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拥有一个由测量师组成的内部团队，其中三名培训用于使用Leica Aibot进行测量任务。AXPO的工程测量师Thorsten Zimmer负责Lai da Nalps的检查项目。

“使用无人机扩大了我们可以用于日常任务的可用设备技术工具箱。亚博5分钟快三总站，GNSS Rovers和较小的无人机对我们来说是常见的工具，因为它是Leica Aibot，”said Zimmer.

大坝检查中的游戏规则技术

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.

用莉卡·艾博特（Leica Aibot）进行调查

计划使用AIBOT每五年进行一次无人机检查。对大坝的检查需要两到三天，总飞行时间约为10个小时。在飞行过程中，Aibot每两秒钟拍摄一次自动图像，使飞行员能够专注于以恒定的高度和距离墙壁附近的距离飞行。检查是一项两人的工作：一个人在无人机上运营，重点是飞行位置，速度和身高，而另一个人则观察周围环境，重点是拍摄的照片的质量和重叠以及电池。

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”评论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. 洪水建模也是无人机减轻与大坝溢出有关的风险的有趣项目。地质学家研究可能滑入大坝湖的潜在危险的土地和岩石。计算这些卷，然后用于仔细的何种分析。
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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