Between the 1980s and 1990s, numerous archaeological operations have allowed us to better understand the organization of the Roman city of Tarragona (Tarraco). The urban layout, the different monumental buildings of the town and its water supply network were all discovered. However, only part of the hydraulic network was analyzed. Indeed, if the water resources and the aqueducts of the capital of Hispania Citerior are globally known, the water evacuation system has not yet been the subject of a systematic study. Thus, I propose to present the "hidden face" of the hydraulic network of the provincial capital. The drainage networks in the ancient period collected both run-off and wastewater (combined network) and functioned solely by gravity (gravity network). They are therefore dependent on topographical and more broadly environmental elements that we will analyze first. Then, I will present the Tarraco drainage system through two levels of study. Firstly, at the neighborhood level, in order to understand the interactions that can exist between the individual drainage networks that are organized at the scale of a building, and the public/collective drainage network of the agglomeration. Secondly, I will focus on the collective drainage network, at the city level, in order to establish the articulations of Tarragona’s management system of its undesirable water. The different drainage installations in the city will be described in the light of the structures found in other Roman towns, particularly in Gallia Narbonensis.