The South lagoon of Tunis, a shallow Mediterranean lagoon, had undergone an important restoration management (during the period 1998-2001), leading to structural and functional changes of this coastal ecosystem. In this work, a comprehensive study of sedimentary dynamic based on hydrodynamic data, granulometric analysis, mineralogy and impact evaluation is presented. Restoration project, especially the establishment of two groups of inlets gates of the channel of Rades and Tunis (recalibrated during the restoration management), imposed an east-west water flow direction, created from high tide submersion, with an average flow exchanged with the sea about 80 m3s-1. This current control local sediment sorting. The grain sizes analysis of superficial sediment shows that the lagoon bottom characterized by fine sediments (<63 µm, 50-90%). The sediment dynamic is controlled by the lagoon water currents inducing an east-west grain-size sorting. The extreme eastern side of the lagoon, close to the inlet gate, is lined by medium sand, moderately sorted and transported by saltation. The central and western sides of the lagoon are covered by fine sand, poorly sorted, and deposed in relatively calm hydrodynamic conditions. Mineralogical results reveal the following association: quartz (13 to 69%), biogenic calcite, (15 to 81%), aragonite (0 to 8%), pyrite (0 to 1,75%) and accessory magnetite, smectite, illite and kaolinite. The lagoon seems to be a protected zone as a result of restoration project that forms a physical barrier for sedimentary materials amount from Gulf of Tunis.
La laguna sur de Túnez, una laguna costera mediterránea poco profunda, se sometió a importantes trabajos de restauración durante el período 1998-2001, lo que ha llevado a cambios estructurales y funcionales de este ecosistema costero. En este trabajo se presenta un estudio integral de dinámica sedimentaria basado en datos hidrodinámicos, análisis granulométrico, mineralogía y evaluación de impacto. El proyecto de restauración, especialmente el establecimiento de la puerta de entrada de los canales de Rades y Túnez, impuso una dirección de flujo de agua este-oeste controlando la clasificación local de sedimentos. El análisis de tamaño de grano del sedimento superficial muestra que el fondo de la laguna está revestido por sedimentos finos (<63 μm, 50-90%). El transporte de sedimentos está controlado por las corrientes de agua de la laguna que inducen una clasificación de tamaño de grano este-oeste. El extremo oriental de la laguna, cerca de la entrada, está bordeado por arena mediana, moderadamente clasificada y transportada por saltación. Los lados central y occidental de la laguna están cubiertos por arena fina, mal clasificados y acumulados en condiciones hidrodinámicas relativamente tranquilas. Los resultados mineralógicos revelan la siguiente asociación: cuarzo (13 a 69%), calcita biogénica (15 a 81%), aragonita (0 a 8%), pirita (0 a 1,75) y magnetita accesoria, esmectita, ilita y caolinita. La laguna parece ser una zona protegida como resultado del proyecto de restauración que forma una barrera física para la cantidad de materiales sedimentarios del Golfo de Túnez.
Coastal lagoons are characterized by high biodiversity and intense primary productions that lead to both ecological and economical considerable importance. Moreover, the close link between human population and coastal zones, consequently coastal lagoons, is particularly evident since approximately two-third of human total population live in these areas. As a matter of fact, since several decades, increasing human wastes have created important problems (Martin Plus
Mediterranean coastal areas suffer from a continuous anthropogenic pressure, resulting from human growth and an increase of industrial and urban activity, (touristic activity, hardboard, and fish farming…) (European Commission,
The coastal lagoons seem to be the most vulnerable ecosystem (Souchu et al.,
In order to counteract pollution and eutrophication risk, an investigation of sediment dynamics of this shallow area (the spatial distribution, composition and size of mineral particles, currents influence, transport and deposition of particles) may be crucial to the interpretation of sediment transportation and accumulation mechanisms. Indeed, the presence of pollutants in sediments is affected by sediment particle size and composition with metals potentially toxic to living organisms often more associated with fine-grained sediments due to their high surface-to-volume ratios and adsorption capability (Szava-Kovats
Tunisian coastal lagoons, like lagoon of Tunis, Bizerte lagoon and Ghar el Melh (southern basin of the Mediterranean Sea) have served for final repositories of anthropic waste (Ben garali,
Giving the high importance of the South lagoon of Tunis, due to its location, near to the capital, the lagoon has received an important government concern and few professional scales have been conned out to find major solution of pollution.
