Case of Monastir Bay (Eastern Tunisia)
Total Page:16
File Type:pdf, Size:1020Kb
Carpathian Journal of Earth and Environmental Sciences, August 2014, Vol. 9, No. 3, p. 231 - 239 EFFECT OF SUBMARINE MORPHOLOGY ON ENVIRONMENT QUALITY: CASE OF MONASTIR BAY (EASTERN TUNISIA) Radhia SOUISSI 1* , Imen TURKI 2,3 & Fouad SOUISSI 1,4 1Institut National de Recherche et d’Analyse Physico-Chimique, Technopôle de Sidi Thabet, 2020 Ariana, Tunisia. [email protected] 2UMR CNRS 6143 Continental and Coastal Morphodynamics ‘M2C’ University of Rouen, 76821 Mont-Saint-Aignan Cedex, France. [email protected]. 3Environmental Hydraulics Institute IH ‘Cantabria’, University of Cantabria 4Université Tunis el Manar, Département de Géologie Abstract: The Monastir Bay is characterized by high organic pollution (up to 6% TOC), along the coastal zone and significant eutrophication. Such degradation seems to be strongly related to both characteristics of the supplied water which is rich in organic particles and the submarine morphology showing a flat extending between Khniss and Lamta and a series of shoals and sand bars between Ras Dimass and Kuriates Islands. Results of the hydrodynamics modeling have shown that the submarine morphology of the bay is responsible for the transformation of the waves coming from the open sea (N, E, SE) to the coastal zone, where reduced energy contributes to the concentration of pollution. Wave modeling has also shown that such concentration prevails in low-depth (0 to -3m depth) areas where both surf and breaking zones are almost absent. This case illustrates an example of the Mediterranean coasts vulnerability to pollution under moderate energy conditions. Key Words: Pollution, shallow water, shoals, hydrodynamics, SMC modeling. 1. INTRODUCTION sedimentological and biological analysis (Sassi et al., 1998a and b; Sahnoun, 2000, Sahnoun et al., 2003; Coastal areas have an important effect on Nouira et al., 2013, Jebali et al., 2011; El Asmi-Jellouli economic, social and ecological issues worldwide. et al., 2001). They are among the most complex and variable The geochemical studies have shown that this marine systems because their dynamics are subject littoral is affected by metallic and organic pollution to the effects deriving from a complex geometry and (Sassi et al., 1998b; Sahnoun, 2000, Sahnoun et al., where the bathymetry plays a crucial role in wave 2003; Nouira et al., 2013). As a consequence of the propagation. Moreover, the wide range of processes weak hydrodynamics prevailing in the bay, nutrient- affecting coastal hydrodynamics such as waves, rich wastewaters discharged in several points along currents, and tides, among others, interact at the shoreline, have progressively led to eutrophication different spatial and temporal scales, thus making of and are directly responsible for the reducing character these zones highly variable environments (Alvarez- of surface sediments (Sassi et al., 1998a). Ellacuria et al., 2010). Coastal areas with shallow Although these studies have identified the water are strategic locations, providing shelter to environmental quality of the coastal areas under many organisms. consideration, the use of numerical models can However, the diversity of anthropogenic provide information on the hydrodynamics of these activities, displays environmental impacts on the coast, areas and allow a comparison with the results particularly in terms of pollution and exploitation of obtained by other techniques. A hydrodynamic study natural resources. The coastline appears as a fragile is developed to determine the reasons that had led to area, specifically where hydrodynamics is reduced, the concentration of pollution in areas with low which is the case at the Bay of Monastir. Previous bathymetry at the Bay of Monastir (Khniss-Ksibet El studies on this bay were based only on a geochemical, Mediouni coastline). The objective of this study is to 231 validate the results of the previous geochemical isobath is at 2.5km from Khniss-Ksibet El Mediouni studies, by using for the first time, the hydrodynamic and at more than 8 km from the coast of Teboulba- modeling based on the use of the results of a recent RasDimass. This bay is also characterized by the topo-bathymetric campaign and waves data. presence of several sandy shoals (the Ras Dimas- Kuriates islands beside Teboulba-Bekalta) 2. STUDY AREA substantially aligned NW-SE, making physical barriers between the shore and the open sea, while 2.1. Description of the site being separated by different passes. We report also that the south-of-Monastir The region of Monastir, located 