Inspecting dams for sustainable hydropower

Author: Anne Pitkaenen

When you approach Lai da Nalps, a reservoir in the municipality of Tujetsch, Grisons, Switzerland, the massive dam structure of 127 metres height and 480 metres width opens majestically already from the distance. The large arch dam is built for hydroelectric power generation at almost 2,000 m above the sea level. Hydropower is considered the backbone of today’s high security of electricity supply in Switzerland.

Lai Da Nalps is located close to the Gotthard tunnel, this and other dams nearby are monitored automatically for acute movements and changes. In addition to the detailed monitoring, an inspection of the dam construction is done visually to document the condition for long-term understanding of the dam’s health.

The dam is operated by Axpo Group, the largest producer of electricity from hydropower in Switzerland. The company is also responsible for the inspection and development of the surrounding area with roads, bridges, a dam lake, the power plant and the dam.

The concrete construction of the dam is designed to withstand extreme loads, which only occur every 10’000 years. This will ensure it won’t risk the lives and livelihood of the people in the valley. However strongly built, the behaviour of the dam and its surroundings and the state of the construction is intensively observed by Axpo personnel.

Axpo has an inhouse team of surveyors, with three of them trained to use Leica Aibot for surveying tasks. Thorsten Zimmer, an engineering surveyor at Axpo, is responsible for the inspection project in Lai da Nalps.

“Using a UAV has widened our toolbox of available equipment technologies we can use for our daily tasks. Total stations, GNSS rovers and smaller UAVs are as common tools for us as it is the Leica Aibot,” said Zimmer.

Game changer technology in dam inspection

Before surveying with UAVs, one option for dam inspection purposes was to take images from below the dam and from the mountain walls surrounding it. For a dam of the size of the Lai da Nalps, the surveyor occasionally had to climb the surrounding hills, have the camera positioned correctly and take the photos, finally covering the entire wall. Another option was to move up and down along the outside of the dam, standing inside a basket or a gondola, and document everything from there. Even with precise planning and the perfect locations, these methods were very time-consuming, sometimes even dangerous, and couldn’t deliver as highly detailed images and quality results as the ones taken with a UAV facing the dam from an approximate 10 m distance.

Before owning a Leica Aibot, the Axpo team had already been a user of Aibotix Aibot X6, which they used for several years for surveying. Now it was the time to upgrade because flight time is started to be a challenge with the previous Aibot. Previously during large inspections - due to much shorter achievable flight times - one person was responsible just for recharging the batteries. With the new Leica Aibot surveyors can use more time and energy focusing on the inspection and not just keeping the system in the air.

Surveying with Leica Aibot

Every dam Axpo operates is planned to have a full UAV inspection every five years using the Aibot. The inspection of a large dam takes two to three days and approximately 10 hours of total flight time. During the flight, the Aibot takes automatic images every two seconds, enabling the pilot to focus just on flying the copter at a constant height and distance close to the wall. The inspection is a two-person job: one operates the UAV with a focus on the flying position, speed and height, while the other person observes the surroundings, focusing on the quality and overlap of the photos taken as well as the status of the batteries.

Axpo uses a Leica Aibot AX20 UAV solution equipped with Sony α7RII sensor and a 55 mm lens. The 42 MP and crisp clear images ensure that data and results are accurate. To create an accurate and reliable point cloud, Axpo uses a Leica Nova TS60 total station to measure control points for post-processing of the data. The dam has several visible elements that can be used as control points, to be able to position and scale the point cloud. Measuring and documenting the control points can be done before, in between or after the flights.

“The conditions at the dam are challenging because of the sheer size or the arching dam and limited access to GPS. We needed a stable UAV that the pilot can easily operate because sometimes one has to fly without the assistance of GPS yet still holding the UAV in a stable and safe position. We use Leica Aibot because it is easy and safe to operate along our large dams and because we are able to have high-resolution images of the outside faces of our dams, which are needed to achieve the level of detail we require for the end result and the final 3d-model” comments Zimmer. As surveyors we usually choose the solution that fits for the task: for some tasks we use for instance a total station, a scanner or a smaller UAV, especially for indoor inspections, and for other projects the Aibot is the right solution – or even a combination of all”.

During the snowless time of the year surveyors are busy with the inspection on the dams. The collected data and images will be processed into a point cloud and analysed by geologists and surveyors during the winter. The results are part of the long-term analysis of the dam to plan if any preventative renovation is needed and when.

Endless possibilities

Surveying UAVs change the geospatial industry and offer enormous potentials for surveyors. Axpo uses the Aibot for the inspection of several active dams and for other indoor and outdoor surveying projects.

1. The company uses Leica Aibot to inspect the inaccessible parts of the dam. Once water levels are low, the team inspects the structures of the dam that are usually under water. The visual inspection of the submerged structure is done by combining unmanned UAV inspection technology and manned inspection from the structure.
2. Flood modelling is also an interesting project for UAVs to mitigate risks related to dam spilling. Geologists investigate potentially risky masses of land and rock that could slide into the dam lake. Volumes of those are calculated and then used in careful what-if analyses.
3. Construction projects and the infrastructure that supports them are large surveying projects for Axpo which can also be supported by UAV captured data.
4. Axpo also uses the Aibot for documentation of river banks by flying along inaccessible areas and for certain planning purposes.

“The Leica Aibot is a solution that fits our purposes, especially for large and detailed inspections of major dams. It is interesting to see how versatile the Leica Aibot is and in how many ways it can be used by our team,” concludes Zimmer.

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