The failure of steel liner of pressure shafts of hydropower plants can have catastrophic consequences. The partial or total loss of the liner support’s rigidity is characterized by a local change of the hydro-acoustic properties and creates reflection boundaries for the water waves. This paper presents a new monitoring method for detecting and locating weak reaches by processing reflected water-hammer waves at the boundaries of the shaft. These water-hammers are generated experimentally by a shut-off valve installed at the downstream end of a test pipe. The weak reaches are modeled by replacing the steel of the pipe’s wall with aluminum and PVC materials. Different test pipe configurations are created by changing the position of these reaches along the test pipe. The monitoring procedure uses the Fast Fourier Transform and wavelet decomposition techniques to estimate the incident-reflection travel time between the pressure sensors and the weak reaches boundaries.