Sediment deposition in major reservoirs in the Zambezi basin

Transcription

1 Challenges in African Hydrology and Water Resources (Proceedings of the Harare Symposium, July 1984). IAHSPubl. no Sediment deposition in major reservoirs in the Zambezi basin INTRODUCTION P. BOLTON Hydraulics Research Ltd, Wallingford, Oxfordshire OXlO 8BA, UK ABSTRACT Fragmentary information from various sources can be used to give order of magnitude estimates of the present rates of sediment deposition in Lakes Kariba and Cabora Bassa. Such estimates are used to identify the possible implications for future reservoir operation and for long-term regional planning. This information may be used to plan appropriate sediment monitoring programmes within the limitations of available resources. The study suggests that sediment deposition in Lake Kariba will have a negligible effect for many centuries whereas in Lake Cabora Bassa its effect may be appreciable within a few decades. Dépôt de sédiments dans les grands réservoirs du bassin du Zambèze RESUME Des renseignements fragmentaires provenant de diverses sources peuvent être utilisés pour obtenir des évaluations des ordres de grandeur des taux actuels de dépôt de sédiments dans les Lacs Kariba et Cabora Bassa. Ces évaluations sont utilisées pour identifier les conséquences possibles, aussi bien pour l'exploitation future des réservoirs que pour la planification régionale à long terme. Ces renseignements peuvent être utilisés pour planifier des programmes appropriés de contrôle de la sédimentation dans la mesure des ressources disponibles. L'étude suggère que le dépôt de sédiments dans le Lac Kariba aura un effet négligeable pendant de nombreux siècles tandis que dans le Lac Cabora Bassa, son effet pourra être appréciable en l'espace de quelques dizaines d'années. Very few reservoirs in Africa have been adequately studied prior to impoundment for the purpose of determining the probable rate of sediment accumulation and its implications. The reservoirs of the Zambezi basin are no exception. The neglect of this question arises in part because the effects of sediment deposition are long term and considered to be negligible within the "economic life" of most projects and in part because the effects are difficult to quantify from the fragmentary information generally available. Nevertheless, order of magnitude calculations based on information and data drawn from a variety of sources can be useful in identifying where future problems may arise, so that account may be taken of these effects in the planning of regional economic development and in the organization 559

2 560 P.Bolton of cost effective sediment monitoring programmes. In this paper estimates are made of the rate of sediment deposition in the two largest reservoirs in the Zambezi basin, Lake Kariba and Lake Cabora Bassa. These reservoirs have been selected for study not because they are in the most immediate danger from siltation (many smaller reservoirs will be affected sooner) but because of their importance within the economies of Zambia, Zimbabwe and Mozambique and their dominant influence over the future development of the lower Zambezi valley. In addition, comparison of the results reveals significant differences between the two cases arising from important contrasts in their design characteristics and operating procedures. LAKE KARIBA Estimated rate of sediment deposition The rate of sediment deposition in Lake Kariba was briefly considered in the pre-impoundment studies (see Central African Council, 1951). A useful life of 1000 years was foreseen on the basis of a limited number of samples taken at Kariba Gorge between 1948 and The calculation of this figure has been examined by Bolton (1983a) who concludes that when allowance is made for an increase in reservoir capacity, in the project as finally built, and when an apparent computational error is rectified, the data from these samples suggest that the period necessary to completely fill the reservoir's "dead" storage capacity should be about 5000 years. To provide independent verification of this estimate, the general characteristics of the drainage basin were studied alongside specific information and data derived from a number of published sources. The area of Lake Kariba's drainage basin is approximately 650 x 10 km. Of this, 480 x 10 km lies upstream of the Victoria Falls. It may be assumed, with reasonable justification, that the Barotse Plain and Chobe Swamps act as sediment deposition zones for virtually all the sediment from this part of the basin. Tributaries downstream of the Victoria Falls, draining directly into the Gwembe Trough, 3 2 comprise the remaining 170 x 10 km of Lake Kariba's drainage basin (140 x 10 km lying in Zimbabwe and 30 x 10 km lying in Zambia) and it is from these that the bulk of the sediment deposited in Lake Kariba originates. Published studies relate only to the drainage basins of tributaries in Zimbabwe. It will be assumed that the sediment yield from the tributaries in Zambia is similar. Ward (1980) and Chikwanha (1980) studied two tributaries of Lake Kariba, the Gwaai and the Umsweswe. between 1975 and Despite significant differences between the characteristics of the two basins upstream of the sampling sites the results suggest that the sediment yields of the two were similar at 2 1 about 40 t km year. The mean sediment concentrations differed significantly (approximately 1600 mg 1" for the Gwaai and 500 mg 1~ for the Umsweswe). In seeking to derive mean values of sediment yield from these results, account must be taken of two important characteristics of the drainage basins: firstly, they lie on the plateau rather than on the steeper escarpment slopes of the Gwembe Trough; and, secondly, neither are undergoing accelerated erosion due

