Central storage of comprehensive dam data with access for all engineers has helped an Austrian company improve safety and management, say Gerald Berger and Fritz Neuschitzer
KÄRNTNER ELEKTRIZITÄTS AG (Kelag) supplies electricity to Carinthia, Austria’s southernmost federal province. The company’s supply is from contracts and part-ownerships with other companies and from Kelag’s own hydro power stations, which were built over the last century.
Kelag’s hydro power embankment dams range from small structures closing compensation basins to a 110m-high rockfill dam with an asphaltic concrete membrane on its upstream face. Most of these are situated at high altitudes. Since the early 1950s Kelag has been involved in dam design and construction. Instrumentation systems for all the dams were developed by Kelag engineers and submitted for approval to Austria’s Staubeckenkommission (dam and reservoir commission), along with the project as a whole.
Defining dam safety
Establishing the safety of a dam and its probability of failure can be a complex process and a very subjective one. Legislators, unable to define safety satisfactorily, have preferred to rely for their final decision on whether or not a dam is safe on the judgement of an individual qualified civil engineer proposed by the dam owner. A law was created (WRG 1959, Nov. 1997, § 23a) providing that a civil engineer, whether employed by a company or independent, will be responsible for the safety of a dam. This responsibility is the major motivation for this dam safety engineer (DSE) to commit themselves to the operation and maintenance of dams and reservoirs.
At Kelag, dam technology compares favourably with international norms on quality and safety.
It is felt by everyone involved in the construction and operation of a dam that structural safety must be such that failure is unthinkable. But during operation, a dam cannot be regarded as an absolutely safe structure. It is by recognising the presence of risk factors that we can take the appropriate action to reduce and eliminate them.
The dam safety engineer decides how to supervise, operate and check the dams, and has responsibility to deal with situations that arise during operation. The state of the art must be investigated for its application to questions of ageing, flood diversion, overtopping, leakage and the operating safety of spillways.
The statutory regulation for dam operation and surveillance provides for a monitoring and documentation system tailored to the requirements of each specific dam and foundation. The instrumentation and surveillance system at Kelag’s dams varies according to the age of the structure. During the last 10 years, efforts have been made to automate safety analysis parameters of dam response.
The company’s central control, monitoring and registration system is at its headquarters in Klagenfurt. In order to cope with an increasing number of data and meet the demand for new functions, a new network control system for load dispatching and network operation was installed in 1992. Since 1997 all the 53 power stations and switchyards have been controlled from this centre. In addition to the existing functions of basic data processing and network analysis programs, Kelag has installed dam surveillance software that includes visual display systems, data administration, archiving, analysis and report programme, as proposed by the dam safety engineer.
During the past ten years, automatic data acquisition systems (ADAS) have increasingly become common practice for dam monitoring. Although these systems have been used in hydrology and water resource management for many years, their application in the dam monitoring sector has been slow, due to the importance of visual inspections, generally involving manual records, and also due to a reluctance to use sophisticated systems, which have greatly delayed a general acceptance of automation.
The main aim of Kelag’s programme is to provide a continuous and instantaneous appraisal of the behaviour of the monitored structure, through acquisition, processing, storage analysis and display of results, which are called up automatically or on special request.
Optimising the safety assessment for a dam requires great experience in monitoring instrumentation. As field data are needed in real time, special hardware is linked into online remote networks using a variety of communication options. The software is capable of reading any data and facilitates the statistical analysis of instrument readings, helping the surveillance staff to evaluate the condition of structures and closely trace their development during reservoir filling or emptying or during floods.
Existing dam surveillance systems have been updated with special measuring devices as follows:
Any unusual rise in seepage rate should be monitored because an abnormal increase may indicate a development within the embankment that jeopardises dam stability. Seepage depends on the condition of the dam and foundation. As grout curtains age, the pressure of water may increase and produce pore water and uplift pressure. One way of measuring seepage flow is by using a calibrated vessel and a stopwatch. The precision requirements are satisfied where the container filling time is at least 10 seconds. Using a measuring flume or weir allows exact measurements of a larger quantity of seepage, but this calls for the installation of sensors such as sonar gauges, graduated scales, pressure balances or other sensors.
