This study presents numerical simulations of advective flow through a composite geomembrane geosynthetic clay liner (GMB-GCL). In the past, GCLs were considered homogeneous materials, but they actually consist of a special layered composite structure that combines two types of materials, geotextiles and bentonite, which are connected together by various processes. One could imagine that, when the GCL hydrates, the different water-retention properties of the geotextile and the bentonite affect the hydraulic behaviour of engineered systems, including GMB-GCL composite liners. To investigate this question, the advective flow through a composite liner modelled as a GCL and a damaged GMB was numerically simulated to evaluate how the hydraulic properties of the unsaturated geotextile and bentonite influences the temporal evolution of advective flow through composite liners. Results are compared with measured water-retention curves of geotextiles and bentonite. The simulation indicates that the reproduced flow rate is influenced by the desaturation of the geotextile that occurs as the bentonite hydrates. The reduction in flow rate is thus governed by the hydraulic conductivities of the geotextile and the bentonite, both of which vary with degree of saturation.