For this purpose, a restoration and development project was carried out between April 1998 and July 2001 by the South Tunisian Studies and Promotion Corporation society, under the control of the Ministry of Infrastructure, Housing and Development planning of the Tunisian Republic. The project was financed mostly by a credit from the European Investment Bank (EIB) and by The Lake Group (group of private banks).
The project consists of enhancing water circulation by widening channels, deepening the lagoon (dredging the bottom sediments), and pumping sea water into the lagoon by building one-way tide gates to remove areas of water stagnation. After this restoration work, the lagoon structure has radically changed and new ecosystem was given rise (SPLT- STUDI/SOGREAH,
In this paper we present a comprehensive study of the new sedimentary dynamics in the South lagoon of Tunis after its restoration. The purposes of this work are:
to characterize the sediments,
to assess the conditions of their transport and deposition,
to determine the natural and anthropogenic factors that affect the surface sedimentary dynamics.
These objectives are obtained through hydrodynamic data, granulometric and mineralogical study.
Such studies would undoubtedly be of paramount importance, not only regarding the sedimentary dynamics of the lagoon artificially modified, sustained development and quality control of the managed area, but also in the prospect of the future to predict the effects of similar restoration’s interventions on other sites to be cleaned and valorized for further use.
The South lagoon of Tunis is the south basin of the lagoon of Tunis. It is a shallow coastal area, located at longitude 10°12’ et 10°16’ E and 36°46’ to 36°48’ N latitude, covering a surface of about 720 hec (
Location map and geological setting of South lagoon of Tunis.
The lagoon is under a semi-arid climate, characterized by a high evaporation rate; it ranges between 62 and 201 mm, in January and July respectively. Wind is a very important meteorological parameter given its hydrodynamic effects. It’s involved in wave generation, propagation and sediment transport. The Rose of the Wind, spanning 2007 to 2011 at the Tunis-Carthage station, shows the dominant winds are from N, NW in winter and S, SE, SW in summer (INM,
Generally, the coastal lagoons formation is related to the latest glacioeustatic variations, by the drop of the sea level and the development of ice caps (Tastetu,
According to Pimienta (
The catchment is represented mainly by the Quaternary alluvium (complex of muddy sand and sandy loam) and by Oligocene (fine sandstones separated by small levels of clay) outcrops at Rades and Bir El Kassâa, and also by the Eocene hills of Jbel Ejellez (Pimenta,
The lagoon has a regular depth of about 2.1 m, except in some restricted areas, on the eastern side, where it reaches a maximum of 5 m.
The hydrodynamic’s lagoon is mainly controlled by the combination of tide and wind effects. Indeed, the tide by the variation of the level of water, control the opening and the closing of the locks. The seawater enters, via Rades channel in rising tide and comes out from the lagoon to the Gulf by channel of Tunis when tide is down. The total exchange of water volume with the sea via the channel of Rades is about 2.57 million m3/day (Jouini,
Its bank lines are straight; they are divergent to channel of Rades and become converging towards the Tunis Channel, favoring better fluid water circulation in the lagoon.
The tidal regime of this area is dominated by semi-diurnal cycle and is characterized by low amplitude that vary from 0.09 m to 0.26 m, with an average 0.20 m (Jouini
According to the studies conducted by Kochlef (
Currents speed and direction of the South Lagoon of Tunis (Kochlef
In order to ensure the proper functioning of the redeveloped lagoon, recalibration works of the lagoon-Gulf hydraulic communication channels (channel of Rades and Tunis channel of Tunis) have been carried out:
Channel of Rades (2 Km length and 4 m deep), ensures the sea water passage from the Gulf to the lagoon. It is protected by a dike that acts against the swell action, the evolution of the shore, and it limits the reintroduction of the materials carried by the marine currents, later it reduces the risk of silting.