3 Sediment deposition in Zambezi reservoirs. 561 to population pressures. Zimbabwe has, for many years, had an active research programme into the implications of soil erosion for arable agriculture. Of particular interest is the study by Stocking & Elwell (1973) of potential erosion hazard throughout Zimbabwe based on various physical and land use parameters. Although it is unlikely that their approach can provide quantitative predictions of the rate of erosion in a given area, it has been useful in identifying regions of the country where erosion rates are potentially high. In Fig.l a simplified version of their results is presented for the tributary basins to Lake Kariba and Lake Cabora Bassa within Zimbabwe. The drainage basins studied by Ward (1980) and Chikwanha (1980) which flake Cabora Bassa Drainage basin of Lake Cabora Bassa Drainage basin of \Lake Kariba Potential erosion hazard based on Stocking and Elwell Drainage basins studied by Ward and Chikwanha Very low to low J Below average to average S = Gwaai U = Umsweswe H = Hunyani Above average to very high FIG.1 Potential erosion hazard in tributary basins in Zimbabwe. are outlined in Fig.l lie largely in regions where the potential erosion is considered to be below average or lower. The value of mean yield found by Ward and Chikwanha is, therefore, likely to provide only the lower limit of the estimated mean yield for the whole area under consideration. Significant parts of the escarpment of the Gwembe Trough have potential erosion rates classified by Stocking & Elwell (1973) as above average or higher. Such regions are likely to have an appreciable effect on the mean yield of the whole area and, since local erosion rates may vary by several orders of magnitude, a safe estimate for the upper limit of the mean yield for the whole area, in the absence of further information, would probably be a factor of 10 greater than the lower limit. In other words, the mean yield of the drainage basin of Lake Kariba downstream of the Victoria Falls lies in the range t km~ 2 year -1. On the basis of the foregoing, admittedly broad, range of values

4 562 P.Bolton for sediment yield, the mean annual input of sediment to Lake Kariba lies in the range 7 to 70 x 10 t, assuming that tributary reservoirs in Zimbabwe trap only a small proportion of the total yield. Ward (1980) suggests that the dry density of submerged sediment deposits in reservoirs in Zimbabwe probably lies in the range t m~. In the case of Lake Kariba the long time-scale involved suggests that most of the sediment will reach a high degree of consolidation. For this reason a dry density of at least 1.0 t m is likely to occur. Thus, the estimated annual rate of loss of storage capacity in Lake Kariba lies between 7 and 70 x 10 m. Effects of sediment deposition The "dead" storage capacity of Lake Kariba is reported to be 116 x m, which represents over 60% of the total reservoir capacity. In view of the concave shape of the reservoir's longitudinal profile and the relatively small drawdown (9 m maximum) it is anticipated that the bulk of the incoming sediment will reach the "dead" storage. At the present rate of input this storage would be filled in years. It is, therefore, reasonable to conclude that the effect of sediment on the operation of the project can safely be ignored. On the other hand, siltation will probably occur in some localized areas principally at tributary inlets. Although those deposits may be small relative to the capacity of the reservoir, their effect on fisheries and navigation may be appreciable. For this reason it maybe necessary to undertake periodic surveys of areas which are at risk. LAKE CABORA BASSA Estimated rate of sediment deposition No quantitative estimate of the rate of sediment accumulation in Lake Cabora Bassa was published in the numerous reports which formed the basis for the design of the project. Furthermore, none of the engineers or officials, contacted by the writer, who were concerned with the project's construction or its present operation considered siltation to be a significant problem. Siltation is widely believed to be occurring at the same rate as at Kariba. As with Lake Kariba, a large part of the drainage basin of Lake Cabora Bassa (which totals 1 x 10 km ) can be neglected as regards the input of sediment to the reservoir. The Kariba Dam regulates approximately 65% of the basin, trapping all its incoming sediment. A further 15% of the area comprises the Kafue basin where sediment is trapped not only by two storage reservoirs but also in the flood plain of the Kafue Flats. The parts of the drainage basin which are likely to contribute the bulk of the sediment input are as follows: the basins of tributaries in Zimbabwe (42 x 10 3 km 2 ), the basins of minor tributaries in Zambia and Mozambique (35 x 10 3 km 2 ); and the Luangwa basin (148 x 10 3 km 2 ). Ward and Chikwanha studied one of the tributaries in Zimbabwe which flows into Lake Cabora Bassa, the Hunyani. However, their study was in the head reaches of the river at a point where the

