Impact of dams and climate on the evolution of the

Journal of African Earth Sciences 142 (2018) 226e233

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Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci

Impact of dams and climate on the evolution of the sediment loads to the sea by the Mejerda River (Golf of Tunis) using a paleo-hydrological approach b, Hamadi Habaieb a, Sihem Benabdallah d, Fatma Kotti a, *, Laurent Dezileau c, Gil Mahe e f Malik Bentkaya , Roget Calvez , Claudine Dieulin b a

Institut National Agronomique de Tunis, 43, Av. Charles Nicolle, 1082 Tunis, Tunisia UMR Hydro-Sciences Montpellier / IRD, France G eoscience Montpellier, Universit e de Montpellier, France d Centre de Recherches et des Technologies des Eaux, BP 273 Soliman, Tunisia e Centre National des Sciences et Technologies Nucl eaires, Sidi Thabet, Tunisia f UMR G-Eau / IRD, INAT, Tunis, Av. Charles Nicolle, 1082 Tunis, Tunisia b c

a r t i c l e i n f o

a b s t r a c t :

Article history: Received 30 March 2017 Received in revised form 11 September 2017 Accepted 4 October 2017 Available online 4 October 2017

The Mejerda River is the one of the major rivers in Tunisia and several studies dealt with the importance of this river in terms of ecology, geochemistry and sociology. However, the sedimentary contributions to the coastal zone from the Mejerda River were poorly observed, and we did not find any continuous monitoring. The main objective of this work is to assess the impact of reduction on sediment transport from the upper basins to the coastal area, after the construction of large dams which may have strong and long-lasting effects on coastal geomorphology and ecosystems. In this paper, a paleo-hydrological approach was applied through the study of sediment cores sampled in the low valley meander on alluvial terrace. We attempt to emphasize, through this approach, the reduction in sediment loads to the sea (Gulf of Tunis) due to the construction of dams on the Mejerda River and to changes in climate conditions. Three cores (C1, C3, C5) were sampled in the lower part of the Mejerda near the hydrometric station of Jdaida with different depths (0.9 m, 1.2 m and 3.0 m) and in different dates (2014e2015). Visible successions of sedimentary layers corresponding to the deposits of successive flood events were identified and the history of the sedimentary contributions to the Mejerda was reconstructed. The thickest layers of sedimentary deposits were related to the exceptional events. They are mainly concentrated on the lower part of the core and are predominantly composed of sands. Since the construction of the Sidi Salem dam in 1981, all of the cores C1, C3 and specially C5 presented mostly a succession of small layers of fine material (silt or clay) without any sand deposits in the downstream River bed. In C5, 18 individual slackwater flood units were identified. A strong decrease in the accumulation rate of sediment was observed based on the length of the flood units and the number of years between flood events. A lower sedimentation rate is observed between 1982 and 2015 of about 2.3 cm/ year, while it was much more important between 1963 and 1981 of about 4.75 cm/year, and around 10 cm/year between 1950 and 1962. The statistical study of precipitation indicated three climatic successive periods between 1951 and 2009 characterized as wet, dry and intermediate. Thus, the sediment rate was higher before 1974 reaching about 8.47 cm/year for the wet condition, of about 4 cm/year for the dry period between 1974 and 1989, and about 1.6 cm/year in the intermediate period afterward to 2009 that also witnessed the construction of the major dams. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Sediment Mejerda River Paleo-hydrology Dam Flood Tunisia

1. Introduction

* Corresponding author. E-mail address: [email protected] (F. Kotti). https://doi.org/10.1016/j.jafrearsci.2017.10.003 1464-343X/© 2017 Elsevier Ltd. All rights reserved.

Sediment transfer and deposition is considered as an important process in the development and maintenance of coastal habitats,

