E-proceedings of the 38 th IAHR World Congress September 1-6, 2019, Panama City, Panama ANALYSING THE EFFECT OF CONFLUENCE HYDRODYNAMICS AND MORPHODYNAMICS ON HYPORHEIC FLUXES. A FIELD STUDY IN TWO SMALL STREAMS I. MARTONE (1) , C. GUALTIERI (1) & T.A. ENDRENY (2) (1) Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Naples, Italy ivo.martone@unina.it; carlo.gualtieri@unina.it (2) Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York, Syracuse, U.S.A. te@esf.edu ABSTRACT Confluences are ubiquitous components of all riverine systems, and are characterized by converging flow streamlines, mixing of flows and a highly complex three-dimensional flow structure located in the so-called Confluence Hydrodynamic Zone (CHZ). The CHZ generally includes a zone of flow stagnation near the upstream junction corner; an area of flow deflection as the tributary enters confluence; shear layer and/or mixing interface between the two converging flows; a possible separation zone at the downstream junction corner(s); flow acceleration within the downstream channel; and flow recovery at the downstream end of the CHZ. The aim of this study was to assess how the above features affect the ecologically important hyporheic fluxes between surface and subsurface waters, and hence guide the management and restoration of the whole river environment. Field investigations were carried out in Onondaga County of central New York, U.S.A. during the 2018 summer to fall seasons. The study case was a 2-meter-wide confluence between Baltimore Brook and Cold Brook forming a 45 degrees junction angle. Field campaigns yielded surveys of channel bathymetry, discharge rates, hydraulic conductivity, and substrate roughness, while probes measured hyporheic zone vertical hydraulic gradient and temperature profile data. Initial findings show spatial variation in the direction of vertical fluxes directions (upwelling and downwelling) during the sampling time period. Confluence morphology and water table gradients beyond the channel domain are affecting magnitude and pattern of hyporheic exchange. These data combined with models could help to advance the understanding of the key hydrological, hydraulic, and ecological issues associated with rivers confluence. Keywords: hyporheic exchange, confluence hydrodynamic, vertical hydraulic gradient, heat tracing, hydraulic conductivity 1 INTRODUCTION In nature several bodies of water merge together forming larger open-channel rivers in which complex hydraulic processes take place (Mosley 1975). This meeting between two streams or rivers (known as river confluence) is a matter of three-dimensional flow structure. The region affected by this merging of two streams is called confluence hydrodynamic zone (CHZ) characterized by strong vertical, lateral and streamwise gradients in velocity forming several distinct hydrodynamic zones (Best 1984). The CHZ generally includes a zone of flow stagnation near the upstream junction corner; an area of flow deflection as the tributary enters confluence; shear layer and/or mixing interface between the two converging flows; a possible separation zone at the downstream junction corner(s); flow acceleration within the downstream channel; and flow recovery at the downstream end of the CHZ. Rivers mixing at confluences, together with bed morphology and sediment transport, might result in highly varied habitat for fish, macroinvertebrates and riparian zones (Fernandes et al., 2004; Grant et al., 2007; Gualtieri et al., 2017). It is well known that water repetitively interact with pore water thanks to stream bed and porous media interface which enter the streambed sediment in downwelling areas, (i.e., downwelling fluxes) and then emerging into the stream in upwelling areas, (i.e., upwelling fluxes): this mixing is known as hyporheic exchange. It affects significantly surface and subsurface water quality, riverine habitat for aquatic and terrestrial organisms and it is an important role in preserving and conserving riverine systems. These ecosystems depend on scales and rates of exchange which may be generated due to spatial and temporal variations of channel features (hydraulic conductivity, stream morphology, streambed pressure distribution, groundwater level, sediment transport, etc.) and it can be longitudinal or transversal to the stream direction (Cardenas et al. 2004; Tonina 2008). Vertical Hydraulic Gradient (VHG) is a method well spread to estimate hyporheic exchange together with the hydraulic conductivity Kv. Hyporheic water exchange is influenced by convective water and heat can be used as a natural tracer by using a one-dimensional model (Anibas et al. 2009; Rau et al. 2010; Gariglio et al. 2013; Song et al. 2017). The advantage in field of such technique has been showed since its practical and inexpensive way to analyze exchange from temperature- depth water profiles.