The role of in-channel aquatic vegetation on hyporheic exchange

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In-channel vegetation, IAV, is ubiquitous in watercourse as plants or algae. It provides many vital ecological and hydromorphological functions and is becoming a major tool in river restoration. Research on IAV has chiefly focused on sediment transport and flow resistance, while its role in inducing hyporheic exchange has been neglected. Hyporheic exchange is the main process connecting streams with streambeds by moving in-stream water in and out of the streambed sediment through the hyporheic zone, HZ, which serves as an interface between surface and ground waters. The HZ promotes important biological and chemical processes, which affect the quality of both stream and streambed waters. Consequently, modeling hyporheic fluxes due to IAV will provide key information for river restoration practices in which planting has been advocated, a $1B per year industry. Additionally, predictions of hyporheic exchange and processes are essential to quantify solute transport and transformation along streams and rivers. Thus the goal of this research is to model hyporheic flow due to IAV. This will provide a fundamental and predictive understanding of the processes that control hyporheic exchange due to flow and IAV interaction. It will advance knowledge on hyporheic flow field near the rhizosphere, the region of sediment directly influenced by root secretions and associated soil microorganisms and the ability to model solute, nutrient, contaminant, and pathogen transport along stream networks by including the effect of IAV. The models developed in this project will be used to predict hyporheic processes in streams with vegetation and will be incorporated into basin-scale transport models of solute and nutrients. A new transformative after school program will be developed to introduce 5th-6th grade students to STEM disciplines through hands-on activities and lessons with a mobile educational flume. In addition undergraduate and graduate Civil Engineering students will be trained. The scientific goal of this project is to mechanistically understand, quantify, and model the effects of IAV on hyporheic exchange as a function of IAV density, patch size, and patch distribution under different flow conditions. IAVs will be considered with three schematizations: 1) interaction between flow and an individual emergent and submerged vegetation stem, 2) interaction between flow and one single submerged vegetation patch, and 3) interaction between flow and multiple submerged vegetation patches. This will be addressed with novel tracer test flume experiments, coupling Matching Index Reflectometry, Particle Image Velocimetry, and analytical and numerical modeling, and by testing three specific hypotheses: hyporheic exchange has an inverse U-shaped (increases to a maximum and then decreases with increasing value of the independent variable) relationship with: (1) IAV density; (2) IAV patch size; and (3) the percent of streambed covered by multiple dense IAV patches.