Author(s): Florian Cordier; Geraldine Nogaro; Nicolas Claude; Bastien Jouy; Sara Puijalon
Keywords: Quatic vegetation; Drag force; Uprooting force; Numerical modeling; Hydraulics; Rhone River
Abstract: In recent years, the development of certain species of macrophytes (i.e., aquatic plants large enough to see with the naked eye) in French streams and rivers has led to recurring problems for users and managers of aquatic environments (fishing, navigation, water pumping, etc.). A major issue is that if these plants are transported by rivers (for example, in the event of high flow), they can induce the clogging of the water intakes of hydroelectric structures. A research program between EDF and UMR CNRS 5023 is currently underway to better understand the drifting mechanisms (uprooting conditions and force) of macrophytes in fluvial environments. The objective of this numerical study is to determine the flow conditions leading to the uprooting of macrophyte meadows (i.e., when the drag force of the plants becomes greater than the uprooting force) and to study their transport within a reach of the Rhône River. This work is based on the use of the two-dimensional Telemac-2D hydrodynamic code (developed within the open-source Telemac-Mascaret system) coupled with a module simulating the uprooting of macrophytes. While previous models only accounted for the drag force induced by isolated plants, the current model considers a new formula calibrated on laboratory flume measurements, to account for the drag force of plant patches. Comparison of these two approaches suggests that uprooting dynamics might be very sensitive to the drag force equation, macrophyte biomasses, and flow rates in the river, but less sensitive to the type of morphology and species of macrophytes. These preliminary results need to be confirmed by field data using a new experimental device for measuring the uprooting force of aquatic plants and tested with long-term data of macrophyte dynamics.