Channel of Tunis (10 Km length and 4.5 m deep), ensures the evacuation of the waters of the lagoon.
The waves generated by wind can affect the function of the locks and the lagoon-sea exchange volume of water. Only eastern winds are important. They promote the supply of seawater into the lagoon to 10% by 10 ms-1 of speed. Other winds directions generate short fetches without effects (Jouini
The present work is of fundamental importance in connection with the protection of this coastal environment from terrestrial pollution that may alter or affect it. Sediments collected from the South Lagoon of Tunis are used to understand the complex interactions between the grain-size pattern and the hydrodynamic process, under urban and natural effects. It would be very interesting to deduce a textural parameter that is characteristic of the lagoon. The distribution of such parameter could be used to predict sediment behavior and its dispersion over the lagoon, in response to new inputs of sediment from natural or man-induced processes.
In order to assess the hydrodynamic characteristics of the South Lagoon of Tunis, 21 surface sediment samples were collected, over the lagoon (
The surface sediment sampling site map.
In order to study samples distribution and surface sediment transport we used grain-size (Folk & Ward,
All samples were wet-sieved (63 µm sieve mesh) to separate the silty and sandy sediment fractions (<63 µm and >63 µm, respectively). The obtained sandy sub-samples were then oven dried at 50°C and granulometric analysis was performed by means of successive sieving from 63 to 2000 µm (AFNOR type). Semi-logarithmic grading curves were established, that permit to distinguish different ways of sediment deposition. Calculations of grain-size parameters such as Units of sediment size mean (Phi), Mean grain sizes (Mz), the standard deviation (ⱷ), skewness (Ski) and kurtosis was carried out based on Folk & Ward (
Then, grain-size types are represented in the C-N Passega diagram (Passega,
Mineralogical analysis for clay and non-clay minerals were conducted via X-ray diffraction (XRD), using X’Pert Pro diffractometer.
Clay minerals were identified, with only 10 samples, on oriented mounts of non-calcareous clay-sized (<2 μm) particles (Holtzapffel,
Results show significant variation for both silty and sandy fractions. Their percentages vary between 33 and 94% for between 6 and 67%, respectively. According to the distribution map of the fine fraction (
The silty Fraction (%) distribution map of South Lagoon of Tunis surface sediments.
Eastern side of the lagoon: is dominated by sandy fraction (> 63 µm) (D50 vary between 140 and 300 µm).
Central zone of the lagoon: sediments are composed by 50 to 75% of silty fraction (< 63 µm).
Western and southern-east sides of the lagoon: are dominated by fine fraction.
The dominance of silty fraction is due to the current flow direction. Fine sediments from marine supply, terrigenous input (watershed erosion, suspended solid contained in the wastewater (ONAS,
Various particle size indices are calculated (
Granulometric indices of surface sediments of South lagoon of Tunis
Samples | X | Y | Sandy fraction (%) | Silty fraction (%) | Mean size | Standard deviation | Coefficient of Skewness (Ski) |
---|---|---|---|---|---|---|---|
C1 | 10.272 | 36.797 | 67 | 33 | 2,78 | 0,81 | -0,15 |
