Concept of Geodetic Controls Network in Dam Structures and Their Under Utilization in the Northern Nigeria’S Dams

concept of Geodetic Controls Network in Dam Structures and their Under utilization in the Northern Nigeria’s dams. Abdulkadir, Isah Funtua Msc. (Nig), mnis. and Mutari A. K. Department of Surveying and Geoinformatics Abubakar Tafawa Balewa University Bauchi. Abstract This paper reviewed the concept of geodetic controls network, their importance and application in monitoring of Dams Structures within the purview of Engineering Geodesy. It highlights the underutilization of these control networks in the monitoring systems of Dams structures in the Northern Nigeria. The paper advocated for strategic manpower development through geospatial education and adequate provision of control points and instruments towards a regular status monitoring of the Structures which shall involve the full utilization of Geodetic Controls Network which was hitherto neglected. Key Words: Geodetic Controls, Dam deformation, Dam Monitoring System, 1.0 INTRODUCTION The need to monitor dams for detection of any possibility of dam failure and flooding has been of great concern to the government and the various communities that lives around the localities where such structures were sited and as well as the downstream communities. For effective control and monitoring, during and after construction, of dam structures, precise control points are normally established. These points constitute the main frame used for detection of any variation arising, such as shift due to deformation, stress and shear of the structures. The points are Locations with established coordinates; latitude and longitude, and often elevation, used for accuracy and precise location of other points and constitute the main frame on which less precise observations may be based. They are as important as planning, design, and execution and “as built”. There are many historical cases of dam failures where early warning signs of failure might have been detected if a good dam safety-monitoring program had been in place. The monitoring program provides the information that is needed to develop a better understanding of the on-going performance of the dam (Barry and Statelier 2010). Sadly however over the past decades, in the Northern Nigeria, despites various instances serious economic and human lives loses due to the absence or ill timed responses by the management of these structures, there is no evidence of the existence of, or of any efforts towards the establishment of an articulated Dam safety monitoring program particularly in the North part of the country. This paper reviewed the significances of the application of geodetic controls in Dams structures monitoring system The objectives is to advance the course for inculcating the culture of short and long term monitoring of Dam structure that include the Geodetic control usage in monitoring the behavior of the structures during the construction period and all along the operation time. Common causes of dam failure include overtopping, foundation problems, structural problems, and piping (internal erosion due to seepage). With an effective monitoring program, these causes can be detected early and repaired or mitigated. There is always an associated high accident risk potential of significant proportions not only for the dam structures themselves, but also for the population living in the area involved as witnessed in the recent year 2012 flood episode in Nigeria principally along River Benue and down stream of River Niger as a results of opening of a gate of a Dam Upstream River Benue in Cameroun Republic. 2.0 THE CONCEPT OF GEODETIC CONTROL The geodetic control consists of geodetic control stations and their related information – the name, feature identification code, latitude and longitude, orthometric height, ellipsoid height, and metadata for each station. The metadata for each geodetic control point contains descriptive data, positional accuracy, condition, and other pertinent characteristics for that point. According to the US Federal Geographic Data Committee, [Part 4: Geodetic Control (FGDC-STD-014.4-2008)] “Geodetic control provides a common reference system for establishing the coordinate positions of all geographic data. It provides the means for tying all geographic features to common, nationally used horizontal and vertical coordinate systems. Therefore fundamental geodetic networks of horizontal and vertical control provide asset in the form of fixed homogeneous coordinate reference systems which forms the basis of all spatially related information. The main features of geodetic control information are geodetic control stations. These monumental points (or in some cases active Global Positioning System control stations) have precisely measured horizontal or vertical locations and are used as a basis for determining the positions of other points. Geodetic control information plays a crucial role in developing all framework data and users’ applications data, because it provides the spatial reference source to register all other spatial data. In addition, geodetic control information may be used to plan surveys, assess data quality, plan data collection and conversion, and fit new areas of data into existing coverages.” The horizontal control is normally provided through Triangulation, Trilateration, and Traversing method while the vertical control will normally be provided by differential leveling, trigonometrical heighting, and Inertial Survey systems or by other techniques. 3.0 DAM MONITORING Dam monitoring relies on the long-term measurement of small structural motions at regular intervals. Traditional surveying techniques and geotechnical instrumentation can effectively monitor one- or two- dimensional modes of these motions. (Stewart and Tsakiri 2001) The monitoring of the movement of dams demands geodetic accuracy over relative short distances and this falls within the purview of Engineering or miniature geodesy. This class of geodesy employs many of the Instruments and practices of normal geodesy and requires exceptionally precise centering to an accuracy of 0.1mm (Bomford 1980). For any structure, monitoring has to ensure the longevity of life and safety of the structure. In the case of the dam structure, it must enable the timely detection of any behavior that could deteriorate the dam, potentially result in its shutdown or failure, in order to implement corrective measures. Therefore the main purpose of dam monitoring is to study whether or not the dam is behaving according to design predictions and to verify design assumption. Monitoring parameters such as leakage pore water pressure, and deformation can provide an indication of the performance of the structure. Consequently a good dam safety monitoring program should be a key part of every dam risk management program. Preventive actions are directly taken by means of installing devices that are able to monitor the behavior of the dam structures. These instruments should be capable of measuring the magnitudes involved, with the adequate precision, for further comparative analysis with the mathematical models provided in the design. According to Barry and Statelier (2010), the scope of the monitoring methods employed depends on the potential risk associated with dam and site characteristics. Such characteristics include: i. Dam height and type ii. Extent of potential damage to people and structure located in flood zone iii. Reservoir and spillway capacity iv. Site seismicity Foundation weakness zones The measurement techniques are generally divided into geodetic and geotechnical/structural methods. a) Geodetic method: conventional & terrestrial survey and space-based method. Global information. Geodetic measuring devices measure georeferenced displacements or movements in one, two or three dimensions. b) Geotechnical/structural: Geotechnical measuring devices measure non-georeferenced displacements or movements and related environmental effects or conditions. 4.0 DAM MONITORING SYSTEM All dams are required to have a level of instrumentation that enables proper monitoring and evaluation of the structure under all operating conditions. Therefore control of a dam requires a wide range of important information coming from the sensors, which are of vital importance for the life of a dam. A specific and sudden decision for correct control of the reservoir, of the dam body behavior and of foundations is often taken. Monitoring is not only carried out by sensors but also involves direct or remote visual inspection as well as topographical measuring. According to Barry and Statelier (2010) the three critical components of a dam monitoring system are: 1) Instrumentation, 2) Data collection, and 3) Data management. Currently, there are many tools available for making improvements in these three areas an optimizing a dam’s monitoring system. The level of technology applicable for a particular dam project varies greatly depending upon the objectives of the monitoring system. 1. Instrumentation : Geotechnical instrumentation at dams consists of piezometers, seismic strong motion instruments, crest survey monuments, tilt plates, inclinometers, and other instruments as deemed necessary to adequately monitor embankment performance. Automated instrumentation systems are currently being installed at each dam to collect and transmit data from the piezometers, seepage measurement devices, pool level sensors and strong motion instrument. The instrumentation are also categorized as civil instrumentation and geodetic instrumentation. The civil instrumentation includes Extensometers, Clinometers,

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