F. Kotti et al. / Journal of African Earth Sciences 142 (2018) 226e233

including wetlands, lagoons, estuaries, sea-grass beds, coral reefs, mangroves, dunes and sand barriers (UNEP/GPA, 2006). In the current context of global environmental and climate changes, modalities of variations in environmental processes including hydrological, sedimentary, and ecological interactions are a sought knowledge. Especially, it is important to distinguish and characterize the processes of change that affected sedimentary equilibrium in coastal areas (Brahim et al., 2014). Approximately 30e40% of suspended matter transported by the world River network is no longer reaching the coasts and it is rather retained in man-made reservoirs, at least for the lifetime of these infrastructures €rosmarty et al., 1997). Also, coastal zone monitoring is an (Vo important task in sustainable development and environmental protection (Alesheikh et al., 2007). However, the sedimentary contributions of Rivers to the coastal zones are poorly measured in semi-arid regions, and there is no chronicles of observations. The history of the sediment transport is not known even for the most important Rivers of the Maghreb, except for some Algerian Rivers (Cherif et al. 2009). Thus, it is not always easy to find in the literature the magnitude and frequency of large floods as well as their sedimentary contributions. However, since the completion of the dams on large Mediterranean watercourses, such as Mejerda in Tunisia, an alarming narrowing of waterbeds has been observed downstream of dams, (Zahar et al., 2008). This adverse geomorphological change is attributed to different factors including the reduction in discharge flows due to reservoir storage. The efficiency of reservoirs at trapping sediment is frequently reported as 70 to 90 per cent of the sediment volume delivered from the watershed (Sundborg, 1992; Toniolo and Schultz, 2005). In general, three impacts were identified by Stanley (1988) on the Nile delta and by Syvitski (2003) at a global scale: (1) the reduction of sediment loads to the sea, (2) the acceleration of coastal erosion, and (3) the modification of deltas and estuaries. In this work, we focus on the sediment contribution by the Mejerda to the coastal zone. It is Tunisia's principal watershed (23,700 km2) and it is one of the longest Rivers of North Africa, sharing its upper basin with Algeria. It is equipped with numerous dams used for water supply and irrigation. Thus, the Mejerda basin is of important interest to the scientific and public communities. It supplies on average 1 billion m3 per year, or 37% of the average annual national surface water resources. Several researchers worked on different thematic related to water balance, flood estimation, water transfer between dams, water quality, and rate of sedimentation within dams (Claude et al., 1976; Jausein, 1971; Barjot, 1952; Abid, 1980; Paskoff, 1994; Ben Mammou and Louati, 2007; Oueslati, 1999; Bouraoui et al., 2005; Louati and Zargouni 2013). Zahar et al. (2008) argued that the dams altered the downstream flow conditions of Rivers, which is confirmed by a previous study (Kotti et al., 2016) and reduced the Mejerda downstream channel capacity and hence inducing an increase of floods since 1996. In 1996, 185 cross-sections of different reaches of the Riverbed were surveyed starting downstream from the Sidi Salem dam (Ghorbel, 1996). Particularly in the gauging station of Jdaida, a section had been previously surveyed in 1976, the cross-section had shrunk by 20% in 15 years after the construction of Sidi Salem dam in 1981 (Zahar et al., 2008). In this paper, the paleo-hydrologycal approach is usually used to reconstruct past flood events on a large time scale. Methods and applied concepts of paleo-hydrology have been described extensively in the literature (e.g., Kochel et al., 1982; Ely and Baker, 1985; Baker, 1987; Benito and Thorndycraft, 2005). The search in this discipline is complex, and requires knowledge across different thematic fields (e.g. geomorphology, hydrology, climatology). The multidisciplinary analysis allows to reveal valuable information on the climatic evolution of the last centuries, on extreme events and finally on the effects of the past

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and recent anthropological actions (Vilanova et al., 2006; Selby and Smith, 2007). Given the lack of documentation on sediment data and the availability of discharge information, we adopt in this work a paleohydrological approach (Dezileau et al., 2014) to determine flood, sediment transport and deposition associated with sediment cores sampled in the low valley meanders on alluvial terraces noted herewith as FD and before the estuary near the hydrological station of Jdaida. A crucial step of the study was the selection of the core sampling point, which should verify several conditions: i) be representative of the total sediment flow to the sea, so being located downstream the last dam and downstream the main structures of water abstraction (for irrigation for instance), ii) be not far from a discharges gauging station, to be able to relate significantly the core deposits with recorded floods, iii) be located in the inside part of a meander showing a natural sedimentation terrace, to ensure regular inundation by each flood, but low water flow velocity to reduce the risk of sediment deposits by erosion, iv) be easily accessible by car to bring the core sampling material, and v) be significantly far from human disturbances. The methodology combines stratigraphic and sedimentological analyses to identify the number of flood units preserved within a particular sedimentary sequence combined with dating techniques to determine the chronology of flood occurrence. We used techniques of stratigraphical analysis and deposits dating (137Cs, 210Pb, geochemistry) to estimate the flood events succession in the study site, and we propose a chronology of floods and deposits. 2. Materials and methods 2.1. Study site description The Mejerda River is the longest of Tunisia, with a 484 km length among which 350 km in Tunisia (Ben Mansoura et al., 2001). The Mejerda drains 23600 km2 among which 7500 km2 are located in Algeria (Fig. 1). The water regime is typically Mediterranean with a low mean annual discharge of 29 m3/s, extreme seasonal variations, and flood peaks around 100 times greater than the mean discharge. Mean annual rainfall in the catchment varies from 400 to 1100 mm (Kotti et al., 2016). Rainfall of over 200 mm can be recorded in less than a day and can therefore lead to devastating flash floods. Since the implementation of the development projects, and more precisely after the construction of the largest dam in 1981, the Sidi Salem dam, the downstream channel has progressively narrowed (Zahar et al., 2008). The downstream part of Mejerda located downward the Jdaida gauging station was frequently modified, either naturally due to floods or by imposed new channels to allow discharge to the sea.

