Studying sediment at Algeria's dams19 September 2007
B Remini and W Hallouche investigate the problem of sedimentation in Algeria's large dams
Algeria's many dams have to deal with the problem of sedimentation. While studying the country's hydraulic structures, the authors noticed that the dams are silting more quickly than ten years ago - and with differing intensity. The useful life of some dams can be as long as 365 years, whereas for others it can be reduced to 60 years. In Algeria, 18 older dams have been seriously threatened due to the acceleration of silting and will reach the end of their useful life shortly if remediation measures are not carried out. Even the newer dams have not escaped the problem - nine news dams are suffering from 45Mm3 of sediment deposited annually. In total 1.1Bm3 of mud was deposited in 2006 behind the 57 large dams in Algeria, which represent 16% of total capacity. As a result, the authors decided to carry out a classification of silting risks for areas in the north of the country. Four classes of risk were obtained: weak, average, strong and very strong rate of sedimentation.
Sediment in Algeria
Characterised by stripped soils and violent rains, the drainage area of north Algeria records the highest values of erosion, with annual tonnage of sediments reaching 180M tons (Demmak A., 1982). Much of this sediment is often trapped behind dams. The authors recorded contents of sediments exceeding 300g/l in the Agrioum Wadi in the east of Algeria (Remini B., 1997) - this is a considerable amount of sediment particularly when compared with French rivers which do not exceed 20g/l (Keller P., and Bouchard J-P., 1983). In the Maghreb, and more particularly in Algeria, it is autumns risings which cause the most erosion and sediment transport. These risings accelerate the silting of dams causing negative consequences such as a reduction in storage capacity and blocking of sluice gates.
Hydrous erosion corresponds to a separation between the particle and its support, not including transport and sedimentation - even if there is always an element of transport involved (Rampon A., 1990). Drainage area erosion is very widespread in the area of the Maghreb, since it experiences all the conditions necessary to start and develop such a process: the climatic irregularities, low density of vegetation cover, soils which are not resistant to water flows and the violence of storms. Several examples testify to the gravity of the problem. In Algeria the specific rate of erosion reaches the value of 5000t/km2 per annum in the catchment area of the Agrioum Wadi (Demmak A., 1989) (photograph 1). In the basin slopes of Martil, Ouergha, Lakhdar, and Tessaout in Morocco, the rate of erosion exceeds 2000t/km2 per anum (Badraoui A. and Hajji A., 2001). In Tunisia, hydrous erosion degrades cultivable grounds, with 1.2Mha seriously affected by erosion in the north and center of Tunisia, This amounts to 25% of the total ground surface (Bouzid A., 1991).
Data of more than 90 dams of the Maghreb is shown on the graph of Brune G.M. (1953) (figure 1), giving the percentage of sediments trapped in a reservoir. It can be seen that for most of the dams studied, the 'trap efficiency' is found to be 90-98%, demonstrating that the amount of silt in the country's reservoirs is considerable.
State of Algeria's dams
The authors followed the evolution of the capacity of dams in Algeria from 1890. On this date there were nine dams with a combined capacity of 61Mm3 and a volume of mud of 2.7Mm3. This capacity reached 1Bm3 in 1962. From this date the number of dams increased considerably, giving a capacity of 6.8Bm3 in 2006. The volume of mud increased to 1.1Bm3 in 2006 (figures 2 and 3).
Presently there are 57 large dams in operation in the country (figure 4) which together silt up gradually every year to 45Mm3, representing a loss of annual capacity equal to 0.65% per year (see photos 2 and 3).
The authors note that the incidence of sedimentation should be considered important, particularly in light of the fact that 1.1Bm3 of sediment settled in reservoirs in 2006, which is a loss of 16% of total capacity. However, they do suggest that the importance of the phenomenon should not be exaggerated - a sedimentation rate of 0.65%/ per year means that many dams can have a useful life of 140 years without intervention. The authors state this should help alleviate the idea that sedimentation condemns dams in Algeria.
They also state that it should not be regarded as fundamental data of hydraulics in Algeria. The sedimentation rate did indeed increase early in 2000, but this was due to the dry spell encountered in the country during the 1990s which was then followed by a number of wet years. This alternation of dry and wet years caused severe erosion of the catchments. Indeed the 1990s were characterised by weak vegetation cover and dissected and loosened soils, while the early years of 2000 were characterised by intense rains which affected the soils. Consequently, the rains of the last five years were the cause of a strong ablation of the surface layer of the soils. According to bathymetric surveys carried out in 2004 by the national agency of dams, sedimentation at certain dams increased considerably compared to that of 1986 (see table 1).
