In this paper, a novel procedure for the optimal design of rural drainage networks, based on the coupling of a Genetic Algorithm with suitable hydrologic and hydraulic models, is presented and demonstrated. The models used, which allow the evaluation of the water depth and discharge through the network, differ for the level of simplifications introduced into the representation of physical phenomena (rainfall-runoff transformation and flood wave propagation through channels). We show their application and discuss how the use of oversimplified approaches, although very popular in practice, can adversely affect the characteristics of the optimal network.