Fig. 1. Tunisia in Africa: Mejerda watershed description, location of dams and main gauging stations, location of the sampling area.

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The lower valley of the Mejerda is a vast plain of deltaic, low and easily flooded origin accusing morphology changes, and new areas gained on the sea due to the filling by sediments deposited by the ancient Mejerda floods (Ben Ayed, 1993). The Mejerda is characterized by low daily flows that are disrupted by flash floods provoked by irregular concentrated rainfall and unevenly distributed on the various seasons. As the flow regime results from the irregularity of the rainfalls, the average values of flow hides an important fluctuation between years, as well as the fact that dry years are more frequent than wet years (Zahar et al., 2008). The water regime was regulated by numerous works built on its main course or its tributaries. Before Sidi Salem dam construction (1981), on average 20 floods per year exceeded 25 m3/s, with 15% of these floods were observed in January, 50% between January and April, and 27% in the autumn (Rodier et al., 1981). The development program of the Mejerda basin was pursued by the realization of several reservoirs: Mellegue (1954), Ben Metir (1954), Lakhmes (1966), Kasseb (1968), Bouheurtma (1976), Siliana (1987), Sidi Salem (1981) and R'Mil (1999) (Fig. 1, Table 1). With the operation of dams, the natural water regime of the River Mejerda is modified. A number of hillslopes reservoirs also exist with a very low storage amount compared to these dams. 2.2. Methodology The sediment (usually fine sands and silts) may be located in protected sites, such as caves and alcoves in the meander and backwater zones behind valley constrictions, in response to flow velocity deceleration (Kochel et al., 1982; Ely and Baker, 1985; Baker and Kochel, 1988; Enzel et al., 1994; Springer, 2002; Webb and Jarrett, 2002; Benito et al., 2003; Benito and Thorndycraft, 2005). For this study, sites of slack water flood sediment deposition were identified along the study reaches. The core drilling site was chosen after several field trips between 2013 and 2015 having for objective the identification of various sites potentially interesting. The core site is located in a meander downstream to the Sidi Salem dam and upstream to the estuary zone. It is located near to the hydrometric gauging station of Jdaida. The gauging station located at almost 7 Km upstream of study sites provides stage observations from 1950. This station provided the potential for correlation between the instrumental and sedimentary flood records. Three campaign of core drilling were taken in the same terrace of the Mejerda River. The selected study site for coring is located in a meander where sedimentary deposits of the river. It is located downstream the last dam on the Mejerda River, before the estuary and near the hydrological station at Jdaida. The first campaign of core drilling and the sampling of the terrace of the Mejerda River were realized on March 2014. A sediment core (C1) of 90 cm was extracted from this terrace. The second drilling campaign was carried out on July 2015. A sediment core (C3) of 1.2 m was

extracted from the same terrace. The last campaign of core drilling and the sampling of the terrace of the Mejerda River were realized on December 15th, 2015. A sediment core (C5) of 3 m was extracted from this terrace. The identification and the highlighting of the various events of floods were realized by the meticulous inspection of every deposit (deduct levels, identification of paleosol, detection of the surfaces of erosion). Sediments grain size were determined using a Beckman Coulter LS 13 320 at the Geosciences Laboratory, University of Montpellier. The geochemical analysis was carried by the XRF Core-Scanner. Dating of sedimentary layers was carried out using 210Pb and 137Cs methods on a centennial time scale at the osciences Montpellier Laboratory (Dezileau et al., 2014). Ge 3. Results and discusion 3.1. Stratigraphic records of flood events on the (FD) Mejerda terrace Numerous deposition units were found in this 3 sediment cores (C1, C3 and C5). The flood deposits consist of fine sand to clay, with many charcoal pieces and ash lens. Median grain size (D50) varies between 5 mm and 232 mm. The sedimentary contributions of C1 and C3 cores are mainly characterized by the silt-clay material intermittently with fine sandy levels. These extracted cores were of only 90 cm and 120 cm length for technical reasons. Different textures are observed the in C1 core with silt at the 34 cm with D50 of 25 mm and at 38 cm with D50 of 35 mm and very fine sand at 71 cm levels with D50 of 84 mm probably corresponding to an exceptional flood event (Fig. 2). For the C3 core, deposits are

