This work was undertaken in Furnas Centrais Elétricas S.A. and presents the results of a research concerning some surge flow in drainage galleries from two dams. Those occurrences were observed during the filling period of their reservoirs. The materials collected from one of the dam’s presented slurry consistency with beige color; in the other dam, the materials consist of several spherical particles of small dimensions, with similar color from the first.

Several laboratory studies were performed using different techniques in order to obtain the chemical, physical and morphological characterization: X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray microanalysis, X-ray fluorescence spectrometry and laser granulometry. As a complement, water, soil and rock samples of the dams were also analyzed in lab.

The results indicated calcium carbonate as the main constituent with some other minor constituents. A high electrical conductivity of the water in contact with those materials was also observed, what differs from the river water of one of the studied dams.

One possibility to explain this phenomenon would be the leaching of concrete residual material from its open voids by the water during the reservoir filling. This would be carried out through the drains, concentrating specially calcium. The observed morphology of the materials can probably be explained on the water movement along the drains, although supplementary studies are necessary to aggregate more information.