To resolve the flooding problems of Ho Chi Minh City, the possibilities for a large barrier downstream of the city are under study. In the final design stage a detailed 1-or 2-dimensional mathematical model is necessary to describe water levels and flow in the branches of the estuary system. The authors plea for the use of simple, first-order models, additional to a detailed model. The use of such models is shown as a tool in the preliminary stages of such a project but also as a reflection tool to get a better understanding of the system. The authors strongly argue that such tools are indispensible since the design process is never smooth and is never a linear sequence of theoretically logical steps. Several examples are being given.