Thus, the increase in storage capacity involves the deposit of more quantities of sediments as shown in figure 1a and b. Following the introduction of 26 new dams - including Beni Haroun dam which has a capacity of 960Mm3 - storage capacity in the country doubled in 20 years, going from 3.2Bm3 in 1986 to 6.8Bm3 in 2006. This capacity increase will involve an increase in the volume of mud. For Beni Haroun dam, it was suggested the annual deposit of sediment could be as much as 9Mm3. The authors suggest however that currently this number is more likely to be around 3.5Mm3 per year since its current capacity does not exceed 350Mm3.
Dams most threatened by silting
Of the 57 large dams in operation, 18 dams are strongly threatened by silting, since the rate of filling exceeds 55% of their total capacity (figure 5). Four dams - Zardezas, Foum El Gherza (photo 4), Fergoug and Ksob - are currently in the process of being de-silted using dredging techniques. The Beni Amrane, Ghrib, and Foum El Gueiss dams are also being heightened through a programme begun in 2000 by the national agency of dams.
The rate of silting at nine new dams is also very high. Their capacity of 1.2Bm3 will record a loss of annual capacity of 1.6%, representing a useful life of 60 years. The problem is not alarming, but it will be necessary for the project owners to regulate the problem, which may include reducing the rate of erosion at the basin slopes.
Dams less threatened by silting
There are 14 large dams in the country which are not threatened by silting. The total capacity of these dams borders 1600Mm3 (figure 7), but the volume of mud deposited on the bottom of these works does not exceed 87Mm3, amounting to a rate of filling of 5.4% in 2006.
A new water policy was adopted in Algeria in 1996. Five agencies were created to manage water resources in certain catchment areas, covering the following basins: Oranie Chott Chergui; Chellif Zhrez; Algerois Soummam Hodna; Constantinois Seybouse Mellegue and the Sahara. As mentioned early, the current rate of filling at the 57 large dams in operation has been estimated at 16% of the total capacity. This rate of filling however varies between the catchment areas. The dams located in the catchment area of Chellif Zahrez, for example, are threatened by the phenomenon of silting, since the rate of filling is around 27% of the total capacity of these dams, reducing their useful life to less that 128 years. This area is however known by the strong rate of erosion due to the weak vegetation cover and the brutality of the risings. On the other hand, the catchment areas of Algerois and Soummam Hodna experience a rate of filling of 9% of the total capacity of the dams. The useful life of these works will exceed 220 years. This weak rate of filling is the consequence of average erosion of soils.
Sensitivity to silting
The authors established a chart of sensitivity to the silting (figure 10) which shows four types of zones of silting; the zone of weak rate of filling (T<0.5%/ per annum), average rate of filling (0.5% per annum<T<1% per annum), strong rate of filling (1% per annum <T<3% per annum) and very strong rate of filling (T>3% per annum). The rate of filling (T) expressed in % per annum represents the loss of capacity of a dam following the deposits of the sediments. This chart has been produced to give project owners an idea of the effect of silting on operational dams, and the need for intervention. It can also give an idea as to how quickly sediment may affect new dams.
Sedimentation is a natural process that can have dramatic consequences for dams, most notably a reduction in storage capacity. Even if the intensity of silting differs from one area to another, the annual rate of sedimentation for the 57 large dams in operation was evaluated to 45Mm3. The loss of the capacity at different dams ranged from 0.5% per annum to 0.65% per annum of total volume over the last ten years, which was probably due to accelerated degradation of the basin slopes caused by the climatic changes. The volume of the mud deposited in the 57 dams is approximately 1.1B m3, amounting to a loss of capacity of 16% of the total reserve. The classification of the risk of silting in four zones of weak, average, strong and very strong should be considered a tool of forecast during planning, realisation and exploitation of dams.
Table 1 Photo 1: Degradation of the Chellif catchment area Photo 1 Photo 2: Over 50% of the capacity of Ighil Emda dam was lost due to silt Photo 2 Photo 3: 100% of the total capacity of Fergoug dam was affected by silt Photo 3 Photo 4: Desilting of Foum El Gherza dam in 2006 Photo 4 Photo 5: De-silting of Fergoug dam Photo 5 Figure 1: Trap efficiency in North African dams (90 dams) Figure 1 Figure 2: Evolution in the time of total capacity Figure 2 Figure 3: Evolution in the time of sedimentation of Algeria's dams Figure 3 Figure 4: Distribution of dams in operation in North Algeria Figure 4 Figure 5: Geographical situation of the dams most threatened by silting Figure 5 Figure 6: Location of new dams showing a strong sedimentation rate Figure 6 Figure 7: Location of dams threatened by silting Figure 7 Figure 8: Presentation of the catchment areas of Algeria Figure 8 Figure 9: Rate of silting in the catchment areas Figure 9 Figure 10: Chart measuring sensitivity to sedimentation Figure 10 Author Info:
B Remini and W Hallouche, Département de Génie Rural, Faculté des sciences de l'ingénieur, Université Saad Dahlab-BP 270, Blida, 9000 Algérie, Email : [email protected], Tel/Fax : 213 25 43 39 40TablesTable 1