An Unstructured Flexible Nonlinear Model for the Cascaded Water-tanks Benchmark * *The corresponding author can be contacted at the (Rishi.Relan@vub.ac.be). This work was supported in part by the IWT-SBO BATTLE 639, Fund for Scientific Research (FWO-Vlaanderen), by the Flemish Government (Methusalem), the Belgian Government through the Inter university Poles of Attraction (IAP VII) Program, and by the ERC advanced grant SNLSID, under contract 320378