1. Department of Infrastructure Engineering The University of Melbourne For more information: Discipline: Geomatics Unravelling the Mechanics of Hyporheic Exchange Alexander Heinrich McCluskey PhD Candidate amccluskey@unimelb.edu.au D417 – Environmental Hydrology and Water Resources Webpage: http://www.ie.unimelb.edu.au/people/rhd.html#McCluskey Supervisors: Dr. Michael Stewardson & Prof. Stanley Grant Introduction The hyporheic zone connects streams to aquifers and their surroundings more generally. Exchange within the hyporheic zone occurs at a range of scales both in terms of orientation (lateral and vertical) and spatially (ranging form reach to catchment scales). The reach scale has been studied in most detail, with a multitude of flume experiments examining the mass transport characteristics across the sediment-water interface. These studies use effective diffusivity to parameterize exchange, yet the physical basis of the exchange process is only in part diffusive. Research Problem There are 3 approaches in describing hyporheic exchange: Research Objectives This research project will address the following questions: References Elliott, A. H., and N. H. Brooks (1997), Transfer of nonsorbing solutes to a streambed with bed forms: Theory, Water Resour. Res., 33(1), 123-136. Grant, S. B., and I. Marusic (2011), Crossing Turbulent Boundaries: Interfacial Flux in Environmental Flows, Environmental Science & Technology, 45(17), 7107–7113. Grant, S. B., M. J. Stewardson, and I. Marusic (in press), Effective difusivity and mass flux across the sediment-water interface in streams, Water Resources Research, xx(xx). Packman, A. I., M. Salehin, and M. Zaramella (2004), Hyporheic Exchange with Gravel Beds: Basic Hydrodynamic Interactions and Bedform-Induced Advective Flows, Journal of Hydraulic Engineering, 130(7), 647-656. Rempfer, D. (2003), Low-Dimentional Modeling and Numerical Simulation of Trnansition in Simple Shear Flows, Annual Review of Fluid Mechanics, 35(1), 229-265. Environmental conditions such as bedforms are known to increase rates of hyporheic exchange (Packman et al., 2004), but also affect roughness, to which hyporheic exchange also scales (Grant et al, in press). Hypothesis: Hyporheic exchange is driven by three different mechanisms, which can be represented to each form their constitutive parts of effective diffusion. 1.Effective diffusivity (D eff ) modelled with Fick’s First Law (Eq. 1) 2.Exchange driven by fluctuating dynamic head (D DH ) according to the “pumping” model (eq. 2) (Elliott and Brooks, 1997) 3.Coherent turbulent penetration (D TP ) (Fig.2) What are the dominant processes of hyporheic exchange? Under which conditions do these mechanisms dominate? How are these processes temporally and spatially distributed? The project will be experimental, combining a range of techniques to simultaneously measure processes in a fume. If each of these mechanism are represented by a constitutive effective diffusion coefficient then the different mechanisms can be represented in eq. 3. Figure 2: Coherent turbulent structures [ a. Grant et al., 2011, b. Rempfer(2003) c. www.hrenya.colorado.edu] Figure 1: parameterization of bedforms (1) (2) (3)