Incorporating Floodplain Functions into River Restoration Engineering: The Role of Floodplain Vegetation on Channel-Floodplain Hydrodynamics

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Because of their distinct role in linking riverine and terrestrial ecosystems, floodplains provide numerous benefits or services to natural and human systems, such as floodwave attenuation, biogeochemical reduction of pollutants, and transfer of nutrients. These services rely on the transfer of water between a river channel and floodplain surface during flooding events, also known as hydrologic exchange flows. Floodplain vegetation plays a particularly important role in controlling river-floodplain hydraulics and hydrologic exchange flows, and therefore can influence many floodplain services and channel stability. However, most river restoration efforts rarely consider the influences of floodplain vegetation because we do not adequately understand the hydraulic controls of vegetation on stable restoration designs nor the degree to which vegetation influences exchange flows and floodplain services. Therefore, the goal for this research is to improve our understanding of the coupled effects of floodplain vegetation on hydrodynamics in restored meandering channels and on hydrologic exchange flows that are important for floodplain services. To meet the above stated goal, this research targets accomplishing the following core objectives: (1) advance conceptual knowledge of the vegetation-flow hydrodynamics related to momentum exchange in meandering channels with natural bed topography; (2) investigate the influence of floodplain vegetation on hydrologic exchange flows on floodplain surfaces; and (3) evaluate relationships between channel hydrodynamics and hydrologic exchange flows on floodplain surfaces pertaining to river-floodplain restoration design. This research will address these objectives using a combination of flume experiments and numerical modeling that are linked to characteristics of local river restoration projects. Flume experiments will be conducted to explore the hydrodynamics of overbank flows in meandering channels while systematically varying floodplain vegetation characteristics. Hydraulic measurements collected during the flume experiments will be used to better understand the influence of floodplain vegetation and channel bedform on in-channel and floodplain hydraulic conditions during overbank flows, including distributions of secondary currents, turbulence characteristics, and hydrologic exchange flows. Results from the flume experiments will be used to inform numerical modeling. The numerical and physical experiments will be complemented by the collection and analysis of field data from local restoration sites on the Cache la Poudre River, Colorado. Work conducted in this research will closely involve the City of Fort Collins, which has already spent nearly $10 million on restoration investments on the local Poudre River and has large goals for transforming the Poudre to provide a greater connection between natural and human systems along the river. This research will engage with stakeholders by participating in numerous local outreach events, including the Poudre River Fest and the City's "The Science Behind the Scenery" program.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.