The application of geosynthetics in hydrotechnical engineering requires an appropriate selection with respect to soil properties and operational conditions of the structures. The paper presents the case of the slope failure of a deep drainage channel caused by the geotextile clogging phenomenon resulting from selecting an inappropriate material with respect to soil parameters. Due to rapid changes of water level in the channel, which will occur during operation, it was required to protect the slope from periodically occurring seepage forces. The slope reinforcement was performed by placing the draining gravel layer between geotextile on the cohesive soil. On the top the openwork concrete plates were laid. Despite the reinforcement, during intense rainfall even before the channel was filled, a long section of the slope slid down and destroyed the ecological reinforcement of channel bottom. In the paper this phenomenon was explained by the analysis of slope equilibrium conditions, along with the laboratory tests of water permeability and the clogging process of the applied geotextile.