5 Sediment deposition in Zambezi reservoirs 563 discharges are partially regulated by a dam. The work of Stocking & Elwell (1973) suggests that Cabora Bassa's tributaries in Zimbabwe may have a higher mean yield than the ones flowing into Lake Kariba. On the other hand, about 10% of their area is regulated by reservoirs, including the Darwendale Reservoir. Offsetting sediment retention in such reservoirs against higher yields, the total sediment discharge from these tributaries into Lake Cabora Bassa probably lies in the range 2 to 20 x 10 s t year -1. The Luangwa basin, in Zambia, is the principal source of sediment to Lake Cabora Bassa. Qualitative study suggests that the yield of this basin will be higher than that of the tributaries considered above because of its physical and hydrological characteristics and because erosion rates have been accelerated by changes in land use in the present century. Although no systematic quantitative study of sediment transport rates has been published, data from an intermittent sampling programme undertaken by the National Council for Scientific Research in Lusaka indicate that sediment concentrations rarely fall below 1000 mg 1 _1. The foregoing evidence provides little basis for deriving an estimate of mean annual sediment yield for the basin. Nevertheless, it seems unlikely that it will be less than 100 t km~ or greater than 1000 t km -. Using these values, the rate of sediment input to Lake Cabora Bassa from the Luangwa lies in the range 15 to 150 x 10 t year". As with the Luangwa, there is little direct information on the sediment yield from basins of the minor tributaries in Zambia and Mozambique, although one of the planning reports for the Cabora Bassa 2 1 Project suggested that it may be of the order of 200 t km year Using a range of values slightly below that used for the Luangwa basin would give a sediment input rate from these tributaries into Lake Cabora Bassa of 3 to 30 x 10 t year -. On the basis of the foregoing estimates, the total rate of sediment input to Lake Cabora Bassa appears to lie in the range 20 to 200 x 10 t year -. Making the same assumptions about the consolidation of deposits as for Kariba, the loss of storage capacity therefore lies in the range 20 to 200 x 10 m year -. Effects of sediment deposition The sediment input rate to Lake Cabora Bassa appears to be a factor of 3 greater than that to Lake Kariba, but its storage capacity is considerably smaller. In the design of the Cabora Bassa Project the minimum drawdown level of the reservoir was taken to be 295 m a.m.s.l. 9 3 The "dead" storage, below this level, is approximately 12.5 x 10 m. This would be filled in a period of years at the rates of sediment input estimated above, if all sediment were to be deposited in the "dead" storage. Mathematical model studies of the project have shown, however, that because of the loss of efficiency in the generating equipment as the hydraulic head falls, there is no net benefit, in terms of energy production, in drawing down the reservoir to this level. A minimum level of 305 m a.m.s.l. or above would be preferable in this respect and would more than double the "dead" storage capacity and hence its siltation life. At the same rate of 9 3 sediment input, the reservoir's total storage capacity (72.5 x 10 m ) would be filled in a period of years.

