The invention relates to a hydraulic plant (I) that defines a path (4, 5, 6) for a forced water flow (E), and comprises at least one machine (1) of the turbine, pump or turbine-pump type, a member (8) provided with at least one deformable wall (81) and defining at least one closed volume (V8) with variable dimensions. The plant also includes means (9, 91) for supplying (Fg) the closed volume with a pressurized fluid. The member with a deformable wall (81) is mounted in a hydraulic duct (6) located downstream from a rotary portion (2) of the machine (1). The deformable wall defines, locally and downstream from the rotary portion (2), a modular section (S63) for the flow (E) passage in the hydraulic duct (6). According to the method of the invention, a pressurized fluid is injected (F9) into a closed volume having variable dimensions (V8) and defined by a member (8) provided in a hydraulic duct (6) located downstream from the rotary portion (2), in order to modulate the passage section (S63).