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 et al., 2006).

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, 1991). These activities have dramatically increased the amount of toxic pollutants, that may exceed the limit of the environmental consumption capacity and lead to a degradation of the ecosystems structure (Diaz, 2001), threating human health and causing imbalance for biota (UNEP/MAP, 2012; Belabed et al., 2013; Zaaboub et al., 2015).

The coastal lagoons seem to be the most vulnerable ecosystem (Souchu et al., 2000). They are considered as dynamic and complex biogeochemical systems (Lankford, 1977; Nicklos & Allen, 1981; Bidet et al., 1982). Continental supply and marine contribution interactions give rise to multiple processes influencing the sedimentology and geochemistry of the lagoon. The lagoon can be naturally fed by sediments transported by the flow of its many streams from eroded watershed outcrops. Indeed, urbanization and industrial expansion have put remarkable ecological stress, by the discharge of the industrial and municipal wastewater and urban runoff. For this reason, the assessment of the lagoon water quality requires understanding the geochemical processes that controlled the exchanges between the different interfaces of this ecosystem. The study of the bottom sediments presents the most useful tool to identify the spatio-temporal evolution of the chemical elements. The latter are in close relation with the sedimentary dynamics and the transport pathways of the chemical compounds towards the receiving environment (Perriaux, 1972; Tricart, 1965; Miskovsky, 1974).

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 2008 Belabed et al., 2013; Zaaboub et al., 2015; Oueslati et al., 2017).

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, 2011; Oueslati, 2010; Zaaboub et al., 2015). The lagoon of Tunis (Northeast of Tunisia) is one of four major lagoons in Tunisia; this lagoon is subdivided into both: North and South lagoon of Tunis (Zaouali, 1983; Harbridge et al., 1976). In This study, we focus on South lagoon of Tunis, which was used to receive raw sewage from different industrial and urban areas that contributed to its degradation (Ben Charrada et al., 2001; Ben Souissi, 2000).

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, 1998; Jouini et al., 2005).

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:

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 (Fig. 1). It communicates with the Gulf of Tunis, via Rades channel and channel of Tunis. Also, the lagoon receives water from draining the urban area of Megrine, runoff of newly developed areas after their planning, as well as a channel called the belt channel (DHU- Equipment Ministry, 2015). It received about 5500 m³/day of wastewater on its south side and south-west side from urban and industrial areas (Jouini et al., 2005).

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, 2012).

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, 1974; Lankford, 1977; Nicklos & Allen, 1981; Bidet et al., 1982). The lagoon of Tunis had taken its current form during the end of the last glaciation until the Holocene.

According to Pimienta (1959), Warmington (1967), Brogan (1967), Craig (1967), Coque (1967), sediment supply from Medjerda river and Meliane river have contributed to filling of the Gulf of Tunis leading the isolation of the lagoon from the open marine system of the Gulf. In 1855 a large channel, Channel of Tunis, was built which divided the lagoon into two lagoons: The North lagoon and The South Lagoon of Tunis.

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, 1959; Paskoff et al., 1983; geological maps 1/50000) (Fig. 1).

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, 2005).

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 et al., 2005).

According to the studies conducted by Kochlef (2003) (Fig. 2), the current velocities recorded in the lagoon are variable. It varies from 65 cm/s to 5 cm/s, going from the channel of Rades to the channel of Tunis, leading to the appearance of little stagnation zones in the northwestern part of the lagoon especially on summer.

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:

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 et al., 2005).

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 (Fig. 3). Samples were collected using a Van Veen grab sampler operated from aboard boat.

In order to study samples distribution and surface sediment transport we used grain-size (Folk & Ward, 1957) and mineralogical (Riviere, 1952, 1953) analysis were carried out.

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.