Caractérisation de la sédimentation des barrages en Algérie Characterization of dams sedimentation in Algeria

Résumé

Le problème de la sédimentation des barrages en Algérie est connu dans le monde de par de son importance. Devant cet état de fait la mesure de l'envasement des barrages présente un intérêt certain pour la quantification des sédiments et l'exploitation des ouvrages.

L'objectif de cette étude est de contribuer à la quantification de l'alluvionnement afin de mieux cerner le dimensionnement des ouvrages hydrotechniques, d'accroître leur durée de vie et de réduire les coûts de réalisation.

L'étude porte sur un échantillon de dix neuf barrages algériens situés dans des bassins versants répartis sur l'ensemble du territoire. Une première réflexion a conduit à identifier la diversité des caractéristiques hydromorphométriques (indice d'altitude, densité de drainage, indice de pente, coefficient d'écoulement,...) et climatiques (sub-humide, semi-aride, aride). L'évaluation des volumes des sédiments à partir des mesures topo-bathymétriques effectuées à deux dates différentes montre une perte dans la capacité initiale, soit 44 % de perte en volume total, qui arrive à la limite de la durée de vie pour certains barrages.

Ces derniers résultats ont été exploités dans l'objectif de mettre au point un modèle simple d'évaluation du débit solide accumulé dans les cuvettes des barrages algériens. A partir de cette réflexion, nous avons cherché, graphiquement, les liens préférentiels entre la sédimentation, la superficie et les différents facteurs hydromorphométriques. On distingue trois facteurs déterminants : la superficie, l'indice d'altitude et le coefficient d'écoulement. Ces derniers paramètres ont permis de mettre au point un abaque de classification en fonction de la continentalité et le contexte géographique. Cette taxonomie a été confirmée par l'analyse en composantes principales des données hydromorphométriques des systèmes étudiés.

Mots-clés:

• Sédimentation,
• barrages,
• modèles,
• facteurs hydroclimatiques,
• durée de vie

Abstract

Deposition of sediments reduces available water reserves by 2 to 5% annually in the man-made reservoirs of the Maghreb countries. The sedimentation problem in the reservoirs in Algeria is known in the world because of its great importance. Measurement of reservoir siltation is important for quantifying deposits and for the management of the hydraulic structures.

The A.N.R.H. (Agence Nationale des Ressources Hydrauliques) first attempted to monitor sediment loads by measuring the transport of suspended particles (solids) at hydrometric stations in a punctual and irregular manner (draw-off by section without measuring the flow velocity), but the real value of a stream's sediment load is not necessarily well reflected by point sampling (TOUAT, 1989; TOUAT 1991). Therefore, small experimental drainage basins (several hectares in surface area) were equipped to study sedimentation in the hydraulic structures solely by slope erosion (RAMDANE, 1989). The specific rate of erosion in catchment areas in northern Algeria is the highest in the Maghreb. Often it is over 2000 T/km_/year in the majority of the Tellian Altas basins, and it reaches 4000 T/km_/year in the Dahra coastal catchments. These latter values are among the highest in the world (DERI and HUSSON, 1976; HEUSCH, 1970). Variation of sediment transport across the wetted cross section was demonstrated by gauging the suspended particles transport (Tss). Measurement of the transport of bed material is not reliable, and often it is estimated as a percentage of the Tss.

The aim of the present study was to improve sedimentation estimates in order to define proper hydrotechnical structures and to increase their lifetime (reducing the costs).

Firstly, we consider information stemming from the topo-bathymetric processing of 19 reservoirs in Algeria. From the temporal point of view, the information is not homogeneous (not synchronous) but has the advantage of integrating all the types of sediments (suspended sediments, bed-load, bank erosion, deposits). The first analysis involved the topo-bathymetric survey of basins done by the A.N.B. (Agence Nationale des Barrages) (DERI, 1977; SOGREAH and A.N.R.H., 1989) whose data were used as the basis for the calibration of our models. These latter data then allow one to predict the lifetime of recent reservoirs. Calibration consists of defining the most influential hydroclimatic or physico-geographic factors affecting the abrasion rate.

