EFFECTS OF STREAM RESTORATION PRACTICES ON HYDRAULIC RETENTION AND STORAGE IN THE HYPORHEIC ZONE AND INSET FLOODPLAINS

Stream restoration has become a common practice in the United States, with goals including both ecosystem rehabilitation and nutrient removal. The practice of placing in-stream structures such as cross vanes can cause interaction of surface water and groundwater in the hyporheic zone inducing both nutrient removal and habitat creation. Floodplains also act as a storage zone for stream water, with most natural floodplains inundated by stream water annually, and inset floodplains (in-channel benches) activated several times a year. The retention capabilities of both in-stream structures and floodplains have been studied separately, but their combined effects on stream retention have not been analyzed. In this study a recently restored 3^rd order stream in Blacksburg Virginia is the basis for hydraulic and transport modeling using MIKE SHE to analyze and compare the storage capabilities of inset floodplains and the hyporheic zone. A sensitivity analysis is performed for hydraulic conditions (e.g., spring, summer, storm flow, and structures), geologic condition (e.g., hydraulic conductivity) and floodplain geometry (e.g, hydraulic impedance and width). A conservative tracer is applied to the stream and the resulting breakthrough tracer curves and total mass stored are analyzed for the stream, floodplain, and hyporheic zone. Results indicate that for storage in the hyporheic zone to occur baseflow stream conditions and relatively low groundwater conditions are necessary, while the flux through the hyporheic zone is directly correlated with the hydraulic conductivity. Inset floodplain storage is independent of groundwater conditions but requires flow substantially higher than baseflow, while the activation flow and extent of storage are dictated by the geometry of the floodplain. Residence times in the hyporheic zone are orders of magnitude larger than those for floodplains, causing substantially less flow through the hyporheic zone than the inset floodplains. These findings indicate that implementing both inset floodplains and in-stream structures in tandem promote storage for both high and low stream flow, potentially improving stream water quality and habitat for a range of flows.