Canopies formed by flexible aquatic plants constitute an important element in river hydraulics as they determine resistance, resilience towards erosion, transport of nutrients and pollutants, and provide the habitat of other species. Due to their flexibility they undergo reconfiguration and closely interact with the surrounding flow. This fluid-structure-coupling poses a considerable challenge to detailed numerical modelling. In the present contribution a recently developed own method is presented capable of numerically simulating such a situation. It is applied to a model configuration composed of very long flexible blades which corresponds to an experiment presently conducted in Lyon. A particularity of this situation, so far considered only rarely, is the length and high flexibility of the blades leading to a Cauchy number Ca around 25,000. The parameters are such that the monami phenomenon is observed featuring synchronized undulating motion of the blades. This very detailed simulation at high Ca is the first of its kind with very long, flat blades typical for aquatic plants. The contribution reports first results for the velocity field in terms of highly resolved instantaneous data and one-point statistics.