When monitoring was started at such seepage weirs in 1980, limit switches were first installed to alert the supervisor. Based on experience gained in the manual measurement of seepage flow during operation, a maximum level of seepage was defined to trigger the alarm. Since 1982 the seepage flow values have been transmitted online to the centre and printed once a day on a spreadsheet. An online printer followed in 1987.
Long-term behaviour of seepage was analysed by hand on an overall plan a PC was used for the first time in 1990. Since 1996 the seepage rates of all the dams have been recorded and backed up on CD. The advantage of the new software is that origin data need not be reduced as filesaving does not require a complicated database we are able to store the origin data in Excel. Since 2000 a graphic display of all automatically transmitted data has been available on Kelag’s intranet. There are more than 800 PCs on the intranet, so the quality of transmission, sensors and accuracy can easily be checked from every Kelag office and by all those involved in safety. The graphic display is updated automatically every half-hour, or by hand if there is an interest in special online data. The updated graphic shows a 21-day history of all the automated measuring devices, so every dam safety engineer or technician is fully informed about seepage changes or data relative to dam behaviour.
Piezometers measure confined water in the dam body and foundation in pressure holes or standpipes, to check the long-term performance of grout curtains. As uplift forces do not represent a major safety risk for our fill dams, open and closed borehole monitoring did not start until 1996. A weekly routine check of the water level distribution in the piezometer, compared to the reservoir level, is made by the dam attendant via a fibre optic or acoustic cable. In closed boreholes pressure is indicated by a high precision manometer.
Field measurements have improved throughout the last ten years. Since 1999 special software has been available to help take manual readings using hand-held computers. These computers have extensive storage and communication facilities and a fully graphic display image, as well as individual programming. In the next upgrade we plan to employ a mobile digital assistant (MDA), a new type of pocket PC.
The instrument readings that are input can be checked for plausibility. This check can be based on more than one value and it is possible to use several values for a regression analysis. Comments on the individual measuring stations or observations supplement the report and are integrated into the stock of measured data and filed. Each entry is provided with a station identification specifying the place, type of data and name of the person making the entry. Transmission by modem to a central evaluator station will soon be replaced by a data carrier or other interface.
Optimising horizontal plate gauges (hose levelling devices)
Settlement has been measured by pipe gauges installed in the dam since the construction period in the 1970s. Settlement inside the dams has decreased since construction and now (eg. after 23 seasonal fillings of the Oscheniksee reservoir) the measuring devices are not accurate enough to show a single-year change. A new technology uses electronic devices to read the pressure difference instead of the fluid level, which gives an accuracy in millimetres and reduces the time required for readings by 50%.
The sensor is installed inside the tube, which is equipped with thin graduated PVC rods and can measure settlement down to 0.5m. Displacement is determined at each steel plate location (every 8m) and settlements every metre to obtain a detailed deformation curve. The curve is shown in Excel and is compared with older measurements.
Using Dataview software
Before the Dataview project, manual readings and measurements were too infrequent to obtain satisfactory correlation with the reservoir level. Improvements in the accuracy and frequency of readings has allowed the earth fill and associated structures to be evaluated.
Nevertheless, we must be fully aware that the installation of ADAS brings additional constraints. The main factors ensuring success are staff training and system maintenance. Personnel who have basic computer skills are in fact easier to find at present than specialists in geotechnics or engineering structures. Furthermore, a team of highly competent and motivated specialists is essential for surveillance, or even the best performing instrument systems will be useless. A great number of sensors have been developed and can be used in the field to obtain information about the behaviour of the dam.
Data acquisition and storage
The data acquisition interface is designed to monitor data sensor contacts. The contact is normally closed and opens in the case of a message. The 4-20mA signal allows a cable failure to be identified.
The owner’s requirement on resolving measured data are met by using analog-digital converters offering 10, 12 or 16-bit best-resolution.
