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The in-plane water flow capacity is the significant parameter for the drainage system design, whether the product is made on site or in a factory. The short-term water flow capacity of drainage systems is measured in accordance with the EN standard for given conditions like compressive load, hydraulic gradient and contact surfaces (soft/soft or rigid/soft or rigid/rigid). The impact of the compressive load and contact surfaces on the water flow capacity depends on the type of components, particularly the drainage core structure and the type of filtration geotextile. The study described in this paper evaluates two types of regression (logarithmic and power regression) to simulate the drainage system performance of drainage cores built with a 3D monofilament structure welded or stitch bonded on both sides to a thermally bonded non-woven geotextile. It allows the assessment of the long-term water flow of the product on the basis of spot measurements of water flow capacities including both thickness reduction of the drainage core and the filter intrusion in the drainage core on the long-term. These measurements were carried out during two compressive creep test durations: up to 20 months by SKZ’s Geosynthetics laboratory and more than 10 years by Low and Bonar.