In the present work, the problems of leakages happened in Tous’s dam (Valencia-Spain) are studied. Tous’s dam is built on karstic terrains with the existence of caverns and strike-slip faults that at the same time become karstic. In this study the techniques of structural geology are applied, so, we deduct the karstic drainage directions, by using determined hypothesis of work. The confirmation of these hypotheses in the course of our fieldwork has permitted to quantify the probability of those leakages for each direction we found. This circumstance has permitted to foretell the places where it would be possible to expect leakages with the increase of the height of the dam foreseen for a second phase of building.