Challenges in African Hydrology and Water Resources (Proceedings of the Harare Symposium, July 1984). IAHS Publ. no. 144. The impact of sediment on reservoir life INTRODUCTION J. D. PITT & G. THOMPSON Binnie & Partners, Artillery House, Artillery Row, London SW1P IPX, UK ABSTRACT Detailed studies of reservoir sedimentation are becoming more commonplace as we are forced to abstract water and generate power from more problematic reservoir sites, often requiring sediment sluicing. In this paper a methodology, evolved whilst carrying out such studies, is presented and some cautionary guidelines are provided for those who are required to predict the effectiveness of sediment sluicing. Impact des sédiments sur la vie d'un réservoir RESUME Les études détaillées de la sédimentation dans les réservoirs sont de plus en plus nécessaires, vu le besoin croissant de détourner de l'eau pour produire de l'énergie à partir de sources plus problématiques, exigeant souvent de procéder à des chasses des sédiments. Cette communication présente une méthodologie conçue ou fur et à mesure de la réalisation d'une telle étude, avec des commentaires destinés aux personnes chargées de prévoir l'efficacité de ces chasses. There are many reservoirs in existance today which cannot perform as designed because much of their storage has been filled by sediment. For hydropower projects loss of storage may not be too serious: the dam still provides the head necessary to generate power although the periods over which the turbine can operate are reduced and the risk of blockage of intakes is increased. For a water supply scheme any loss of live storage increases the risk of supply failure and this is often unacceptable. Therefore, when either studying an existing reservoir or when planning a new project sediment studies are required to predict the loss of storage as a function of time. In this paper a general approach for reservoir sedimentation studies is described. However, these studies cannot be carried out in isolation, but must be integrated with economic and operational studies for the project. CLASSIFICATION AND GENERAL APPROACH The approach adopted to study.a particular reservoir depends upon the size of the reservoir in relation to its inflows, the sediment regime in the river and the purpose of the reservoir. There are several guidelines which one can use to decide how critical sedimentation is likely to be in a particular reservoir. 541
542 J.D.Pitt & G.Thompson For instance a stream power in excess of 10-9 N m s -1 indicates an ability to pass significant quantities of sediment (Rooseboom, 1975). Alternative guidelines can be found in the form of trap efficiency curves, the more notable of which are those produced by Brune and Churchill. These relate the proportion of the inflowing sediment load which is retained in the.reservoir, to the volume of the reservoir and the volume of the mean annual flow. These can then be used to relate reservoir size and the sediment concentration of inflows to "reservoir half life" (i.e. the time taken to fill 50% of the reservoir storage with sediment). Figure 1, based on Brune's curve, shows that reservoir half life varies roughly in proportion to reservoir size and the inverse of the sediment concentration. Assuming an average sediment inflow Ratio of reservoir volume to volume of annual inflow FIG.l Reservoir half life as a function of the average sediment concentration of the inflow, reservoir volume and mean annual inflow (assuming no sediment sluicing). concentration in the range 1000-4000 mg l -1, most reservoirs with storages in excess of 50% of the mean annual inflow will have half lives measured in hundreds of years. For storages in the range 5-50% of the mean annual inflow, half lives will be measured in decades whilst for the smaller reservoirs the half life may be less than 20 years. These three classes of reservoir provide a basis for selecting the study approach. For large storage ratio reservoirs with half lives in excess of 100 years, the main effort in a sediment study will be to check that
The impact of sediment on reservoir life 543 possible land use changes will not dramatically affect the sediment inflows and to investigate the effect of internal sediment flows, such as density currents. Flooding of upstream areas is not likely to be of major importance until the reservoir is at least half full of sediment. Degradation downstream may well be a major effect. Except for reservoirs with large storage to inflow ratios, it will be necessary to consider sediment sluicing. There are other methods of reducing sediment deposits, for example by dredging or providing check dams above the reservoir. However, in most reservoirs the cost effective way of maintaining storage is by sluicing. Examples where sluicing has been effective include the Bhatgarh Dam in India; the Old Aswan Dam on the Nile, Sanmenxia Reservoir, China and the La Pena Dam in Spain (USDA, 1943; Zhang Qishun & Long Yuqian, 1980). The effectiveness of the sluicing is dependent upon the duration and degree to which the reservoir is drawn down and the discharge capacity of the sluices. Effective sluicing has generally only been observed where the drawdown level is below about the half height of the dam and the sluice capacity exceeds the mean annual flow by at least a factor of 2. Smaller sluices have sometimes been found to be effective in releasing density currents, but in general they are only effective in creating a local scour hole and cannot restore reservoir storage. For the small storage ratio reservoirs, with half lives of less than 20 years, measures for sluicing sediment are almost certainly essential. These measures may consist of both structural works, such as low level outlets and operational measures which allow significant reservoir drawdown during part of the year. Sluicing will be carried out during the early wet season, to pass the heavily sediment laden water and to scour some of the existing deposits. The studies involved in designing such a reservoir and sluicing arrangements include: carrying out field work to identify the characteristics of the sediment and the quantities involved; and undertaking numerical and physical model studies to ensure that the sluicing arrangements will remove sediment, control flooding