The focus of this paper is on applications (e.g. some leachate collection layers in landfills) where a drainage system consists of a geocomposite overlain by a sand layer. If the geocomposite does not have sufficient flow capacity to convey all the collected liquid, a fraction of the liquid flows in the sand layer. In such drainage systems, the maximum liquid thickness should be calculated to check that the flow capacity of the sand is not exceeded, and the maximum head should be calculated to check that it is less than the maximum head prescribed by regulations. This paper presents a method for calculating the maximum liquid thickness and the maximum head in drainage systems composed of two layers, with the lower layer being a geocomposite