Reservoir sedimentation surveys using Global Positioning System
K.K. Agarwal & K.C. Idiculla
Central Water Commission
Ministry of Water Resources
R.K.Puram, New Delhi-110066
Tel. No. 11-6105019
Fax : 91-11-6106849
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.
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:
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.
GPS for position information
GPS is a satellite-based global navigation system that enables users to accurately determine 3-dimensional positions (x,y,z) worldwide. While GPS is clearly the most accurate worldwide all-weather navigation system yet developed, it still can exhibit significant errors. GPS receivers calculate its position from distance measurements to the satellites that are determined by how long a radio signal takes to reach the receiver from the satellite. Position accuracy depends on the receiver’s ability to accurately calculate the time it takes for each satellite signal to travel to earth. This is where the problem lies. There are primarily four sources of errors which can affect the receiver’s calculation. These errors consist of (i) time, because of clock differences, (ii) ionosphere and troposphere delays on the radio signal, (iii) signal multi-path,errors caused by signal arrival by different paths usually due to signal reflecting from obstacles, and (iv) quality of GPS receiver. The combination of these errors, can limit GPS accuracy to 10 to 16 meters. These errors are eliminated through a technique known as “Differential”.
Differential positioning, which requires at least two receivers, does provide the means for high accurate surveying. DGPS determines the position of one receiver in reference to another and is a method of increasing position accuracies by eliminating or minimising the uncertainties. Differential positioning is not concerned with the absolute position of each unit, but with the relative difference between the positions of the two units, which are simultaneously observing the same satellites. The basic principle being that errors calculated y GPS receivers in a local area will have common errors. The inherent errors are mostly concealed because the satellite transmission is essentially the same at both receivers. One GPS receiver is programmed with the known coordinates and is stationed over a known geographical benchmark. This receiver, known as the master or reference unit, remains over the known benchmark, monitors the movement of the satellites, and calculates its apparent geographical position by direct reception from the satellites. The inherent errors in the satellite position are determined relative to the master’s programmed position and necessary corrections or differences are applied to the mobile GPS receiver on the survey vessel.
There are different ways to apply DGPS collection methods to hydrographic surveying which includes real time and post processing GPS. For real time survey a constant link between the base station and rover either radio signal or other communication techniques that broadcast the differential corrections is required. The weakness with all real-time collection systems is the communication link between the master and mobile GPS receivers. Communication problems can occur with all the systems when surveying in areas with obstructions such as mountains, cliffs, vegetation etc. When these situations occur the master receiver will have to be shifted to new locations, but at times more costly and time consuming. Post-processing DGPS is therefore, used in which instead of applying the differential correction to the rover via radio contact, it is applied after the day’s survey is completed. The GPS survey information is stored in each receiver’s memory and downloaded to a computer at the end of the day. Once both receiver’s have been downloaded, the correction factors can be applied to the rovers’ data and an accurate position obtained. The limitations of a post-processed survey include uncertainty of real-time positional accuracy and the inability to accurately navigate a precise pre-planned survey route.
A minimum of four satellite observations are required to mathematically solve for the four unknown receiver parameters ( latitude, longitude, altitude and time). For hydrographic surveying the altitude, the water surface elevation parameter, is known which realistically means only three satellite observations are needed to track the survey vessel. But to obtain highest accurate positioning the survey vessel tracks all available satellites and monitors geometric accuracy of its positions.
The GPS can also be used for vertical measurement allowing not only greater accuracy in the hydrographic survey, but also more productive data collection. The antenna is mounted a fixed distance above the transducer. This provides absolute positioning for the depth measurement without the need for a water surface measurement to define a reference datum. It also eliminates the concern for a changing water surface elevation or boat heave (vertical displacement) due to wave action during the survey.
Depth Measuring Unit
The present hydrographic survey system use sonic soundings to record continuous profiles of the bottom of small and large reservoirs. The basic components are the recorder, transmitting and receiving transducer and power supply. The manually operated sounding lines and poles used in the past are virtually outdated and replaced by modern sonic sounders which have the capability of recording continuous profiles of the reservoir bottom and providing an analog bottom profile chart and digital record stored on the computer system for later processing. The echo sounding instrument consists of the recorder, the transmitting-receiving transducers mounted in the hull or sides of the survey boat, and a power source of either a battery or generator converter combination. The depth of water is recorded continuously at hundreds of soundings a minute on chart paper and the depth digital signal is recorded by the computer at a prescribed interval. Calibration of depth measurement from the echo sounder is critical in assuring high quality depth measurements by the hydrographic survey system. The largest correction results from the variability of the sound velocity in water due to density, salinity, temperature, turbidity, and depth of water. In fresh water at 60 degree F, echo sounders are generally calibrated for a sound velocity of 1463 m/s. The indicated depth by the echo sounder needs to be corrected for the known depth or water conditions. This can be accomplished by different methods with the bar-check being one of the commonly used methods. A bar check consists of lowering an acoustic reflector, such as a flat metal plate, to a known depth below the transducer and adjusting the instrument to produce and equivalent depth reading. The bar check must be conducted in fairly calm water with minimum wind conditions. Mild to strong wind will shift the sounding vessel so that the calibrating bar will be suspended at an angle from vertical causing the narrow signal beam from the transducer to miss the bar. The survey crew should make a bar check and record results on a depth chart and log sheet. Comparisons are recorded during both descend and ascend of the bar, at a pre determined intervals through out the depth range of the survey. Any adjustments to the speed of sound of the eco sounder should be noted on the chart. With careful calibration and correct collection techniques, a high degree of bottom profile accuracy can be maintained. The accuracy of the equipment is 0.1% of the water depth.