problems and not exacerbate problems downstream. The sediment and operational studies are interactive; reliable sluicing constraints are often not acceptable for either economic or operational reasons, and therefore the finally adopted rules may well have to be an acceptable compromise. Beni Amrane Dam in Algeria is typical of dams in this category. For this reservoir it was necessary to carry out a combined sediment and operation simulation to prove the viability of the scheme. DATA COLLECTION AND ANALYSIS Data collection is an extensive subject; for a detailed approach to the problem the reader is referred to Vanoni (1975). Available data on sediment loads are generally less extensive and less reliable than data on flows. Whereas it is common to find flow records covering several decades with daily values, it is rare to find sediment records covering more than 10 years. Data that are available are often only available as points on a sediment rating curve. Ideally one requires detailed measurements at two or three
544 J.D.Pitt & G.Thompson day intervals during low flows and more regularly during high flows with not only sediment concentration but also particle size distribution being recorded. Such information is expensive to collect and rarely available. 800 600 5 400-3.-I Lakhdaria 19.9.79-23.9.79. 400 g -300 2 200 S c O 100 40 60- Time (hours) 80 19.9.79 ' 20.9.79 21.9.79 22.9.79 23.9.79 FIG.2 Flow and sediment concentration vs. time for a major flood event on the Oued Isser, Algeria. Examples illustrating the need for such detailed data are given in Pigs 2 and 3. The hydrograph shown on Fig.2 is typical of floods in arid areas where the time of rise to peak is very short. The peak concentration occurred well before the peak flow and well before the magnitude of the flood was apparent. By taking daily fit lines monthly data bserved points 10000 FIG.3 Example of seasonal variation in sediment rating curves for the Oued Isser, 'Algeria.
The impact of sediment on reservoir life 545 samples or by only sampling near the peak flow the sediment runoff for this event could have been in error by 50%. On some rivers a single sediment rating curve is adequate; however, on a great many rivers this is not so. For example, on the Euphrates in Iraq the sediment load is a consequence of the operation of upstream storages and is not correlated with river flow. This must be the case in many regulated rivers. Figure 3 shows the sediment rating curves for a natural river in Algeria. The best fit line for each calendar month is plotted for the 8 years of record. A single concentration could occur within a range of flows covering three orders of magnitude. The use of a single sediment rating curve in this situation would be meaningless in terms of estimating sediment yield and developing operational criteria. Sediment data show trends more clearly than flow records. If a catchment is being developed or its land use changed, then sediment loads can vary by an order of magnitude, whereas the flow will not change by more than 100%. Due to this sensitivity, care is required when using past sediment records, although past flow records may well be acceptable. A prerequisite of any sediment study is that the drainage basin should be inspected and a time scale given to major changes which may have affected the sediment runoff. Bed load is seldom measured in routine sediment monitoring. In general an allowance for this can be made by evaluating the bed load component using empirical formula and adding this to the measured suspended load. The alternative is either to carry out the measurement in an extremely turbulent part of the river or to install expensive bottom withdrawal type sampling stations. Data collection must be specific to each case, since every river has its own peculiarities such as high sediment concentrations or snowmelt runoff. Field work is the key to understanding the sediment processes of a river. Besides routine monitoring to determine flows and sediment runoff, periodic surveillance is also required, covering both the general area and the specific drainage basin. Sampling material along the river bed, in point bars, from deposits in flood zones and from nearby reservoirs is very important and can reveal features that are not picked up by routine monitoring. USE OF MATHEMATICAL AND PHYSICAL MODELS The effect of sediment on reservoir performance can often be studied using a simple mathematical model of the reservoir. Such a model could include mass balance techniques together with trap efficiency concepts and a concept of sediment sluicing efficiency. This model could rapidly scan a range of sluicing scenarios and bracket what is acceptable from an operational viewpoint. Such models have the advantage of being cheap and easy to develop and run and can be updated later in a study by more precise data on trap and sluicing efficiency. More detailed studies should include both physical and mathematical modelling. Physical modelling can be used to define the sediment deposition patterns in the vicinity of the dam site and intake structures. However, for overall reservoir response, mathematical models are commonly used. Care must be exercized in selecting models
546 J.D.Pitt & G.Thompson as both mathematical and physical models include approximations and assumptions. Typical of these assumptions is a commonly used technique in mathematical models known as the fixed mobile bed width concept. This concept assumes the channel width to be constant in time. Over the operating range of levels in a reservoir the channel width can change by an order of magnitude. This can result in both the quantity of material available for scour during drawdown and the sluicing efficiency being overestimated. However, more accurate modelling can be achieved by allowing the width to vary with water level. An example of results from this type of detailed modelling is o 0.2-10 20 30 1(0 ibuu -, (b) Distance From dam x (miles)! ' 11)00-1300 - 1200 - " \ AN 'AN ; \N * %M A/ AN * -, "'A\Model *\\ 1979 Survey V\ / ^ \'t I* I' ll ik fa _!, 1100 - i i i 0 2 k 6 8 10 Width (1000 ft) FIG.4 Examples of results from the HEC6BP model: (a) distribution of deposits in Tarbela reservoir (1974-1979); (b) typical cross section- geometry in Tarbela Reservoir (1974-1979).
