# Impact of Arch Dam Cracking on Monitoring Data
## 1 Introduction
### Monitoring of Arch Dams  
The identification of any anomaly is fundamental to the safety assessment of dams. Historically and to this day, the method for measuring ordinary alterations in a dam and detecting potentially dangerous abnormal changes has been performed by employing monitoring devices located in the dam. The first step to understanding how dam monitoring devices identify different types of cracks is to establish the ability of the devices to measure dam deformations. In the early 1970s, the development of automated monitoring systems started for dams, which reduced the tedious work involved in processing and displaying the data. It was not until 1982 that ICOLD first addressed the issue of automated dam monitoring by publishing a comprehensive review of the automation of monitoring systems [14]. It should be noted that monitoring systems (both automatic and manually operated) suffer from accuracy limitations in addition to systematic errors, random errors, and noise [15]. The most commonly monitored parameters in dams are dam body displacement, foundation displacement, water leakage, joint and crack movement, concrete temperature, and external parameters such as stream flow, seismic movement, ice thickness, air temperature and relative humidity, pore pressure, snow and precipitation, water level, and water temperature [16]. As a general rule, concrete dams and hydroelectric power plant structures demand five times more accurate monitoring measurements than embankment dams and slopes enclosing reservoirs [17]. Sensors employed in monitoring measurements are generally divided into geodetic sensing techniques (terrestrial and space) and geotechnical/structural instruments (e.g., pendulum and extensometers). Additional information on the instrumentation used for each of these parameters can be found in [18]. Traditionally, the most precise and widely installed monitoring device to record concrete dam movements has been the pendulum. Pendulum technology has advanced recently, making the monitoring devices installed in some older dams obsolete. From manual sight-reading pendulums, the focus has shifted to devices that employ sensors to automatically measure and continuously send those data to a control center. On the other hand, there is a main branch in geodetic engineering to determine the elevations between two ground points, either directly or indirectly. Geodetic approaches provide data on absolute and relative displacements (changes in coordinates) to derive the displacement and deformation ranges of the monitored structure. Thus, geodetic surveys provide global information on the behavior of the investigated object [17]. These geodetic devices, which traditionally have not been accurate enough to detect deformations less than 1 mm in size [19; 20; 21; 22; 23], have improved in the 21 st century to the point that it is nowadays possible to employ digital, optical, or laser devices that reach the required accuracy. This work compares various methods and instruments currently used in precise measurements to assess displacements. Table 1 summarizes the different categories of monitoring devices used in arch dams. The type of monitoring devices used in a dam, as well as the number of devices and their position inside the dam, is usually decided based on previous experience in the safety of similar dams, as well as on the budget for the design of the new dam. It is important to consider that the installation of surveillance devices a posteriori inside the dam is not a common practice, as it is usually a complex and costly process. There is a necessity to establish new criteria for the selection and location of dam instrumentation in conformity with the paradigm shift in measurement devices, measurement automation, and evaluation methods. Therefore, this study attempts to shed some light on the implications of the position and type of monitoring devices in an arch dam.