PHYSICAL MODELING OF THE FEEDBACKS BETWEEN A REED CANARYGRASS (PHALARIS ARUNDINACEA) PATCH, HYDRAULICS, AND BEDFORM EVOLUTION (Invited Presentation)

Reed Canarygrass (RCG) is an aggressive invader in aquatic ecosystems that swiftly modifies the environment and decreases biodiversity. A physical model was used to investigate the mechanisms by which RCG spreads. Experimental trials were implemented to examine feedbacks between a mid-channel patch of the flexible RCG, hydraulics, and bedform evolution as level of submergence increased and plant deflection occurred. The objectives of the study were to (1) document wake length scales and depositional patterns with increasing plant submergence, (2) characterize variability in wake characteristics with depth and (3) identify possible mechanisms for RCG expansion based on the observed positive and negative feedbacks. In this study, longitudinally-streamlined patch expansion was promoted at low submergence levels through the positive feedbacks of greater wake zone deposition and a long wake length scale. At low submergence, lateral scour served as a negative feedback for lateral patch expansion. Increased submergence resulted in stem deflection and introduced vertical shear turbulence in the upper portion of the wake profile. This turbulence shortened wake length scales and diminished wake zone deposition, resulting in a negative feedback for streamlined patch expansion. However, diminished lateral velocities and near-bed stem deflection at increased submergence created a feedback for lateral patch expansion. Understanding of these feedbacks helps to inform management priorities and anticipate the geomorphic impacts of RCG invasion such as diminished habitat diversity, decreased flood conveyance and channel incision.