep slope. Large section of the study area is mapped as moderate sensitivity coasts. This class was reported for wide range of coastal sections corresponding mainly to sheltered sea cliffs, rocky shores which are advanced by a large platform. High sensitivity index is attributed mainly for beaches (Gabes) fully exposed to wave action and backed by dunes and low inland. Sediments depletion is ensured by the cross-shore components and littoral drift. Coastal slope favors energy dissipation in the large surf zone. Coastal transects ranked with very high sensitivity includebarrier and sandy spits and occur along the eastern coasts of Jerba Island, which are exposed to oblique waves. The volumes of sand eroded from proximal beaches (source zone) are transferred laterally by littoral drift and deposited beyond morphodynamic system limits. If these volumes are not renewed sufficiently in the proximal zone, a negative sedimentary budget takes place and enhances littoral erodibility.The lateral transfer of sediments from source beaches toward end spit section presents a significant parameter to evaluate the coastal sensitivity.Such coasts have undergone recession over recent two decades and may be subject to further landward retreat during storm surges.Thus, littoral planningin these areas is not recommended because morphodynamic equilibrium is tied to littoral drift.