6 564 P.Bolton In the time-scales of economic planning it would appear, at first sight, that, although the loss of storage capacity is occurring much more rapidly than in Lake Kariba, sediment deposition in Lake Cabora Bassa can also be neglected. However, study of the shape of Lake Cabora Bassa (Fig.2), suggests that a significant proportion of the incoming sediment will accumulate within the "live" storage of the Distance from dam (km) FIG.2 Lake Cabora Bassa: outline (at normal maximum level) and longitudinal profile. reservoir and thereby affect its operation much sooner. The longitudinal profile of the reservoir is convex and the bulk of the incoming sediment passes into a broad shallow basin, the Mucangadze Basin, which lies wholly above 295 m a.m.s.l. and which is separated from the deeper parts of the reservoir by a narrow reach constricted by islands. These characteristics will tend to prevent appreciable quantities of sediment from being transported into the "dead" storage until a later stage in the reservoir's development. A detailed study of the hydrology of the project (Bolton, 1983a), has shown that, with the present installed hydroelectric capacity, the principal constraint in its operation is not the generation of power but the regulation of flood discharges. The maximum discharge capacity of the dam's spill gates is less than the mean 3 month inflow of the year design flood. The operating procedures for the project must, in consequence, follow a flood rule curve which provides an annual drawdown in advance of the wet season to provide sufficient flood storage to prevent the dam from being overtopped. The appropriate value to use for the lowest drawdown

7 Sediment deposition in Zambezi reservoirs 565 level of the flood rule curve is a matter of some dispute; values between 315 and 325 m a.m.s.l., on 1 February each year, have been proposed in consultants' reports but rigorous application of British design recommendations (Institution of Civil Engineers, 1978) would require even greater values of drawdown. To illustrate the possible effect of sediment on the project, it will be assumed that an annual drawdown level of 316 m a.m.s.l. is adopted and that approximately one half of the incoming sediment is deposited in the reservoir zone between 316 and 326 m a.m.s.l. Thus, at the end of 50 years the loss of flood storage will be in the range 0.5 to 5 x 10 m. To compensate for this loss of storage capacity the annual drawdown would have to be increased. Calculated on the basis of the pre-impoundment reservoir capacity curve the extra drawdown required would be in the range m. However, values calculated in this way would in practice need to be increased both because the incremental capacity provided by the extra drawdown would itself be affected by sediment deposition and because, if the drawdown were to be increased, it would be necessary to further increase the flood storage to compensate for the reduced discharge capacity of the spill gates at the reduced reservoir levels of the early flood season. Taking account of these effects, the necessary increase in drawdown after 50 years, to provide the same degree of flood protection for the dam, would be in the range m. It should be stressed that the results of the above calculations are dependent both on the form of the flood rule curve finally adopted by the project's operators and on assumptions about the distribution of sediment in the reservoir. Nevertheless, at the higher rates of sediment input which may occur within the range estimated above, sediment deposition is likely to have an appreciable effect on the project's operation within its "economic life". In particular the additional drawdown needed to compensate for the loss of flood storage will, in all probability, restrict the peak power output of the project during the flood season and may also jeopardize its firm power production. The effect of sediment accumulation will be greater if, as proposed, additional generating capacity is installed in a North Bank Power Station; the increased drawdown will then also have a significant effect on the maximum guaranteed power output since the reservoir level will be lower at the start of critical dry periods. In the original plans for the Cabora Bassa Project considerable stress was placed on its potential multipurpose benefits. In the implementation of the plans the emphasis was changed to that of optimizing energy production to provide the revenue necessary to finance the undertaking; although such benefits as navigation, fisheries, agricultural development and flood mitigation were regarded as potentially important. Progressive siltation directly affects fisheries and navigation on the reservoir and indirectly affects other multipurpose benefits by reducing the flexibility of operation of the project. In particular the degree of attenuation of downstream flood discharges which can be achieved is strongly influenced by the available storage capacity. Since this capacity will decrease with time as sédiment accumulates the risk of inundation to development projects in the lower Zambezi floodplain will gradually increase. This factor must be considered