150km south peninsula; the El Enf sand spit circumscribes the of Tunis, is marked by the extension of lands having Lagoon of Monastir which has sheltered the fish- low topography, but with a slight variation in local farming station between the 1970s and the 1990s. topography, represented mainly by the Mio-Plio- This lagoon, in which maximum water depth is 1.5m, Quaternary cliff of Monastir and the Tyrrhenian hills has two passes, water enters from the northern pass of Ksibet el Mediouni, on the one hand, and by and is discharged from the southern pass, loaded with collapsed areas represented mainly by the Sebkha of organic matters from the former aquaculture station. Monastir, on the other hand (Amari & Bedir,1988; South of the El Enf sand spit, a few hundred meters Kamoun, 1981; Paskoff & Sanlaville, 1983). wide flat sea floor extends from Khniss to Teboulba, The bay of Monastir is characterized by a very where sediments are composed of very fine sands low-water depth (Fig. 1). The 3m isobath is at 2km rich in decaying organic matters (Sassi et al., 1998a), from the coastline of Khniss-Ksibet El Mediouni and that are indicative of highly anoxic conditions. at 900m from the coast of Lamta-Sayada. But the 5m 664000 672000 680000 4 5 3 5 - , - , 1 4 8 - - 4 5 3 - -5 - 37 0 3 -1 - - 5 2 5 , , - 3 5 3 4 - , - 4 2 - - 5 1 , - 1 - 3 -1 -1 3960000 5 3960000 2 Monastir - . 1 - 4 -4 - ,5 1 5 - 5 , , -4 0 - - - 4 8 2 ,5 , - -8 5 -7 3 -4, - -12,5 5 5 -5 , - 0 ,5 5 3 2 1 -7 , - 5 - 2 , ,5 - 3 5 - 0,5 - -9,5 0 -1 -1 1 -1 - - -1 2 1 - 1 ,5 1 9 0 1 -1 9 -1 2 - - ,5 6 -1 , -6 -9,5 1, -1 5 ,5 5 0 -12 -4 5 -1 -1 -1 , -6, - 1 1 0,5 -1 5 7 5 , - 5 - 8 -8 1 2,5 ,5 5 5 ,5 - - -8 -9 8 - 6 Khniss 5 6 -7 , -2 , - -7 ,5 -8 -5,5 5 1 - -5 - 5 -6 ,5 -3 9 3 -5 -6 -6 ,5 -5 -5 ,5 9,5 - - -7 -6 -2 - 5 5 ,5 -2 -4- - -4 4 , - 2 3,5 8 - 3 -8 7 -0 - ,5 3952000 ,5 - 3952000 -0 ,5 -1 7 -7 -7 ,5 -0 -6,5 ,5 -0,5 -0 -5 -6 -5 -4 - ,5 3 ,5 5 -1,5 - - 1 7 ,5 -0,5 , 1 ,5 5 -6 - - Ksibet El Mediouni 0 - -0,5 - 1 2 4 1 , -4 ,5 - - ,5 5 3 -5 ,5 4 -4 ,5 3 -5 - -0 -0 5 - ,5 -1 , -5 1 ,5 -2 , 5 - 1 1 1 4 5 - - , , 5 5 4 -4, -1 - 1 5 - 3 5 Lamta -4, 6 - -6 3 ,5 -2 -4,5 -7 -0 -5 Sayada ,5 ,5 - -7 6 -6 -0 -5 ,5 -0 -5 ,5 ,5 ,5 -1 0 - ,5 -1 -0 - Wastewater discharge Teboulba ,5 -1 1 ,5 -0 -2 -4 Rainwater discharge ,5 -1 -0,5 -4 Profile 2 - 3944000 3944000 5 0002 500 0 m 664000 672000 680000 Figure 1. Map of bathymetry in the Bay of Monastir and location of wastewater and rainwater discharge. 232 The overall rainfall in the region is not The average annual level of the tide on the significant with a 20-year annual average of 316mm, eastern coast of Tunisia is 35cm, while the during the period 1970-1990. However, exceptional maximum level of the extraordinary vivid waters is events are probably responsible for a large 50 cm. The total tide is of semi-diurnal type, contribution of water and sediment on the coast. resulting from astronomical and meteorological tides, showing that the sea level is determined by the 2.2. Wind effect of the meteorological tide (Fig. 3). Statistical analysis of local winds (INM, Fig. 2) shows that the prevailing winds are characterized by a speed between 1 and 5 m/s, and come from three main directions of W, NNE and E with an occurrence probability of 10.7%, 8.5% and 7.6%, respectively. 2.3. Tide Astronomic tides cause the water levels to rise and fall and yield large scale current patterns, sometimes with high velocities. Tides directly affect coastal morphology, navigation, fisheries habitat and recreational activities (Kamphuis, 2002). Figure 2. Wind rose (direction and frequency), (1997- 2006) (INM). Figure 3. Astronomical and meteorological tides in the study Figure 4. Wave rose ((a: wave height, direction and frequency) and (b: wave period, direction and frequency)) in the Bay of Hammamet. 233 2.4. Wave climate Khniss, implanted in the early 1970s and designed for the disposal of rainwater in urban and industrial Wave rose (Fig. 4a and b) shows that the areas bordering the Bay (Fig. 1). predominant waves are the ones coming from ESE to The drain of Khniss is the most important SSE. The occurence frequency of these waves is work to be executed. Since its inception, the work 45.5%. The mean and stormy wave height takes a has had a very negative role on the bay, not only due value of 1 m and 2 m, respectively. The mean peak to the fine sediments carried to the Bay, but also to period is 7 seconds and the stormy peak period is 10 the introduction of huge freshwater quantities, seconds.The significant wave-height probability polluted by significant organic and metallic loads.