Momentum and Scalar Exchange Between Channels and Vegetated Banks

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

0125056 Nepf Wetland systems often consist of a branching network of channels cutting through regions of dense vegetation. While the channels provide most of the flow conveyance, the vegetated regions provide most of the bioremediation and particle trapping. Thus, exchange between the channel and vegetated zone may control the overall wetland function, in particular with regard to water quality and sediment trapping. A similar relationship exists between the channel and vegetated bank of a river system. Laboratory experiments will be used to quantify the exchange of momentum and mass between a channel and a zone of emergent vegetation of equal or lesser depth (a bank), and to develop a model that predicts this exchange from representative vegetation and flow parameters. In the flume model, the exchange will be observed by tracking the evolution of dye plumes released from different locations within the channel and vegetated zone. The model vegetation will be designed using geometric and dynamic similarity to real vegetation. A preliminary flow visualization study will be used to assess major mechanisms, to define ranges of behavior across different geometry, and to refine the experimental system. Subsequent detailed studies will simultaneously measure concentration and velocity to directly quantify turbulent fluxes of momentum and mass. The study will draw from existing models for floodplains, including the effects of vegetation on hydraulic roughness and floodplain geomorphology. After the completion of this project, future phases of this research will integrate the new model for exchange between channels and vegetated zones with existing models for channel flow and vegetated flow to construct a comprehensive wetland model capable of predicting wetland circulation and transport. This model will provide an important tool for managing wetland systems by enabling stake holders to predict the changes in wetland function that will arise with proposed development and land-use changes.