ABSTRACT

Hyporheic zone refers to the interface between the bed and surface water in streams, rivers, and other aquatic ecosystems. Contaminants and nutrients are constantly being exchanged between surface and subsurface water in the hyporheic zone, controlling water quality, the metabolism of benthic microbes, and the associated biogeochemical cycling. Vegetation and turbulence, which are ubiquitous in aquatic ecosystems, affect surface-subsurface exchange and, as such, impact water quality and stream biogeochemical cycling. However, how vegetation and turbulence impact exchange remains unclear, making it difficult to predict contaminant transport and biogeochemical cycling in streams, lakes, and coastal areas. The proposed study aims to combine laboratory experiments in a water-recirculating flume, numerical simulation, and field experiments in an outdoor stream to quantify the impacts of vegetation and turbulence on flow and solute transport in the hyporheic zone. The results from this study will help improve predictions of contaminant transport and biogeochemical cycling in streams and other aquatic ecosystems, as well as help ecologists design stream restoration projects that use vegetation to increase the retention and degradation of contaminants in sediment. The proposed project will also train next-generation scientists, including two graduate students and undergraduates from underrepresented groups. Further, a demonstration hands-on activity will be developed and used in outreach events for K-12 girls and teachers.

The goal of the proposed research is to quantitatively characterize the role of turbulence and in-channel vegetation on hyporheic exchange and propose a multi-scale modeling framework for predicting hyporheic exchange and solute transport in meandering channels with bedforms, turbulent flows, and in-channel vegetation. Systematically controlled experiments with refractive index matched sediment and vegetation, as well as fluorescent dye imaging, will be conducted in flumes to directly visualize and quantify the turbulence and vegetation-induced hyporheic exchange. A physics-based theoretical model will be developed to predict hyporheic exchange as a function of turbulent kinetic energy and vegetation stem size, volume fraction, and drag coefficient. The theoretical model will then be incorporated into a multiscale numerical modeling framework to investigate the combined effects of the important drivers (including near-bed turbulence, vegetation, bedforms, and channel meanderings) on flow and solute transport in complex meandering streams. The modeling results will be further validated by field tracer experiments in an outdoor meandering channel with bedforms, turbulent flows, and vegetation.

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.

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