Systemic analysis of sediment connectivity in the Wadi Leben watershed, Tunisia (Mediterranean): Between morpho-structural potentialities, originality of the hydrographic network and societal reorganizations

The Kneiss Islands and the outlet of the Wadi Leben watershed (Tunisia), is an internationally recognized site, classified ZICO (1990) and Ramsar (2007) for the high fish productivity of its shallow waters, which makes these areas a favorable breeding ground for migratory birds. The difficulties to access to the sea make these areas slightly degraded. However, the sustainability of this site is now being jeopardized with anthropogenic pressure becoming progressively stronger on this littoral zone and the upstream watershed.

Our objective is to show the original hydro-sedimentary functioning of this Mediterranean watershed, shared between physical characteristics and intrusive agricultural practices in full reconversion (from local/traditional to international/globalized): (i) The upstream zone of the watershed is subject to sediment retention related to the implementation of a hydro-agricultural and anti-erosion system (water and soil conservation works); (ii) The intermediate zone, by its morpho-structural and sedimentogenetic characteristics, has favored the accumulation of sediments in the valley bottoms, which makes it not only a transfer zone but also a sediment-recharge zone that can substitute for the upstream; (iii) The downstream zone, which could be expected to be sensitive to these upstream conditions, which may have altered its morphosedimentary equilibrium and caused a retreat of the coastline and the estuary, and accelerate soil salinization in the coastal plain, records a completely different hydro-morphological behavior, where the role of high-magnitude, low-frequency events (i.e., exceptional floods) is highlighted to understand its recent geomorphological dynamics.

In a methodological point of view, this research is based on a systemic analysis used to better understand the hydro-sedimentary connectivity. Topographic field measurements were carried out on a small representative watershed (7 km²) in order to estimate sediment storage induced by agricultural practices. Results were then extrapolated at the watershed scale (1215 km²) according to a modeling approach. Channel adjustments including the linear and lateral erosion and sedimentation processes were then estimated according to twenty ¹³⁷Cs and ²¹⁰Pb dating carried out in the transfer zone as well as in the wetlands (Sebkhas and shotts) located at the outlet of the watershed.

As a conclusion, we indicate that Wadi Leben can be proposed as a model of hydro-morpho-sedimentary functioning showing the efficiency of sedimentary recharge as a key point of equilibrium of the watershed as soon as the river is capable of lateral displacements, thus joining the question of the re-dynamization of rivers in the perspective of the river-bed restoration. A balance sheet leading to the assessment of the sustainability of economic practices (agriculture, industry, tourism, fishing) in the context where change factors (water resources, hydrological regime, sedimentary stock) may affect the hydrosystem in its watershed in the more or less long term is also proposed.