Etude du transport solide en suspension dans l'Oued Mouilah (Nord Ouest Algérien)

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.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 dataThe 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: LE=0.0009P1.69 (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

Production et transport des matières solides en suspension dans le bassin versant de la Haute-Tafna (Nord-Ouest Algérien)

Dans ce travail on s'intéresse aux sédiments transportés en suspension par le cours d'eau Oued Tafna à Beni-Bahdel. Une analyse de la variabilité de la charge en suspension de l'eau quant aux fluctuations des régimes hydro-climatiques, pour la période allant de septembre 1988 à août 1993, montre que la production des sédiments est très forte en automne mais de faible ampleur au printemps. En été, des crues éphémères fortement chargées sont parfois observées. En automne, le flux des matières solides en suspension représente 44% du flux annuel estimé à 286000 tonnes, alors que les apports en eaux ne représentent que 17% de l'apport annuel moyen évalué à 30,9 millions de m3. Au printemps, malgré l'augmentation des débits (60% de l'apport total annuel), la charge en suspension diminue de façon significative.Cependant, les sédiments ramenés par le cours d'eau sont produits soit par ruissellement superficiel sur les versants soit arrachés du lit et des berges du cours d'eau. On montre alors que la contribution de ces deux mécanismes d'érosion reste remarquablement constante d'une année à une autre. La production du ruissellement superficiel en matières solides représente alors 62% du flux annuel alors que l'érosion du cours d'eau contribue par 38%.Knowledge of suspended sediment loads carried by rivers provides insight into the underlying erosion mechanisms. In Maghreb rivers there are few data available on suspended sediment transport (PROBST and SUCHET, 1992). In this context knowledge of suspended sediment loads in the Oued Sebdou River (Mediterranean Algeria), which is a main stream in the Upper-Tafna basin, is of interest. The purpose of this study was to determine periods of active erosion and estimate the contributions of slope erosion and channel erosion in this basin.Situated in northwest Algeria, the Upper-Tafna basin covers an area of 256 km2 (Fig.1). Rising in Ouled Ouriach, the upper reaches of the river flow through Jurassic soils at altitudes up to 1400 m. These tributary streams join on the Sebdou plain (900 m), which is composed of Plio-quaternary alluviums. The soil is composed of calcareous-marnes, calcareous and Jurassic dolomites (BENEST, 1972; BENEST et al., 1999) until the Beni-Bahdel dam.The study zone is characterized by a semi-arid climate. Analysis of the rainfall data at the Beni-Bahdel station (X=34°42'33"; Y=01°29'48"; Z=660 m) from 1939-1940 to 1997-1998 (Fig. 2) demonstrates that the rain supply has decreased greatly since 1975, with more than an 18% decrease in rainfall amounts.Hydrological data and methodology The study used instantaneous water discharge and suspended sediment load values (1257 values) measured and supplied by the National Agency of Hydrologic Resources [ANRH]. Five annual water years have been investigated during the period from September 1988 to August 1993. The discharge values (QL, in m3/s) were determined from the rating curve for the measured heights of the water at the gauging station. The suspended load (g/l) was determined on water samples taken from the stream channel banks. The number of samples taken was adapted to the hydrological regime (i.e. every other day or during flood periods) and samples were collected as frequently as increments of 10 cm.The suspended sediments were carried from the slopes into the river by surface runoff or by stream channel erosion caused by flowing water. In order to express the contribution of these two mechanisms, we applied hydrograph separation methods used by ETCHANCHU (1988), ETCHANCHU and PROBST (1986), KATTAN et al. (1987) and PROBST and BAZERBACH (1986).Balance sheet of solid and liquid contributionsThe annual liquid contributions from 1988 to 1993 were evaluated to be 30.9 million m3. The abrupt slopes of the basin cause water to move quickly with a lot of force. In turn a large amount of suspended sediments (estimated at 286,000 tons) is generated, which corresponds to a soil erosion rate of 1120 tons/km2/year. This value is comparable to those estimated by WALLING (1984) where the erosion rates ranged between 1000 and 5000 tons/km2 /year.As seen in Table 1, the annual suspended sediment load varies dramatically. In 1990/91, the soil erosion rate was estimated to be 4283 tons per km2 ; this amount was 180 times higher than the sediment load in 1992/93 (24 tons per km2). Furthermore, there is no relationship between annual suspended sediment load and water discharge or annual rainfall. The annual precipitation (355 mm) was recorded during the year 1988/1989, and generated a soil erosion rate evaluated at 1072 tons per km2. For a similar rainfall, 345 mm recorded during the year 1992/93, the erosion rate was only about 24 tons/km2 /year.Autumn liquid contributions represent only about 17% of the mean annual supply (Fig. 4). Furthermore the heavy rains produce higher peak flows as the rain encounters dry soil with less vegetation cover and produces large quantities of solid by heavy rain splash erosion. The mean seasonal suspended sediment concentration was higher representing approximately 44% of total annual production. Spring corresponds to a dilution in sediment concentration caused by a large groundwater contribution. The volume discharged by spring flow is about 60% of total annual river discharge. The solids transported during this period are mainly eroded from the stream banks and represent 36% of total annual sediment load.Calculated values of monthly suspended sediment load and water supply are reported in Table 3. The monthly suspended sediment load carried by the river was very irregular. The majority (94%) of the total sediment load was transported in only three months, 45% in September, 30% in March and 19% in July. The monthly amounts varied from year to year, and this can be explained by the occurrence of floods (Table 4).The river suspended sediments were carried from the slopes into the river by surface runoff water or by stream channel erosion. Using a hydrograph separation method, the contributions of these two mechanisms are summarized in Table 4. According to these values we observed that : - The contribution of slope erosion processes represents an average of 62% of the total river-suspended sediment transport. - The second contribution from channel erosion represents at least 38% of the total river transport. This percentage is comparable with those obtained by ETCHANCHU and PROBST (1986) for the Garonne River basin (30%). This contribution is higher than those estimated by KATTAN et al. (1987) (22% for the Senegal River basin) and by ROBINSON (1977) for some American rivers. - This contribution was far less than those estimated by DUYSING (1985) for a forest stream in Luxemburg, where sediment produced by surface runoff was about 62% of annual suspended sediment loads. - This estimate is comparable to those estimated by where the erosion rates range between 1000 and 5000 tons/km2/year. - Although the annual amount of suspended sediment was variable, the relative contributions of the slope and channel erosion were less variable from year to year