The output from GPS receiver is available at two seconds interval, whereas a fathometer can take soundings at much higher frequencies, as high as 20 times per second. The computer program monitors the GPS serial port for incoming data, and every time a GPS data string is received, the program immediately retrieves a depth reading from the second serial port. The survey software has many other capabilities. They are listed below.
Qualified and experienced personnel are essential for an efficient and productive field operation. The key personnel include a ydrographic crew chief experienced in all phases of the field operations and knowledgeable about the computation and report needs of the study and a crew member or members capable of assisting in the operation and maintenance of the field instruments. GPS system allow collection of data by one person, but for safety purposes and assistance, a minimum of two field personnel are recommended. For larger reservoirs, one or two additional crew members may be necessary to support the survey and for the safety of the operation.
The development of the present collection system has made the contour method the preferred method for data collection and analysis. Prior to these collection system the surveys were scheduled when the reservoir were near empty. Now they are usually scheduled when near full capacity to reduce the collection time and cost. If the reservoir areas are not aerial surveyed, then the bathymetric survey should be scheduled when the reservoir is as full as possible.
Brief description of the method adopted for topographic and hydrographic survey is given below.
The survey is conducted in a rapid and efficient manner. The basic equipment required for a GPS hydrographic survey are a boat, two GPS receivers, fathometer, and a laptop computer with data-logging software. The boat is equipped with the bathymetric equipment, the GPS system mounted on board and a lap-top computer while its reference station is positioned on a known geographical benchmark. The survey software enables fixing of grid lines and interfacing of bathymeter and DGPS and taking X, Y, Z values at required interval/ grid. Boat navigation is also controlled by the software so that boat tracks the grid line accurately. The surveys can also be carried out at random mode, rather than attempting to locate and survey pre-defined sediment range lines. The GPS receivers output position information is available at every two seconds for hydrographic survey works. Survey grade fathometers take soundings at much higher frequencies, as high as 20 times per second. Therefore, the survey software monitors the GPS serial port for incoming data, and every time a GPS data string is received the program immediately retrieves a depth reading from the second serial port. Alternatively, to collect the data simultaneously each port can be monitored continuously and data values time-stamped. The programme then uses interpolation to define simultaneous data from each port.
For accurate land survey, control points were established using DGPS all along the periphery of the reservoir. Land survey covered the entire area between water spread area and MWL. The survey is carried out using Total Station having internal memory to store up to 3000 points and facility to transfer data on PC.
Data Reduction and Analysis
The bathymetric data is first transferred to reduced levels format, after removing all collected points without differential correction. Entire data from Total Station available in digital format is then merged with bathymetry data, after doing required formatting. Contours for the reservoir areas are computed from the compiled data using the TIN (Triangular irregular network) surface modeling package. A TIN is a set of adjacent, non-overlapping triangles computed from irregularly spaced points with x, y coordinates and z values. In this method triangles are formed between all collected data points including all boundary points preserving all collected survey points. The contour surface areas for the reservoir are computed from the TIN at selected elevation intervals for the complete vertical elevation of the reservoir. Grid interpolation method can also be used for working out contour surface areas. The basic elevation verses surface area data developed from the survey become the input data for the computation of revised detailed area and capacity tables. The following outputs can be obtained using suitable software. There are several software packages available for this.
Advantages of GPS survey over other hydrographic survey techniques
The conventional hydrographic survey methods are based on typical land survey techniques and are labour intensive. The survey vessel is required to be brought to stationary position every time observation is taken. In DGPS hydrographic surveying the observations are taken from a moving survey vessel, which allows faster data acquisition with better accuracy. The line-of sight from the base station to the boat is not necessary, as required in the conventional survey. A GPS survey can be completed between control points (even on opposite side of a mountain) without having to traverse or even see the other point. A large number of reference stations/ range monuments and Bench Mark pillars are required to be erected, all along the periphery of the reservoir, for conducting conventional survey, whereas in GPS survey only a few control stations are required. Other advantages are the ability to achieve greater accuracy and the ability to efficiently collect large amounts of data. For data collection, the grid generally adopted for a very large reservoir in a conventional survey is 1000x100m, and in GPS survey a 50x50m grid is adopted ie. the intensity of data collection in GPS survey is 50 times in a conventional survey. The hydrographic survey can be carried out at random mode without specifying range lines. The data collecting system with GPS is compact and can be accommodated in smaller boats.