Sediment Problems in Reservoirs. Control of Sediment Deposits
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NTNU Trondheim Norges teknisk-naturvitenskapelige universitet Doktor ingenioravhandling 1997:114 Institutt for vassbygging IVB-rapport B2-1997-5 vi i 5 a> m! . CD % B t g CZ) 50 c-i" i—i. o PL P ■S'S % $E < 3 GO o GO DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Norwegian University of Department of Hydraulic and Science and Technology Environmental Engineering NTNU SEDIMENT PROBLEMS IN RESERVOIRS Control of sediment deposits By Tom Jacobsen A dissertation Submitted to the Faculty of Civil Engineering, the Norwegian University of Science and Technology, in partial fulfilment of the requirements for the degree of Doctor Engineer Trondheim, Norway, November 1997 IVB Report B2-1997-3 Summary SUMMARY Water is stored in reservoirs for a number of purposes such as hydropower generation, irrigation, water supply, flood control, recreation and navigation. When a reservoir is formed on a river, the river will tend to lose its sediment transporting capacity, and sediment will deposit in the reservoir. The sediment load in many rivers, in particular in regions with arid, semi-arid or monsoon climate, is of such magnitude that it severely limits any reservoir development. Estimates of annual sediment deposition in the world ’s reservoirs vary from 60 - 120 km3, or 1 - 2 % of the total water storage capacity. The objectives of this study were therefore to study the problem of reservoir sedimentation and to investigate methods of removing of sediment from reservoirs. Various aspects of reservoir sedimentation are discussed. Anthropogenical impacts seem to have a major influence on the erosion processes and it is estimated that 50 % of all erosion is accelerated. It is concluded that temporal distribution is uneven, mainly because of the important role of the very large flood events. A world map showing the Reservoir Capacity: Annual Sediment Inflow ratio (CSR) for reservoirs with volume equal to 10 % of annual inflow has been prepared. Comparison with reported cases shows acceptable agreement. The map shows that regions where sedimentation problems generally are severe include western parts of North and South America, eastern, southern and northern Africa, parts of Australia and not least, most of Asia. In particular, the development of medium-sized reservoirs is difficult, and sometimes not feasible at all. Medium-sized reservoirs in this context are defined as reservoirs with a capacity of between 3 and 30 % of annual water inflow. The reason that medium sized reservoirs, in which water can be stored with limited social and ecological impacts, cannot be built is that they are too large for conventional flushing techniques, and too small to store the sediment that accumulates in the course of the economic lifetime of the reservoir. A key benefit of a numerical model is its ability to simulate fine-sediment behaviour. As physical modelling of fine sediment is difficult, such models may be the only option other than direct measurement in assessing fine sediment behaviour. A 2D/3D numerical model, SSIIM, was applied to a case study of two flood drawdown trials in Lake Roxburgh, New Zealand. The sediment computation was carried out per size fraction (non-uniform modelling), which allowed changes in bed grain size distribution to be modelled. Actual bed levels, l Summary grain size distributions, water levels and inflows were used as input to the model. Good results were obtained in modelling erosion and deposition in the reservoir as well as sediment outflows during the two events. It has been shown that SSIIM can be used for trap efficiency calculation, as an alternative to well-established empirical methods. Two techniques that permits controlled suction of sediment and water into a pipe have been developed, the Slotted Pipe Sediment Sluicer (SPSS) and the Saxophone Sediment Sluicer (SSS). The techniques exploit the inflow pattern through a slot in a pipe. An equation describing this inflow pattern was derived and verified experimentally. The SPSS is fixed near the reservoir bed and sediment that deposits on top of it is removed in the sluicing process. The SSS, on the other hand, sluices sediment from the surface of the sediment deposits. Both techniques are characterised by that they can sluice sediment at a concentration close to the sediment transport capacity of the outlet pipe, without causing the pipe to block. They are suitable for sediment removal, as the sediment can be transported out of the reservoir through a pipe, using the natural head as the driving force. The techniques are simple. Both the SPSS and the SSS can be manufactured from circular pipes and the only movable part is the valve on the outlet pipe. Laboratory experiments, field experiments and field experience all show that the SPSS and SSS can meet the requirements listed below: 1. Sluicing of sediments should be possible without interrupting water supply from the reservoir. 