C2 | 10.266 | 36.797 | 45 | 55 | 1,85 | 1,11 | 0,13 |
C3 | 10.266 | 36.795 | 52 | 48 | 1,99 | 1,12 | 0,14 |
C4 | 10.257 | 36.79 | 7 | 93 | 2,3 | 1,08 | 0,08 |
C5 | 10.255 | 36.795 | 33 | 67 | 2,19 | 1,06 | 0,1 |
C6 | 10.254 | 36.8 | 6 | 94 | 2,16 | 0,96 | 0,16 |
C7 | 10.242 | 36.801 | 52 | 48 | 2,35 | 0,98 | 0,04 |
C8 | 10.243 | 36.796 | 50 | 50 | 2,24 | 1,06 | -0,01 |
C9 | 10.243 | 36.79 | 40 | 60 | 2,37 | 1,01 | 0 |
C10 | 10.236 | 36.79 | 14 | 86 | 2,14 | 1,05 | 0,11 |
C11 | 10.235 | 36.795 | 6 | 94 | 2,12 | 0,94 | 0,09 |
C12 | 10.235 | 36.8 | 41 | 59 | 2,26 | 1,06 | 0,05 |
C13 | 10.228 | 36.799 | 23 | 77 | 2,21 | 1,09 | -0,05 |
C14 | 10.228 | 36.795 | 38 | 62 | 2,15 | 0,92 | 0,22 |
C15 | 10.231 | 36.789 | 10 | 90 | 2,56 | 0,89 | -0,07 |
C16 | 10.222 | 36.788 | 9 | 91 | 2,76 | 0,82 | -0,14 |
C17 | 10.222 | 36.791 | 24 | 76 | 2,58 | 0,92 | -0,07 |
C18 | 10.221 | 36.796 | 26 | 74 | 2,46 | 1 | -0,11 |
C19 | 10.216 | 36.791 | 15 | 85 | 2,67 | 0,82 | -0,15 |
C20 | 10.216 | 36.788 | 6 | 94 | 2,57 | 0,91 | -0,1 |
C21 | 10.212 | 36.791 | 19 | 81 | 2,94 | 0,87 | -0,43 |
Cumulative curves reveal presence of two facies types (
Example of cumulative curves of South Lagoon of Tunis surface sediments Tunis.
The cumulative semi-logarithmic curves show S-shaped facies: found in the eastern side of lagoon (off the channel of Rades) this indicates that these sands are transported within a relatively turbulent environment, corresponding to the gulf sediment supply.
The cumulative semi-logarithmic curves show hyperbolic-shaped facies: were found in the majority of the samples from the South lagoon of Tunis. This indicates that these sands are transported within relatively calm hydrodynamic conditions. These sediments are deposited by settling and they show a predominance of the fine fraction.
From the cumulative curves, we calculated the particle size indices surface sediments of the South Tunis lagoon (
Mean grain size range from 1.85 to 2.94µm. These facies consist of fine to medium sand (Folk & Ward,
Medium sand is present in samples from the eastern side of the lagoon.
Fine sand is found in the majority of the lagoon.
The standard deviation values, ranging from 0.18 to 1.12, indicate that the most of sand of the lagoon is moderately sorted to poorly sorted in the eastern side of the lagoon, near to channel of Rades.
The skewness of the surface sediment varies from -0.43 to 0.22. Skewness variation shows a decreasing trend from the east towards the west. Its variation coefficient indicates:
Very asymmetric frequency curves of coarse sands, for samples located near to the two locks.
Symmetric frequency curves of coarse sand for the central part of the lagoon.
Asymmetric frequency curves of coarse sand, for samples located in western zone of the lagoon. It seems to be an intermediate facies area between the two extreme zones.
Asymmetric frequency curves of fine sand, mainly in the eastern part and near the water discharge points.
The Passega diagram (Passega,
Passega diagram: Surface sediment transport modes.
Sediments transport by thrust, represented by PO segment, it concerns sediments taken near the banks of the lagoon.
Fine sand is transported by saltation, represented by the QP segment. It concerns almost the majority of sediments in the central areas of the lagoon and the west side near to the lock.