Sensitivity analysis of urban flood flows to hydraulic controls

Water Qualit

STATIONS D’EPURATION A LITS FILTRANTS PLANTES DE MACROPHYTES WASTE WATER TREATMENT PLANTS WITH MACROPHYTES

L’utilisation de marais pour l’amélioration de la qualité de l’eau est une technique très ancienne. Depuis que l’homme rejettedes effluents pollués dans le milieu naturel, les marais ont été impliqués – volontairement ou non – dans l’épuration de ceux-ci.De nombreuses recherches sont actuellement menées afin de mieux comprendre les phénomènes intervenant dans le sol pourl’épuration, l’influence des conditions climatiques, les tailles et formes des lits, la charge hydraulique et le type de plantesutilisées. Le présent travail se veut une contribution à la mise en valeur des techniques d’épuration des eaux usées domestiquespar lits plantés de macrophytes

Experimental investigation and performance analysis of Archimedes screw generator

The generation of renewable energy with Archimedes screw generators (ASG) transforming potential energy of fluid flow into mechanical energy is a growing technology suitable for low-head hydraulic sites. This paper presents an improved theoretical model linking screw performance to screw geometry and flow conditions. This model takes into account leakages, friction losses and variable fill levels. The modelled values of torques and efficiencies are in a fairly good agreement with experimental results obtained for a laboratory-scale screw. The downstream screw immersion is shown to impact ASG efficiency and an optimal immersion level is proposed. It has been found that fluid friction on the screw is not negligible. The analysis shows that a single value of the friction coefficient is suitable for modelling the screw performance under various flow conditions. The leakage phenomenon at under-filling flow conditions and friction forces in complex turbulent flows need to be further studied

Lagrangian model for deposition from low-concentrated particles-laden turbulent jets

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.This work presents a simple model that accurately predicts the deposition from low-concentrated particle-laden turbulent jets in different scenarios. Our proposed model is a developed Lagrangian model that takes advantage of the preferential concentration phenomenon. The unidirectional coupling (fluid- sediment) is used in the modelling. This choice is adopted because when the concentration of solid particles is small enough, it does not affect the hydrodynamic development of the jet. The deposition criterion states that the particle deposits when its settling velocity is greater than the vertical component of the entrainment velocity. Six experiments chosen from the available literature are used to validate the model. These experiments cover the cases of horizontal and inclined buoyant jets in stationary ambient, horizontal buoyant jets in coflow current and nonbuoyant horizontal jets in stationary ambient. Good agreement between the experiments and the obtained simulations is revealed. A sensitivity analysis study is conducted in order to investigate the role of the model main variables. We found that the most important variables are respectively the settling velocity of solid particles, the jet initial velocity, ambient velocity and the buoyancy forces.dc201

A graphical method to study suspended sediment dynamics during flood events in the Wadi Sebdou, NW Algeria (1973-2004)

Small sub-basins are numerous in Mediterranean area and global sedimentary budgets cannot be obtained without a detailed understanding of the hydroclimatic processes that govern sediment fluxes in these small river systems. In this study, the shape of the relationship between sediment concentration (C) and water discharge (Q) during flood events of a 31-years period (1973-2004) was analyzed at the outlet of the Wadi Sebdou basin (256 km(2)) in northwest Algeria, using a new graphical analysis method based on features of hysteresis loops. Each flood was decomposed as successive stages - each of which being characterized by a sediment rating curve - and used to quantify the contribution of diverse sediment sources within the basin. Seven common classes of hydrological events (single valued and six hysteresis loops: clockwise, counter-clockwise, eight-shaped, single valued plus one loop, either clockwise or counter-clockwise, and single valued plus at least two loops) were explored. Sediment supply from locally derived sources (or "base load") was high and reached 77% of total sediment yield for the study period, and was attributed to weathering of bed material or gullies. The remainder portion was derived from hill-slopes, re-suspension of fresh fine deposits in the river bed, or bank collapse. The ratio of suspended sediment load derived from active sediment source varies considerably from one flood to another depending on many factors, such as seasonality and antecedent type of flood. The simpler floods were the most frequent ones but produced less sediment. The most frequent floods were clockwise or anticlockwise (52% of floods) but brought only 34% of the total sediment flux. The 18% of the most complex floods (single valued plus at least two loops) produced more than 53% of the sediments, mainly supplied by base load (84%). Intra-annual variability was very high. Over 31-years, the five biggest floods cumulated 64% of the total sediment flux and were of three classes, the most complex floods being dominant. The largest contribution occurred during one single 33-days long hydrological event with complex form which represented 26% of the total sediment flux. Apart from the five biggest flood events, the base load represented 53%. The biggest floods favour base load as compared to the smallest ones