This International Standard specifies a method for determining the constant-head water flow capacity within the plane of a geotextile or geotextile-related product. NOTE 1 If the full water flow capacity characteristics of the geotextile or geotextile-related product have previously been established, then for control purposes it can be sufficient to determine the water flow capacity at two loads and both gradients. NOTE 2 The compressibility of the product over time will substantially influence the in-plane water flow capacity. Test methods for assessing the compressive creep behaviour of geotextiles or geotextile-related products are described in ISO 25619-1. The test report is judged in conjunction with the long-term compressive creep behaviour in order to assess the long-term flow capacity.