Abstract
Historically, hydrographic survey on rivers and reservoirs was often completed using line-of-sight techniques to survey a monumented range line. The reservoir surveys have been carried out using the conventional equipments e.g. theodolite, plane table, sextant, range finders, sounding rods, echo-sounders and slow moving boats etc. The surveys conducted by this method are time consuming and sometime it takes upto three years to complete the survey of a major reservoir like Hirakud. During such long time of survey, the siltation pattern and the bed levels also get changed. The degree of accuracy of data collected is also very poor. From the data collected by CWC on the capacity survey of various reservoirs for the last forty years, it is also observed that the rate of sedimentation between capacity surveys in most of the reservoirs shows large variation. So, the need to update the sediment measurement techniques and to introduce latest technology available in the field was felt to overcome the difficulties faced in the conventional method especially in major reservoirs and to drastically reduce the time requirement of surveys besides increasing the quality of data. With this objective Central Water Commission formulated a scheme for conducting capacity survey of 30 important reservoirs in the country. An automated collection system comprising of Computer, Global Positioning System and Echo-sounder is being used for conducting the hydrographic survey. The development of Global Positioning System is revolutionising the way both land and hydrographic surveys are carried out. Using GPS it is feasible to collect enough coordinate data to effectively map the entire reservoir bottom, and this data can then be used to develop a digital terrain model (DTM). With a DTM and suitable computer program, reservoir sedimentation changes from one survey to the next are easily and accurately calculated and mapped. With this system it is possible to complete survey of a large reservoir in a period of two to three months. Capacity survey of nineteen reservoirs viz. Srisailam and Nagarjunasagar (AP); Konar, Tilaiya, Tenughat and Getalsud (Bihar); Ukai and Dharoi (Gujarat); Linganamakki and Ghataprabha (Karnataka); Kakki and Idukki (Kerala); Gandhisagar and Minimata (MP); Jayakwadi (Maharashtra); Balimela (Orissa); Emerald Avalanche (TN) Matatila (UP) and Mayurakshi (WB) have been taken up under this scheme. Capacity surveys of another eight reservoirs are proposed to be taken up in the current year. The methodology adopted for conducting the survey, analysis of data to obtain area-elevation-capacity table/curve, contour map, cross-sections, l-sections etc. and also the advantages of conducting hydrographic survey using GPS are explained in this paper.
Introduction
Reservoir surveys are necessary to get more realistic data/estimate regarding the rate of siltation and to provide reliable criteria for studying the implication of annual loss of storage over a definite period of time with particular reference to reduction of intended benefits in the form of irrigation potential, hydropower, flood absorption capacity and water supply for domestic and industrial uses etc; and periodic reallocation of available storage for various pool levels. It will also help in proper estimation of loss of storage at the planning stage itself besides evaluating the effectiveness of soil conservation measures carried out in the catchment area of River Valley Projects. Since the major cause of storage capacity change is sediment deposition the monitoring program can determine:
- storage depletion caused by sediment deposition since closure of dam
- annual sediment yield rates
- current location of sediment deposition
- sediment densities
- lateral and longitudinal distribution of deposited sediment
- reservoir trap efficiencies
It is generally recommended to carry out survey of reservoirs periodically so that quantity of sedimentation taking place can be assessed and timely remedial measures taken. The systematic surveys of Indian reservoirs were started in the year 1958 when a co-ordinated scheme was formulated and reservoir sedimentation surveys were entrusted to several research stations in the country. Later, this work was independently undertaken by reservoir project authorities. As a result of these steps a large number of reservoirs have been surveyed. In India, so far conventional methods of hydrographic survey using echo-sounder along range lines is adopted. Measurements are typically taken from a boat, which can be a large cabin enclosed vessel for off-shore work or a small open boat for work on inland waters. The measurements required are a depth measurement and position measurement. Depth measurements are typically made with a fathometer, which is a sonar-based device measuring depth based on the velocity of the speed of sound. Position information locating the boat at the time of the depth measurement is typically based on common land survey technology. The inflow and outflow method involving measurement of inflows into and outflows from the reservoir comprising discharges and sediment concentrations is also being used in some cases. Of late, use of Hi-Tech methods has been started in which hydrographic surveys are carried out employing computerised methods both for data collection and analysis. In this method fast moving boats are used and it is possible to obtain data at closer intervals.
Over the last ten years, the use of Global Positioning System have revolutionized the way surveyors perform geodetic and control surveys on land. GPS is a positioning system based on a constellation of satellites orbiting the earth. One of the greatest advantages that GPS provides over traditional land-based surveying techniques is that line-of-sight between control points is not necessary.