8 566 P.Bolton in planning the long-term development of the region. Likewise the progressive lowering of the annual drawdown level of the reservoir will affect the planning of development projects on its shores. APPROPRIATE METHODS FOR SEDIMENT MONITORING The shortcomings of the results presented in this paper as a basis for official policy decisions are readily apparent. The purpose of the study has been to identify those effects of sediment deposition which are likely to be most significant in planning the medium and long-term development of the region. The actual extent of the effects can only be determined by undertaking appropriate field surveys. In the case of Lake Cabora Bassa two aspects of sediment deposition require further examination: firstly, to determine more precisely the rate of sediment inflow; and, secondly, to identify the principal regions of deposition in the reservoir. In smaller reservoirs it is possible to evaluate both these factors by undertaking periodic hydrographie surveys along established survey lines. This approach would not be applicable in a reservoir the size of Cabora Bassa because of the large number of sections which would have to be surveyed for a given level of accuracy and the difficulty of achieving accurate horizontal fixes on an open surface which frequently exceeds 20 km in width. In the absence of hydrographie data, the only reliable method of estimating the total rate of sediment inflow is by monitoring the discharge of the principal sediment carrying tributaries. For Lake Cabora Bassa the bulk of the inflow could be measured by establishing one permanent sediment monitoring station at a suitable location on the Luangwa River. Hydraulics Research, Wallingford, has evolved a method of monitoring sediment discharges by exploiting the physical differences between the transport of "wash load" and "suspended bed material load" (Bolton, 1983b). Wash load must be monitored continuously since it is supply dependent; this may be achieved through the installation of an adequately calibrated silt sensor (Fish, 1983). By contrast, the discharge of suspended bed material load and bed load can be correlated with river stage; this may be achieved by an intensive programme of pumped sampling over a single period of, say, three months. Such a monitoring procedure provides reliable estimates of the long-term sediment discharge without requiring a heavy long-term commitment of resources and staff. However, in the case of the Luangwa River, its success would depend on achieving close cooperation between the authorities in Zambia and Mozambique. At a later stage other tributaries of Lake Cabora Bassa could be monitored if it were found to be necessary. The problem of accurately determining the distribution of sediment in Lake Cabora Bassa remains unresolved although hydrographie survey of a small number of sections in the Mucangadze basin would provide some indication of the rate at which the "live" storage was being filled. A more accurate value could be obtained from study of a sequence of aerial photographs of this basin taken at, say, 5-year intervals when the reservoir level is at a minimum. The disadvantages of this method are its cost and the difficulty of providing accurate

9 Sediment deposition in Zambezi reservoirs 567 ground control in such an inaccessible region. Alternatively it may be possible to gain some indication of sediment distribution by studying the change in shape of the shore line at known drawdown levels on satellite images. However, with the present level of resolution and the difficulty of obtaining cloud-free images it is unlikely that this method will provide the required information. As with many projects in which it is necessary to quantify the effects of sediment deposition, the final choice of monitoring procedure should be based on an assessment of the purpose for which the data are required. At present such an assessment can only be made subjectively. To develop a more objective procedure would require both the development of appropriate methods of economic analysis and a knowledge of the level of accuracy which can be achieved in different types of sediment survey. Further study of the latter question is currently being undertaken at Hydraulics Research. ACKNOWLEDGEMENTS The initial research on which this paper is based was undertaken at the University of Edinburgh. I am indebted to my supervisor, Mr H.Dickinson, and to the authorities in Mozambique for their encouragement and assistance. The preparation of the paper and the development of the final section were carried out at Hydraulics Research, Wallingford in the Overseas Development Unit headed by Dr K.Sanmuganathan with funds provided by the Overseas Development Administration of the British Government. REFERENCES Bolton, P. (1983a) The regulation of the Zambezi in Mozambique: a study of the origins and impact of the Cabora Bassa Project. PhD Thesis, Univ. of Edinburgh. Bolton, P. (1983b) Sediment discharge measurement and calculation. Tech. Note 0D/TN2, Hydraulics Research, Wallingford, Oxfordshire, UK. Central African Council (1951) Report on Kariba Gorge and Kafue River Hydro-electric Projects. Inter-Territorial Hydro-Electric Power Comm., Salisbury. Chikwanha, R. (1980) Sediment Yield from Rhodesian Rivers: Season. Hydrological Branch, Min. Water Dev., Salisbury. Fish, I.L. (1983) Partech turbidity monitors. Tech. Note OD/TN1, Hydraulics Research, Wallingford, Oxfordshire, UK. Institution of Civil Engineers (1978) Floods and Reservoir Safety. ICE, London. Stocking, M. & Elwell, H.A. (1973) Soil erosion hazard in Rhodesia. Rhodesian Agric. J. 70 (4), Ward, P.R.B. (1980) Sediment transport and a reservoir siltation formula for Zimbabwe-Rhodesia. Civ. Engr in S.A. 22 (1), 9-15.