Etude du transport solide en suspension dans l'Oued Mouilah (Nord Ouest Algérien) Study of the suspended load at the river Mouilah (North West Algeria)

Résumé

Les résultats d'analyse graphique des valeurs instantanées des débits solides en suspension dans le cours d'eau de l'Oued Mouilah et leurs relations avec les débits liquides, durant les compagnes de prélèvements de 1977 à 1993, ont permis de montrer l'existence de deux périodes d'érosion actives. Le flux des matières solides en suspension dans le cours d'eau est très variable d'une année à une autre et la dégradation spécifique moyenne annuelle sur les 16 années d'étude est estimée à 126 tonnes par km². Cette valeur est relativement faible par rapport à celles trouvées pour d'autres régions à régime hydrologique similaire.

Mots-clés:

• Pluie,
• débit liquide,
• débit solide en suspension,
• dégradation spécifique,
• bassin versant,
• Oued Mouilah,
• Algérie

Abstract

The extent and rates of alluvial deposit and dam siltation caused by sediment deposition from Maghreb streams have prompted a number of attempts to quantify and explain the complex mechanisms of suspended sediment transport. In Algeria, a country with scarce water resources, deposition of sediments in dams is estimated to average 20 million m³ /year, which contributes to a 0.3 % yearly loss of storage capacity from a total capacity estimated at 6.2 billion m³.

Of interest in this context are suspended sediment loads in Mediterranean Algeria's Mouilah River, on which is built the Hammam Boughrara, a 117 million-m³ capacity dam put into service in 1998.

The Mouilah River basin, situated in northwest Algeria, covers a 2650-km² area and has a 230-km perimeter (Table I).

The Mouilah runs along 124 km, rising at an altitude of 1250 m in Algeria then flowing into Morocco. It is ephemeral; perennial flow sets in near Oujda (Morocco), below which it re-enters Algeria near Maghnia (Figure 1).

The study zone is characterized by a semi-arid climate. From 1977 to 1993, annual mean temperature was 16.7 °C. Rainfall was relatively scarce and unequally distributed throughout the year, with an inter-annual average of 300 mm over the same period (Figure 2).

Analysis of hydrological data

The study used instantaneous water discharge values (m³/s) measured at the mouth of the Mouilah from September 1977 to August 1993 (results calculated and furnished by the National Agency of Hydric Resources [ANRH]). For measured values, suspended loads (g/l) were evaluated using samples taken from the river: total suspended loads were calculated as the product of these concentrations and water discharge. The number of samples was adapted to the hydrological regime: They were taken every other day or, during flood periods, as frequently as quarter-hourly.

Analysis of the instantaneous discharges showed that suspended loads were related to discharge by a power law (Figure 3).

To study the responses of the basin over the hydrological year, we grouped the results - 16 years' worth of data - according to season, and analysed the relationship between liquid discharge and suspended sediment load (Table II).

Graphical analysis of Figure 4 reveals that autumn and the spring are distinguished by strong river discharges leading to important transport of solids. The maximum flow of solids was about 104 000 kg/s, resulting from a water discharge of 1880 m³/s in November 1986. By contrast, winter and summer discharges were much smaller; values did not exceed 220 m³/s in winter and 83 m³/s in summer.

After the dry season, the first rains of autumn encounter dry, hard and barely erodible soil. The response of the basin in terms of suspended-solids generation is therefore very small. It is the heavy rains of October and November that remove large quantities of solids transportable by streams.

After the very dry and cool winter and a succession of freezes and thaws, spring rains fall on poorer soil, leading to relatively high loads, though still lesser than those of autumn.

Summer is marked by very dispersed values encompassing the smallest discharges of the year and some relatively high suspended loads associated with low discharges, the latter arising from seasonal storms.

In summary, stream discharge is very variable throughout the hydrological year. Suspended sediment transport in the Mouilah River basin occurs principally during flood periods. We distinguished two periods of active erosion, one in autumn and another, lesser period in spring.

Annual contributions

An annual balance sheet of solid and liquid contributions shows that these two parameters vary regularly and as a function of rainfall (Figure 5). Annual liquid contributions from 1977 to 1993 were evaluated at 48.7 million m³, which corresponds to a mean flood depth of 18.4 mm, and a low flow coefficient of 6 %. Due to very dispersed and extreme values and variability, rainfall-discharge relations are varied. This leads to inter-annual irregularity for flood depths: consequently, a relationship with annual rainfalls (Figure 6) was difficult to establish. However, we noted a tendency of the form: L_E=0.0009P^1.69 (R=0.74).

The annual mean contribution of suspended sediment at the mouth of the river was estimated at 335 000 tons, which corresponds to a soil erosion rate of 126 tons/km²/year. This value is moderate compared to other basins of the region, such as the Mazafran (Algiers) and Isser (Lakhdaria) river basins, where erosion rates are about 1610 and 2300 tons/km²/year, respectively (Table III).

The inter-annual solid and liquid contributions contrast markedly. Indeed, for liquid contributions, the first and third quartiles are 21.8 and 64.7 million m³, respectively, which shows that 25 % of the annual moduli representing humid years were three times more important than those representing dry years (Figure 7). Furthermore, in annual loads expressed as stream turbidity, variation between the temperate and arid reaches of the stream is apparent.

Figure 8 shows that the highest annual liquid contribution, 117.8 million m³, was recorded during the year 1979-80, and generated a solid contribution of 670 000 tons. This liquid contribution is higher than that of 1986-87, evaluated at 106.4 million m³, which carried 2.69 million tons - an erosion rate of more than 1000 tons/km²/year.

Keywords:

• Rainfall,
• discharge,
• erosion,
• suspended load,
• erosion rate,
• Mouilah,
• Algeria