Fibre optic sensing for dam monitoring6 April 2020
Michael Mondanos & Sam Johansson from Silixa explain how distributed fibre optic sensing enhances dam safety by providing early alerts of potential problems
Dams used for hydropower, irrigation or mining are considered an installation containing dangerous forces with significant human, economical, and environmental consequences when they fail. Unfortunately, such incidents have happened.
XSeepT, a high-precision, fibre-optic sensing-based seepage monitoring system, provides continuous understanding of dam conditions, taking dam safety to a higher level. Developed by HydroResearch, a Swedish company, XseepT utilises high spatial resolution temperature data from Silixa’s ULTIMA DTS, distributed temperature sensor, to detect small seepage flow changes and to calculate seepage rates in a dam structure. Any change in the seepage flow rates can be an indication of erosion, one of the most common causes of dam failure.
The concealed cause of failure
Tailings dams are built in complicated geological environments and are often partly made of mining tailings. They face many challenges due to external loads such as heavy seasonal rainfall, water level changes and seismic disturbances due to mining activities or naturally occurring events.
Seepage occurs in most embankment and earth dams as the impounded water seeks the path of least resistance through the dam and its foundations. Excessive seepage becomes a concern and may be a threat to the integrity of the structure that could lead to the failure of the dam. Mapping seepage paths by continuous and real time monitoring requires a dense deployment of sensing networks, and high-tech surveillance systems to capture and understand subsurface processes and prevent erosion. Distributed fibre optical sensing is a key technology that can be applied to monitor critical geophysical parameters such as temperature and strain with a sub metre resolution over several km.
Distributed fibre optic sensing offers the advantage of a cost-effective high spatial monitoring coverage.
Figure 1: Example of location of optical fibres (red dots) in a new dam, also used for research with more fibres than normally used
In the past few decades, several new dam monitoring methods have been developed, amongst them XSeepT, the seepage measurement-based monitoring solution that analyses the natural seasonal temperature variations.
As the fibre is the sensor, measurements can be taken along the full length of the fibre optic cable enabling operators to gain detailed information along the entire dam. Distributed sensing makes it possible to detect very small, localised changes in the seepage flow rates that would otherwise go unnoticed. It delivers temperature readings with the precision of point sensors with the undisputable advantage of fibreoptics: the highest possible spatial coverage. As no specialised fibre is required, installation costs are relatively low-cost.
The seepage evaluation method uses the natural seasonal temperature variation that occurs in all surface water. The seepage flow causes a seasonal temperature variation inside the dam. This seasonal temperature variation can be measured in the water reservoir and used to evaluate the seepage flow. The level of the seasonal temperature variation inside the dam depends on the seepage flow rate. By measuring, and comparing these two seasonal temperature variations, it is possible to detect small seepage changes and to evaluate the seepage flow.
The temperature field inside the dam will thus be a result of the seasonal temperature variations on the surface of the dam and the seepage flow rates passing through the dam. With increasing distance from the interface boundary between the reservoir water and the embankment there will be a time lag of the seasonal variation.
The approach is sometimes called the “passive method” to distinguish from the “active method” where heat is added via resistive cables and the thermal conductivity is evaluated.
Figure 2: Installation of optical fibres in the downstream part of the dam during construction
Sweden’s pioneering approach
Measurements based on Distributed Temperature Sensor (DTS) monitoring give information along the entire dam with high spatial resolution and high temperature accuracy. This method has already been applied extensively on tailings dams in the Scandinavian mining sector since 2000.
Sweden has enthusiastically embraced the deployment of fibre optics, where approximately 100 hydropower and tailings dams are equipped with optical fibres. Installations in tailings dams have been made at the five largest mining sites, or in about one third of the total number of tailings dams. Six units are in operation in those tailing dams with a further 8 in hydropower dams
These installations have provided the sector with a wealth of experience and are considered a success. Hence, fibre optic monitoring in new dams has become the de facto standard.
One key component of the increasing number of permanent instruments is the ever-increasing performance of the DTS monitoring units. Silixa’s ULTIMA DTS and XT-DTS distributed temperature sensors provide the world’s most precise measurements, with sampling resolutions of 12cm (over 5km) and with temperature resolution as low as 0.01oC. These units have been in use at a number of dams worldwide, including the US, Canada, Norway, and Finland
Figure 3: Installation of cables in vertical standpipes
Installations in new dams
The optimal location for seepage monitoring is below ground water level inside the dam or at just upstream the drainage system in the dam toe, and on the crest for measuring movements or to be used for dam breach verification (see the green dots in Figure 1).
The seepage flow through an embankment dam can be detected by temperature measurements in the upstream filter (where inflow occurs) and with measurements in the downstream part where the seepage will pass on its way downstream (see the cable installation in Figure 1, the red dots). The same cable location can be used for both passive and active measurements. Cables can also be installed on the crest for measuring movements or to be used for dam breach verification.
Cable installations in new dams offer excellent possibilities to optimal place the cable for optimum measurements, considering the actual dam design as well the foundation properties. Most of new dams are therefore equipped with optical fibres.
Seepage measurements in existing dams
Most installations in Sweden are made in existing dams by installing the cables in the dam toe, upstream of the drainage system, or just downstream of the dam. Additionally, two installations have been made where normal seepage collection can’t be used, such as the situation where there is a high water level downstream. Cables are then installed in boreholes where the temperature is measured. The high special resolution with Silixa’s ULTIMA DTS allows detailed seepage flow information both in the dam (Figure 4) or in the foundation, if the standpipes have been installed there. Two more installations of this type are planned for 2020.
Figure 4: Temperature measurements in 11 standpipes (black vertical lines) in the downstream toe
Seepage detection based on optical fibre distributed temperature sensing has been established as a key technology and has significantly improved monitoring capabilities of dam operators. The installation of optical fibre sensing networks offers additional advantages as the capability of fibre optic distributed sensing develops further. HydroResearch and Silixa are developing additional measurement techniques based on the same optical fibre cable to map seepage below the drainage system and identify density changes in the subsurface geology. These new monitoring techniques involve the development and evaluation of distributed acoustic sensing using both passive and active seismic surveys.