Results of the semi-quantitative mineralogical analysis are given in
Relative percentages of non-clay minerals in South Tunis Lagoon sediments
Samples | Quartz | Calcite | Aragonite | Pyrite | Magnetite |
---|---|---|---|---|---|
C1 | 69.49 | 30.51 | 0 | 0 | 0 |
C2 | 49.85 | 42.69 | 0 | 0 | 0 |
C3 | 40.21 | 59.79 | 0 | 0 | 0 |
C4 | 43.7 | 50.42 | 0 | 0 | 0 |
C5 | 45.56 | 48.79 | 0 | 0 | 0 |
C6 | 43.7 | 56.96 | 0 | 0 | 0 |
C7 | 56.81 | 42.7 | 6.57 | 0.94 | 0 |
C8 | 60.71 | 35.67 | 3.31 | 0.49 | 0 |
C9 | 67.76 | 25.3 | 8.2 | 0 | 0 |
C10 | 51.71 | 48.03 | 0 | 1.28 | 0 |
C11 | 46.46 | 48.19 | 0 | 1.18 | 0 |
C12 | 57.55 | 35.94 | 8.02 | 0.94 | 0 |
C13 | 51.97 | 44.48 | 0 | 1.31 | 0 |
C14 | 48.77 | 35.74 | 7.79 | 0.82 | 0 |
C15 | 12.9 | 81.94 | 0 | 0.65 | 0 |
C16 | 46.12 | 44.57 | 0 | 1.16 | 0 |
C17 | 59.22 | 34.47 | 0 | 0.49 | 0 |
C18 | 53.63 | 31.55 | 2.7 | 1.31 | 1.35 |
C19 | 33.7 | 49.02 | 0 | 1.75 | 1.35 |
C20 | 46.64 | 43.08 | 1.58 | 0.79 | 1.35 |
C21 | 50.83 | 38.75 | 5 | 0 | 0 |
Clay and non-clay minerals distribution map of the South Lagoon of Tunis surface sediments.
Quartz is the most abundant mineral, with relatively high contents ranging from 13 to 69%. Its content increases from the east lock to the west of the lagoon. The quartz in the lagoon originated mainly from Quaternary lithological formations outcropping in the catchment.
Calcite is abundant in all samples, with proportions varying from 25.3 to 81.94%, which may have originated chemically through direct precipitation of biogeochemical limestone (Harbridge
Pyrite is present in majority of collected samples with low level ranging from 0.49 to 1.75%. This mineral is generally associated to fine fraction and characterize reducing conditions in the depositional environment.
Aragonite is present in some sediment samples collected in the central zone of the lagoon and close to the banks, at percentage of 8.2%. Aragonite is common in biogenic marine sediments.
Magnetite is present in few samples close to the wastewater discharge points at a percentage of 1.35%, reflecting the important role of water discharge as a source of iron input.
For clay minerals, the lagoon sediments consist mainly of kaolinite (ranging from 27 to 72%.), illite (between 15 and 48%) and with a few samples containing less than 40% of smectite (
Before the restoration project, the lagoon was covering 1300 hec. Its shape was irregular. The bathymetry was less than 1m. Water circulation in the lagoon was dominated by wind and was very sluggish. Anthropogenic activities generate most of the lagoon sediment. Sludge is the most abundant sediment (Harbridge
The restoration project (1998-2001) has put remarkable structural modifications on the lagoon (
Plan of restoration works.
Sediments derived from various source rocks have different mineral assemblages (Chen et al.,
Similar case to the South lagoon of Tunis, the lagoon of Venice suffered from environmental problems caused by human pression and the exponential industrial development in its watershed (Brambati
Those actions have not complied the expected goals. A negative sediment balance was established: appearance of erosion phenomena, in a side, and filling of the channels in other side of the lagoon, unlike the South lagoon of Tunis that seems to be equate equilibrium.
Hence, the choice of intervention depends on the objectives of restoration, also the success of any project must be discussed regarding different parameters of the ecosystem. A preliminary multidisciplinary study can help in orienting further restoration activities in order to fill the major gaps and to improve opportunity to success in long terms.
The analysis of grain size shows that the fine fraction (>63 µm) cover the majority of the bottom lagoon.
The calculation of the fine fraction proportion and the interpretation of the semi-cumulative curves of coarse fraction lead us to a subdivision the lagoon in two zones: the extreme eastern side of the lagoon that is covered by medium to fine sand, and the rest of the lagoon, characterized by fine fraction. This grain-size sorting is governed by the east-west lagoon currents. The sediment transport is related to lagoon water current, causing appears of relatively calm areas that can be place of suspended materials accumulation.
The limited amount of sand is under the effect of infrastructure construction (the dike of channel of Rades) which acts as a physical barrier. Thus, it noticeable that after the restoration project the sedimentary behavior of the lagoon seemed to be fairly stable.
The mineralogy of the studied samples showed that calcite and quartz are the main non-clay minerals and illite, kaolinite and smectite, as clay mineral, are dominants. The lagoon is essentially fed by sedimentary supply from the surrounding areas.