Peti dam: 22 new cracks in a year, and how we count them
A year after the baseline, we went back to the Peti dam in Minas Gerais from 12 to 15 September 2023, this time to compare. The 2022 reference counted 4,052 cracks with a cumulative length of 2,023 m; the operator expected this second campaign to list what had changed. That requires two campaigns that can be superimposed to within less than a millimetre, and a way to tell a change in the concrete from a mere change in lighting.
The same projection surfaces
The 2022 protocol was repeated exactly: GNSS base on the same benchmark on the left bank, a general flight of 787 photos with the drone carrying the 45-megapixel sensor, an overall orthophoto at 1.2 cm including the crest, then a 3D flight plan of 1,649 photos 6.5 m from the downstream face, i.e. 0.8 mm per pixel. The upstream face was covered with a compact RTK drone. The photos were reprojected onto surfaces strictly identical to those of 2022, the cylinder of the arch and the planes of the side blocks, and the face orthophotos were registered on the baseline.
Two categories of change
From this second campaign onwards, the comparison has used two categories drawn against the baseline: new or longer cracks, and cracks or sections that have become more visible. The opening measurement stayed constant, within what the pixel size allows; the width of 400 cracks was measured at one point.
A grid of 68 cells
So that the operator’s engineers can find changes quickly, the downstream face was divided into 68 cells referenced by a letter and a number. A general view flags the cells where a change was recorded, and each cell has its own comparison plate, with the new and more visible cracks drawn on the face orthophoto. This format was defined with the operator in October 2023, based on its proposal, and approved in November.
Result of the second campaign
Between 2022 and 2023, 22 cracks appeared or grew longer, totalling 4.358 m: +0.54% in number and +0.22% in length compared with the baseline. 54 cracks or sections became more visible, totalling 13.1 m. Only the length of a few cracks increased; the measured openings remained constant. Two factors complicate the reading from one year to the next: the dam’s setting, which governs how it is lit, and dirt on the face.
The report, dated 8 January 2024 and delivered with the year’s face orthophoto, the shapefiles of both categories and the 68 plates, set the format for the following campaigns: 4 new or longer cracks in 2024, 6 in 2025.