The impact of sediment on reservoir life 547 shown in Fig.4. The example is a simulation of sedimentation in 9 3 Tarbela Reservoir, which has a capacity of 14 x 10 m and storage to annual inflow ratio of 0.2. Sediment inflows consist of medium to fine sands and silts with an average concentration of 4000 mg l -. Since initial impounding in 1974, sediment deposits have built up and been moved forward by reworking during drawdown periods. Operation levels during drawdown are typically half of the height of the dam. The simulation covers the 6-year period from initial impoundment and includes the reworking periods. The agreement between model and prototype is good. This simulation was carried out using the program HEC6BP developed from the US Army Corps of Engineers program HEC6 by Binnie & Partners. The developments included: (a) Allowing the cross section width over which scour and deposition occurs to vary with the width of the water surface. (b) Inclusion of a general capability to model a river network including several branches and several dams. (c) Use of simple flow routing through reservoirs and storages to reproduce the effects of drawdown and impoundment. (d) Provision for the resuspension of silts and clays. (e) A re-evaluation of the methodologies used to calculate armour layer stability and sediment availability. The program is currently being used to study a proposed reservoir on the Indus in Pakistan. The effectiveness of various sluicing options is being considered together with their effect upon: upstream flooding problems; downstream degradation; project operation, yield and economics. Programs such as HEC6BP are extremely powerful tools but they must be used with care. The use of such models highlights the déficiences in existing data, because the conclusions of a study may be sensitive to inaccuracies in data relating to particle size distributions or total load. Also the model may well contain the correct theory but the interaction of effects may produce unrealistic conditions. The output from such a model should therefore be monitored and any unforeseen results investigated before being accepted. CONCLUDING REMARKS Sediment studies are becoming an increasingly important part of the feasibility studies for a reservoir. Different reservoirs require studies which concentrate on different aspects. Problems that can arise because of reservoir sedimentation include: loss of storage and consequent loss of yield; increased flooding upstream; blockage of intakes; abrasion of turbines; and downstream problems involving both aggradation and degradation. Eeservoir sedimentation can sometimes be alleviated by sluicing. However, studies to prove the viability of sluicing require good field data and should include reviewing case studies, mathematical modelling at both a simple and detailed level and physical modelling. However, even when all of these studies have been carried out, the only real proof of the efficiency of sluicing is the operation of the prototype. Sufficient flexibility should therefore be included in the design
548 J.D.Pitt & G.Thompson so that the operator can alter his rule curves according to experience. ACKNOWLEDGEMENTS The authors wish to thank the Water and Power Development Authority of Pakistan for their encouragement in the development of the program HEC6BP and for providing the data for Tarbela Reservoir. REFERENCES Rooseboom, A. (1975) Sedimentneerlating in damkomme (Sedimentation in reservoirs). Tegniese Verslag nr 63, Dept Waterwese, Republic of South Africa. USDA (1943) The control of reservoir silting. US Department of Agriculture, Miscellaneous Publication no. 521. Vanoni (1975) Sedimentation Engineering. ASCE Manuals and Reports on Engineering Practice no. 54. Zhang Qishun & Long Yuqian (1980) Sediment problems of Sanmenxia Reservoir. In: Proceedings of the International Symposium on River Sedimentation, Beijing, China, 707-716.