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Sediment Deposition in Major Reservoirs in the Zambezi Basin

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Sediment Deposition in Major Reservoirs in the Zambezi Basin Challenges in African Hydrology and Water Resources (Proceedings of the Harare Symposium, July 1984). IAHSPubl. no. 144. Sediment deposition in major reservoirs in the Zambezi basin 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. INTRODUCTION 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 559 560 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 1950. 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 1979. 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 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 0 100 flake Cabora Bassa Drainage basin of Lake Cabora Bassa Drainage basin of \Lake Kariba Potential erosion hazard Drainage basins studied based on Stocking and Elwell by Ward and Chikwanha Very low to low S = Gwaai U = Umsweswe J Below average to average 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 40-400 t km~2year-1. On the basis of the foregoing, admittedly broad, range of values 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 0.64-1.28 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 9 3 10 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 1600- 16 000 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.
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