Fig. 2. D50 for the sediment cores C1, C3 and C5 in alluvial terrace FD of the Mejerda. Table 1 Characteristic of the largest storages built in the basin Mejerda (DGBGTH, 2016). Dams

Year of construction

Catchment area (km2)

Initial total capacity (Mm3)

Actual siltation volume (Mm3)

Percentage of lost capacity (%)

Mellegue Beni M'tir Kassab Sidi Salem Bouheurtma Siliana Ain Dalia Lakhemes R'mil

1954 1954 1966 1981 1976 1987 1987 1968 1999

10300 103 170 17885 457 972 206 127 221

182 62 81 814 117 70 82 8,22 4

121,94 (on 2000) 1,24 4,97 170,88 (on 2002) 5,46 36,56 (on 2002) * 1,00 (on 2000) *

66,93 * 6,08 20,99 4,65 52,23 * 12,17 *

*Unavailable Data.


usually composed of fine materials with D50 less than 30 mm. There are peaks of very fine sand between 63 and 125 mm observed at 41 cm and 90 cm, indicating a more energetic hydraulic regime generally corresponding to exceptional events (Fig. 2). For the C5 core, deposits are composed of fine sands silt boundary, observed at the 18 cm with D50 of 65 mm, 126 cm with D50 of 49 mm, 147 cm with D50 of 44 mm. The fine sand will start at 212 cm with D50 of 212 mm, 252 cm with D50 of 231 mm and 296 cm levels with D50 of 224 mm. All of deposits are probably corresponding to an exceptional flood event (Fig. 2). Thus, for technical reasons and depth limit of C1 and C3 cores, we were not able to propose a flood chronology based on sedimentological, geochemical and geochronological analysis. There have not chronological tracers for C1 and C3. More particularly, there was no record in flood units with the high 137Cs activity that can be associated to the maximum atmospheric production in the mid-1960s (around 1963). Hence, there has been a quest for deeper core C5 in the same alluvial terrace FD. A visible succession of sedimentary layers corresponding to the deposits of successive floods on the study site has been determined with C5 core. The stratigraphy of C5 consists of 18 individual slackwater flood units (Fig. 3). For the geochemical analysis 3 elements were used: the Rubidium (Rb), the Titanium (Ti) and the Zirconium (Zr). The Rubidium (Rb) element is associated with clays, and is inversely proportional to the percentage of sand. Zr and Ti are present in medium and fine sands. Therefore, the geochemical analysis shows that the lower stratigraphic slackwater deposits units (FD1, FD3, FD4, FD5, FD8, FD10 and FD12) probably correspond to exceptional events. However, the geochemistry of the profile exhibits very high variation between the base and the top of the terrace (Fig. 3). The 137Cs activity is recorded in all flood units FD1 to FD18, with maximum values of 8,2 mBq/g measured in units FD7 (Fig. 4). The accumulation of 137CS in sedimentary deposits into the environment began during early nuclear weapon tests and some nuclear accidents as early as the mid-50s (Popp et al., 1988). The first flood event post-1950, identified by the first trace of 137 Cs activity in the profile, is that of FD1 indicating that the all flood deposits FD1eFD18 can be post-dated to this period. More particularly, the high 137Cs activity recorded in flood units FD7 at 172 cm for a peak value of 8,2 mBq/g can be associated to the maximum atmospheric production in the mid-1960s (around 1963) as shown in Fig. 4.

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Fig. 4. Daily hydrographs at the Jdaida gauging station between 1950 and 2014, and 137 Cs chronology for the sediment core C5 in alluvial terrace FD of the Mejerda.

The 210Pbex activity is recorded in all flood units. The 210Pbex helps confirm a number of results produced using 137Cs dating technique. In particular, the first trace of 210Pbex activity in the profile is that of FD1, thereby indicating that the all flood deposits FD1eFD18 are recent and post-dated approximately to the end of 1910 (