Drainage modules are storage units for rainwater harvesting and are used to promote groundwater recharge, storage, quality and quantity control as well as reuse of surface runoff. Different assessment methods were applied in the past to assess the efficiency and performance of subsurface drainage modules including laboratory experiments, field and numerical. However, very few methods had been identified to use computational fluid dynamics (CFD) techniques to assess the performance numerically. In this paper, CFD was applied to study the flow parameters of Rainsmart modules. Among the findings of this study is that the module used alters the pattern and creates flow resistance with a higher Manning's roughness coefficient of 0.10 at the US and a lower one of 0.02 at the DS which signifies the effect of the module. The modules pattern had been observed to be the major cause of the variation with the retardation occurring at the blocked parts and the increase across the porous openings. The study, however, recommends the application of numerical techniques to study the effect of the module under different slope and gate opening and closing conditions. © Published under licence by IOP Publishing Ltd.