WHEN DOES STRUCTURE-INDUCED HYPORHEIC EXCHANGE MATTER? ANALYZING HYDRAULICS AND HEAT FLUX IN COARSE STREAMBEDS

In-stream structures such as log steps or debris dams induce hyporheic exchange and influence stream temperature, in turn potentially impacting aquatic organisms and ecosystem processes. However, previous research is inconclusive: in some cases structure-induced exchange significantly impacted downstream surface water temperatures, while in other cases it did not. Here we examine the pivotal role of hydraulic conductivity (K) in controlling whether such thermal effects are significant. We modeled hyporheic water and heat exchange induced by a single weir (representing debris dams, log dams) during summer with the computational fluid dynamics program Ansys CFX. CFX simulates several key processes that are critical for accurately modeling hyporheic exchange in high K situations. These include a fully 3D simulation of surface water including turbulence, 3D groundwater hydraulics including non-Darcy flow, and fully coupling the two domains. We calibrated the groundwater model by varying K until the heat transport model matched site-specific temperature data from a low order stream near Blacksburg, VA. Our results reveal two countervailing trends that together determine hyporheic influence on surface water temperatures. The first trend is that increasing K increases hyporheic flow relative to surface flow. The second trend is that increasing K decreases the temperature difference between downwelling and upwelling zones and therefore decreases the average cooling, buffering, and lagging. The net effect of these two trends is a maximum average cooling effect from structure-induced hyporheic exchange at an intermediate K (or “sweet spot” of K=5e-5 m/s). The fully coupled surface water-groundwater model reveals the tradeoff between surface flows over the weir and hyporheic flows, as well as the onset of 100% hyporheic flow. Within coarser sediments, pore scale Reynolds numbers indicate that inertial forces are important relative to viscous forces and hyporheic flow rates deviate from Darcy’s law. Hydrodynamic dispersion increases mixing for higher K’s and can alter stream temperature dynamics.