We therefore considered 19 Algerian reservoirs located on basins distributed over a large part of the country. A first analysis led to the identification of the variability of the hydromorphometric (elevation index, drainage density, slope index, flow coefficient, etc.) and the climatic (subhumid, semiarid, arid) characteristics of the studied sample. The evolution of the volumes of sediments from two topo-bathymetric measurements, which were carried out on two different dates, shows a variation between 0.017 Mm3/year and 2.490 Mm3/year. This sedimentation resulted in a loss of initial capacity, which caused the termination of life for some reservoirs. The initial capacity of the 19 considered reservoirs was about 1900 Mm3; it was reduced to 1070 Mm3 in 1986. This constitutes a loss of 44% of the initial volume.

These latter results were exploited with the aim of setting up a simple model for estimating sedimentation and classifying the Algerian reservoirs. To this end, we graphed the preferential links between sedimentation, surface area, and the different hydromorphometric factors. Three determining factors were identified: the surface of drainage basin, the elevation index and the flow coefficient. These latter parameters allowed a spatial partitioning of the examined sample into three groups relating to the continentality (elevation index) and to the geographical context: class Ia represents the catchment basins of the high plateau zone (hard and semiarid climate, a low abrasion rate); class Ib corresponds to an intermediate position (with a moderate physico- geographic characteristic, the cross between two climates: semiarid and subhumid); and class II shows the coastal drainage basins that are influenced by the subhumid climate (an important abrasion rate). Essentially, sedimentation within the hydraulic structures depends on the precipitation, the surface area of the catchment, the inflow (flow coefficient) and the relief (elevation index).

In order to validate these first results, we further analyzed the data table corresponding to the hydromorphometric factors and the abrasion rates. The study of such a data table is generally carried out by interpreting the information by means of multidimensional statistical methods (principal components analysis, factorial discriminant analysis, factorial correspondence analysis, etc.). These descriptive methods, which disregard part of the initial information, yield an explanation of the structure of the data table in terms of hydrokinematics (ROLET and SEGUIN, 1986a and b). Principal components analysis, based on multivariate analysis, implies a mode of deductive reasoning (MUDRY, 1991). Application of this approach showed the presence of sub- groups, based on hydromorphometric criteria, which resembled those obtained earlier in the graphical analysis. This method also corroborated the formulated observations in the classification analysis (graphical analysis).

Calibration of the mathematic functions, based on the minimization of errors by the least squares method, on the data series of "abrasion rate surface" according to the observed data of deposits, showed a significant correlation between the observed and calculated values of the abrasion rate. These latter models did not integrate all the determining factors, but they can be useful for predicting the lifetime of recent reservoirs. These last tasks complete the results obtained from the previous models based on the classification (graphical, principal components analysis) and reveal great efficient means.

To validate our models, we extended our application to the other sites presenting neighbouring conditions. We then inserted in our abacus the deposition values for seven Tunisian reservoirs and sixteen Moroccan reservoirs. The three countries (Algeria, Tunisia, Morocco) are situated in the same physico-geographical conditions. The Tunisian and Moroccan reservoirs behave similarly to those in Algeria and present a high abrasion rate.

Another task is the application of the taxonomy already defined (classification abacus model) to recent reservoirs to determine their lifetime. Regarding the obtained results, it turns out that the reservoirs that have a short calculated lifetime must be fitted out first, and a specific protocol for their exploitation must be established.

Finally, we can imagine the use of our abacus to include the bathymetric results of the reservoirs in order to analyze the evolution of the abrasion rate in small catchment areas. It is suggested that further investigation be undertaken in order to better understand the hierarchical organization of the factors that govern sedimentation in the Maghreb.

Keywords:

• Sedimentation,
• reservoirs,
• models,
• classification,
• statistics,
• lifetime