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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-189
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 May 2018

Research article | 07 May 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

How to determine the effective discharge and its return period in a semi-arid basin? The case of the Wadi Sebdou, Algeria (1973–2004)

Abdesselam Megnounif1 and Sylvain Ouillon2 Abdesselam Megnounif and Sylvain Ouillon
  • 1Laboratoire EOLE, Département d'hydraulique, Faculté de Technologie, Université Aboubekr Belkaid, Tlemcen, Algeria
  • 2LEGOS, Univ. of Toulouse, IRD, CNRS, CNES, Toulouse, France

Abstract. Over a long multi-year period, flood events can be classified according to their effectiveness in moving sediments. Efficiency depends both on the magnitude and frequency with which events occur. In this study, the efficiency of the Wadi Sebdou (North-West Algeria), in a semi-arid environment, is examined through its histogram of sediment supply by discharge classes, established from 31-years of measurements. The effective (or dominant) discharge is the one whose class corresponds to the maximum sediment supply. Three types of subdivisions into discharge classes were compared. The subdivision in classes of equal amplitudes and the subdivision with equivalent discharges were those which allowed a correct distribution of frequencies and supplies of water and sediments. The effective discharges for these two subdivisions were close and almost equal to the half load discharge, i.e. to the flow rate corresponding to 50% of the cumulative sediment yield. The substitution of the flow histogram by a probability relationship and the use of a sediment rating curve enable to infer a theoretical value of the effective discharge. In this basin with strongly irregular flows, the introduction of a probability distribution was tested and assessed, analytical solutions are provided, but the Log-normal and Log-Gumbel laws highly underestimated the effective discharge. Return periods, estimated from the annual series of maximum discharge and half-load discharge, were compared. The former gives the period between hydrological years with discharges higher than the effective discharge (around 2 years), and the latter shows that more than half of the yearly sediment supply is carried by flows higher than the effective discharge only every 7 hydrological years. The study finally emphasized that the distribution of suspended load as a function of liquid discharge was sensitive to the basin and its forcings. On the Wadi Sebdou, the distribution of the sediment load, bimodal before 1988, became essentially monomodal after this date.

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Abdesselam Megnounif and Sylvain Ouillon
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Abdesselam Megnounif and Sylvain Ouillon
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From a classification of river discharges, the effective or dominant discharge is the one whose class corresponds to the maximum sediment supply. The efficiency of the Wadi Sebdou (NW Algeria) was examined over 1973–2004. Two subdivisions allowed a correct distribution of frequencies and supplies of water and sediments. Their effective discharges, close to the half load discharge, was compared to theoretical values based on probabilities and rating curves. Return periods were also explained.
From a classification of river discharges, the effective or dominant discharge is the one whose...
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