Hydraulics and sediment transport around vegetation patches
This study examines the effects of the invasive riparian ecosystem engineer Phalaris arundinacea, or Reed Canarygrass (RCG), on riverbed form evolution and subsequent feedback cycles. The linkage between the interdependent processes in riparian biological communities and fluvial geomorphology works to alter the aquatic landscape as described in the biogeomorphology concept. It is found that many riparian vegetation species act as ecosystem engineers by creating pioneer landforms that impact channel morphology. This alteration of the river's physical landscape results in accelerated reach scale morphological changes such as an increase in flow velocity, alterations to the Manning’s roughness and an increase in the availability for riparian colonization sites. In the case of P. arundinacea, propagation results primarily from rhizome shoots, floating rhizome mats and seed dispersal in a wide range of ecological conditions. With the highest germination rates in saturated soils, RCG can easily spread in many aquatic habitats, including rivers where the plant can form mid-channel patches. This research will investigate the invasive Reed Canarygrass’ control of fluvial landform development by quantifying the change in bed form topography as a result of changing depths of submergence in a hydraulic flume. Using Froude similitude, vegetation is scaled to flume channel geometry using depth as characteristic length. Through the implementation of a physical model, feedbacks associated with bed form evolution that further link biological and physical processes will be explored. A primary hypothesized feedback is as depth of submersion increases and plant becomes fully deflected, P. arundinacea decreasingly contributes to bed form development in the low velocity wake region, which leads to a decreased surface for further colonization behind the patch. A second possible feedback of the bedform expansion includes the narrowing of the river channel and increased lateral velocities , maintaining a transport mechanism for further downstream rhizome and seed dispersal but also limiting patch expansion laterally. The model is based on a prototype reach on the Sprague River near the former Chiloquin Dam site. The results of the proposed research will ultimately contribute to the fundamental understanding of the linkage between fluvial geomorphology and vegetation, as well as the management of invasive species, specificall within disturbed areas due to channel restoration.