Physical modeling of the feedbacks between a patch of flexible Reed Canarygrass (Phalaris arundinacea), wake hydraulics, and downstream deposition

Abstract

Reed Canarygrass (RCG) is an aggressive invader in aquatic ecosystems that swiftly modifies the environment, resulting in biodiversity loss and reduced flood conveyance. However, because so little research has been conducted on RCG along riverbanks and islands, the mechanism by which it so rapidly colonizes rivers is not well understood. This study examined how colonization of RCG, via the expansion of an individual patch through wake deposition, may be driven by differences in flow depths. A set of mobile-bed experiments were conducted in a physical model to examine the influence of submergence depth on wake characteristics and deposition around a mid-channel, emergent patch of flexible RCG. At low submergence (31%) depths, low wake velocities promoted streamlined deposition behind the patch that is associated with longitudinal patch expansion. At deeper submergence (75%) depths, the introduction of vertical shear over a portion of deflected stems produced less overall deposition dispersed across the channel. Thus, it was found that, as submergence increased, positive feedbacks for longitudinal expansion of patches decreased. These results indicate that, from a management perspective, targeting removal of RCG patches at shallower submergence depths, and/or modifying the hydroperiod for deeper submergence of the plant, is likely to be more effective than targeting plants subjected to deeper flows.