The aim of this work is to present an application of recent methods for solving the l1 design problem, based on the Scaled-Q approach, on a high-order, non-minimum phase system. We start by describing the system which is an open-channel hydraulic system (e.g.: an irrigation canal). From the linearization and discretization of the set of two partial-derivative equations, a state-space model of the system is generated. This model is a high-order MIMO system (five external perturbations w, five control inputs u, five controlled outputs z', five measured outputs y, 65 states x) and is non-minimum phase. A controller is then designed by minimizing the l1 norm of the impulse response of the transfer matrix between the perturbation w and the output z=[z' z'], where the five additional variable z' are defined as z'=Du u. Considering this additional transfer (w to z') in the minimization problem leads to a better posed problem and provides much better robustness margins. Time-domain template constraints are added in order to force integrators into the controller. The numerical resolution of the problem proved to be efficient, despite of the characteristics of the system. The obtained results are compared in the time-domain to classical PID and LQG controllers, both on linear and non-linear simulated plants. The results proved to be very good in terms of performance and robustness, in particular for the rejection of the worst-case perturbation.