A sheathing of elastically deformable waterproofing sheet material (11) is applied and sealingly secured to a surface area of a hydraulic structure (10; 31; 36, 39) to be protected. The seeped water, which collects behind the waterproof sheathing (11), is discharged by gravity through one-way drainage valves (13), provided in pre-established drainage positions of the waterproof sheathing (11); the drainage valves (13) are automatically opened and closed by the differential pressure of the water acting on opposite faces of a flexible sheet like flat valving member (M) of the drainage valve (13).