This corresponds to a resolution of 0.1%, 0.02% or 0.015% of the maximum sensor value. Data are acquired through a remote-control system where every data point transmitted is provided with clear origin information. The status of the entry system, the time of origin, and information on potential errors caused during transmission are included in the transmission. The remote-control system is designed as a network and can be routed via different paths – fibreoptic, power line courier or radio – to prevent transmission interruption causing complete loss of information. The information is time-stamped as it arrives, helping to assign responsibilities where quick action is required.
Arriving information is protected in one-day data tables instead of in a single large table. Apart from the resulting high working speed, this allows information to be distributed on a decentralised basis, improving safety. Also, it is possible to encode all information (even keyboard entries) via a 128-bit code.
There is a very high level of accuracy for data filing, using cyclic and spontaneous methods, and this allows multiple information storage over many years with a relatively little hardware. So there is no need to thin out recorded data as is necessary for many of the old systems.
Complete checking of the information filed in the data tables is possible because a fixed record format is used, with longitudinal and horizontal checksums for both individual records and overall records. The above mechanisms cannot be addressed by the network users.
Thanks to this system, data acquisition speed is not affected by work with the filed information.
The data entry computer, where the automatic remote-control data accumulates, works at the same time as the so-called pool computer for dam monitoring, so the processed data are available. The computer is equipped, via a second hard disk, with a connection to the Kelag network. The first hard disk acquires the data from the power stations and retrieves internet information for the Dataview.com service.
As the computer is equipped with a network operating system for the acquisition and pool level, it is possible for Kelag’s dam surveillance staff to specify users and access passwords.
There is no access to the first hard disk, so automatically acquired data can be copied in real time without thinning-out. It is easy to use the Dataview program elements and updating, and the improved information flow improves dam monitoring documentation and enhances the staff’s sense of responsibility.
Some graphical displays and information are made available as prepared information on the Kelag intranet. This service allows responsible engineers and dam maintenance staff of Kelag to call up data from the past thirty days for routine assessment. This access to prepared information is possible from each of Kelag’s PCs without requiring the installation of special Dataview software.
Data are normally updated every day at the start of work, but can be renewed at any other time. This easy access allows each user to deposit comments, which include detailed documentation of maintenance work and evaluation of individual events. The comments are password-protected.
Producing safety reports
Workstation computers offer comprehensive access to the intranet and allow individuals to view and assess information and prepare safety reports. The workstations have access to the pool level and have capabilities for dam monitoring, graphical display, messages and reports (including early-warning limits for measured data).
Each workstation can be automatically updated from the pool level. They have identical installations, so that extension is easy. It only requires authorised access to the acquisition and pool level from a new computer. The pool computer also has information for document and master data management available. Data can be exported to all the usual word processors, spreadsheets and databases.
Kelag is testing a mobile digital assistant (MDA) for dam monitoring. In the case of dam failure, flood or other hazards the MDA provides a huge database including the latest monitoring results, facility information and reports, without requiring a connection to the host. However, the resolution of the MDA display is still limited, so the engineer has to preselect the monitoring period. The MDA’s information content is one sixth that of a conventional display screen.
The MDA serves as an information platform as well as a new type of data acquisition device during dam inspection. An input instrument reading can be checked for plausibility. To help identify measuring stations a bar code may be installed and a key card will be possible. Comments and photographs relating to the measuring stations, or observations, supplement the inspection tour and are integrated into the stock of measured data and filed. The results can be sent immediately to the office via data carrier or mobile telephone. New technologies (transponders, USB-sticks, hard USB flashdisks etc) will soon allow a substantial extension of decentralised data gathering.
There is no doubt that monitoring systems contribute immensely to our ability to ensure dam safety. Automation offers new methods of observing dam behaviour and speeds up emergency assessment. The reservoir level data immediately available provides a basis for decisions in extreme situations such as floods, earthquakes or structural failure. Further development of automated data acquisition systems will speed up the creation of expert training systems and networks such as the “Hydro Power Competence Network” or the “Integrity Assessment of Large Concrete Dams” on the World Wide Web, although the dam engineering profession has not yet reached agreement on these systems.