2. The technique should be reliable and simple to implement and operate. The chances of maloperation causing severe problems, such as blocking of the pipeline, should be small. 3. A minimum of water should be required for the transport of sediment out of the reservoir, and the necessary investment should be low. 4. It should be possible to operate the system without any input of external energy if the head between the water surface and the outlet is sufficient. 5. It should be possible to use sediment for construction, for land reclamation, as a soil improver in irrigation water or for regaining the natural sediment regime in the river. To obtain efficient removal of sediment through a pipe or an open channel it is crucial to know the behaviour of the sediment-water mixture. Hydraulic transport of sediment in pipes is widely used and several methods for computation of such flow exist. Three methods for computation of limit deposit velocity and two for u Summary computation of headless were studied and found to compare reasonably well with laboratory and field experiments. However, when hypothetical slurries were studied, considerable differences between the methods were discovered. Transport of sediment in open channels is a field in which relatively little work has been done. Unlike sediment transport in pipes, no method for computing open-channel sediment transport seems to be generally accepted. An experiment was performed where a sediment-water mixture, which properties could be controlled, was tested simultaneously in a pipe and an open channel. It was found that the energy gradient required in the open channel was less than 2/3 of that in the pipe. On the basis of the experiments, methods for computing limit deposit velocity and head loss in open channels have been derived. The applications for which the SPSS and SSS were developed, removal of sediment from reservoirs and bypass of sediment are described. The sediment transport capacity of pipelines and open channels has been computed for different sizes of conduits and flow situations. It has been shown that sediment deposits theoretically can be removed at a rate of 64,000 m3 per hour through a 1.5 metre- diameter pipe. Due its simplicity it is suggested that the SPSS can be used in hoppers and pressurised sand traps. An experiment was performed, which demonstrated that coarse sediment (e.g. for construction purposes) can be extracted from a pipe carrying a sediment-water mixture. Some technical and economic conditions affecting the economics of sediment removal from reservoirs have been identified and studied. It is concluded that the benefits from increased dry-season power production can justify removal of sediment from a medium-sized reservoir. However, of more importance is the finding that the economics of such an undertaking are highly dependent on a number of these conditions. The removal of sediment from small reservoirs is discussed, and it is argued that this can be highly beneficial, because of the frequency with which the reservoir volume is used. Several additional benefits apart from the purely economical aspects are likely to be obtained, adding to the arguments for implementing a sediment removal system. m Preface PREFACE This report gives a summary of studies on sediment problems in reservoirs and a series of experiments undertaken to develop methods for removal of sediment from reservoirs. The work was undertaken during the years from 1993 to 1997, when I was enrolled as a doctoral student at the Norwegian University of Science and technology (NTNU), Department of Hydraulic and Environmental Engineering. Scholarship for the doctoral studies has been financed by NTNU. Thanks also go to to the research fund financed by NORAD and handled by the international office at NTNU, Veidekke and the Norwegian Research Council (NFR) for financing. A number of persons have been of invaluable help in this work, and my gratitude is hereby expressed to - the employees of Butwal Power Company (BPC) and BPC Hydroconsult in Nepal for welcoming me and helping me in every possible way to perform field experiments at Jhimruk Hydropower Plant. - the Kristiansund project office, Stolt Comex Seaway ASA. The onshore test and the offshore use of the Saxophone Sediment Sluicer provided valuable knowledge and experience. - the employees of the National Institute of water and atmospheric Research (NIWA) in Christchurch, New Zealand, for welcoming me and making my five month stay such a valuable experience. In particular I want to thank Jeremy M. Walsh for the co-operation during and after my stay. Tanks are also due to Contact Energy ltd, for providing excellent reservoir data. - Nils Reidar Olsen at SINTEF, who developed and helped me using SSIIM. - Marimette Rynning at Titania AS for providing data for pipeline sediment transport at the Titania mines at Tellnes. - friends and colleges at the Department of Hydraulic and Environmental Engineering, NTNU, for making it such a good place of work and Svend Halstadtrp and Hans Frisvold, for construction of the experimental equipment which was always better than promised.