A CRITICAL APPRISAL OF DENSITY CURRENTS & SOIL CONSERVATION AT MEASURES TO PROLONG THE LIFE OF RESERVOIRS

Abstract

ATURE OF SEDIMENTATION FRCBIENS The process of sedimentation embraces the processes of erosion from the parent material, entrainment, transportation, deposition and its compac-tion with age. These natural phenomenon have been taking place throughout the ages and have shaped the physical features of the earth as it is today. The important dynamic agents constitutirg the erosion of sediments are water, wind, ice and gravity. In most parts of the world the dynamic action of water plays the ma jor part in the erosion, even though each may be of vital importance locally. The erosion may be defined as the process of separation of rock and soil fagments from their parent material by the dynamic action of forces, mentioned above. Since the present discussions are confined to sedimentation and measures that can be taken to control the same from the reservoirs, only the action of water will be analysed in its different aspects in the sedimenta-tion process. 1 .01 .02 The erosion and deposition of the transported sediment cause varied problems . The relatively imperceptible sheet erosion from the cultivated lands gradually reduces the depth of the fertile soil thereby adversely affectir ; the produce. Uncontrolled erosion of this topsoil results in the increased hours of labour and money in reachixg down the necessary thickness of topsoil. In fact, in advanced cases of sheet erosion, it may modify the farm fields to an extent cultivation is no more profitable, or even physically possible. Sediment in transportation makes the water unfit for human consumption. Unfiltered water for domestic purposes can tolerate only very traces of sediment in rain water. A higher concentration of sediment makes the filtration obligatory. For most of the industrial uses also the concentration of sediment would have to 1/2 be controlled. An increase in this concentration beyond the critical value would always male filtration of this raw water necessary resulting in higher operation costs. 1 .01 .03 Problems created by the deposition of sedimsnt are many. Sediment accumulated in channels' reduce their discharging capacities and result in overflowing of banks thus damaging the adjacent properties. Deposition of silt in navigable waterways, irrigation and drainage channels, reservoirs and harbours and in multifarious structures entail costly maintenance in the removal of silt. Otherwise it results in reduced service utility if the silt deposition is not cleared, as for example, in reservoirs. 1 .01 .04 Thus, from the above paragraphs we can conclude that it is of vital importance to the mankind to control soil erosion. With a rapidly increasing population and ever increasing demand f food and other products derived from soil and water it is but necessary that the exploitation of soil resources is controlled and a more careful and wise planning of future use is embarled upon. 1 .02 RESERVOIR SEDIMENTATION Itoservoirs are constructed to store water so that it can be utilised at the time of need. The river water is intercepted by the dams and the result-ing storage is used for town and industrial water supply, irrigation, power, flood control and other various purposes. If this engineering feat had no side effects, there would not have arisen any problem of the progressive economics of reservoir usage. The river feeding the reservoir, on the way, picks up silts of varying sites and in varying quantities. When it meets the reservoir which is a still pool, the velocity of the flowing water gets reduced from some distance upstream of the reservoir de to the formation of backwater and the silt carrying capacity of the flowing water reduces. This results in the gradual deposition of silt that was being carried by the river upto the 1/3 mouth of the reservoir. The sediment-charged water with a partly reduced charge in the backwater reach may get partly or fully mixed into the still water of the reservoir or continue as a definably separate entity along the reservoir bed. The silt particles which get distributed in the reservoir gradually settle on the bed of reservoir . That part of the silt laden flow which continues to flow over the bed gets deflected upwards when it encounters an obstruction like the dam structure . This brings about turbulence in the reservoir flow and thus the sediment charged water gets mixed with still water as its counterpart did before . The feeble flow, if at all any, will continue vertically and when it reaches the water level in the reservoir, is forced to change its direction. This will further result in eddies and by this time the flow has practically lost all force which enabled the movemnt of water and results in the ultimate mixing with the reservoir water. The silt particles carried by the flowing water inside the reservoir bed start settling and continue to do so until disturbed by either a fresh stream of water or opera-tion of sluices, which again create turbulence. Otherwise this process continues all through the river flow period and the silt particles continue to accumulate in the bed of the reservoir. The foregoing lines describe, in brief, the process of sedimentation in reservoirs. It is evident that the sedimenta-tion of a reservoir constructed on a natural water course is inevitable, But the point of concern is the rapidity of this process and the duration of time that elapses before the usefulness of the reservoir is seriously impaired or damaged. The replacement cost of storage lost to sediment accumulation in American reservoirs amounts to millions of dollars annually.1 An example of extreme sedimentation is shown in Fig. 1 .01 . In a very short span of about 11 years the sediment filled the Koyadaira reservoir to such an extent that hardly any storage remained at the end of this period. A diagrammatic sItch of the deposition of sediment in reservoir may 1/4 be seen in Fig. 1.02, which shows the sediment deposited in different reaches along the longitudinal section. One layer represents the settlement of silt particles of one flood season. The demarcation between individual floods may not be very distinct, as, in a rainy season , silt settlement due to the floods overlap to a good extent. But '. there is sufficient time for the silt particles to settle without disturbance in between flood seasons and the exposed silt surface settles and gets compacted as and when the water is released from the reservoir. 1.02.02 Distribution of sediment in reservoirs : The actual distribution of sediment in reservoirs is of vital importance in evaluating the loss of storage space for different purposes in a multi-purpose reservoir. In such a reservoir, where different allocations are made at different elevations for the various purposes, the sedimentation normally does not follow a uniform law, which results in modified capacities as and when sedimentation occurs. The variation is not in proportion to capacity at different elevations. This Hakes the problem of allocation of storage very complex. The Fig. 1 .03 is a representative diagram of this aspect. The accumulation of sediment in the live storage in various proportions in various regions is shown in the sketch. A knowledge of distribution of sediment in reservoirs is also helpful in locating outlet sluices. Silting • at the mouth of outlet is one of the very common occuranoes, which is a constant source of headache to the operation arxi maintenance crew. Chow1 has quoted an example of Schoharie reservoir, New York whose intake mouth got excessively silted which is requiring periodical clearance of the silt to ensure tbcdesired discharge. 1/5 1.03 PROBIEMS OF RESERVOIR SEDII1;NI`ATI0N The problems arising out of reservoir sedimentation nay be grouped into three categories, namely : i (a) Effect above the reservoir (Aggradation); (b) Effect inside the reservoir; and (c) Effect downstream of the reservoir (Degradation) .....