The reservoir of Saint-Denis has two goals. Fight against flooding by optimizing draining and treat the pollution by settling the water.

Flooded corridors of the Giraud-Morel drain network.

Most of the inhabitants of Lyon know that subsoil of the Croix Rousse and Fourvière is traversed by numerous galleries. Before the various prohibitions (and even after) to walk the subsoil of Lyon in the late 1980s, students were fond of underground walks. Moreover, there were few entries directly into schoolyards (now closed).

Fish bones (also abbreviated ADP) are part of a network of much larger gallery, called Fantasques, due to the name of a road, above the galleries. As its name suggests, the network looks vaguely to a structure of a wheat. In fact, it is a network of parallels arranged around two dorsals (or main roads) with an orthonormal structure, much like certain Nazca archaeological symbols, or even a TV antenna, but definitively not like fish bones.
The main feature of these undergrounds is their purpose was not to drain water, unlike the other two parts of the network (The Colinettes and large galleries). Most galleries are straight, flat, millstone, with dry soil, with the notable exception of the main corridors, steep slope with many stairs.

A piece of aqueduct joins the other drain corridors.