Preferential and Non-Darcy Flows in the Hyporheic Zone: Surface Water-Groundwater Hydraulics and Effects on Stream Functions

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Virginia Tech

Surface water-groundwater interaction can provide various stream functions including temperature regulation, nutrient cycling, pollutant attenuation, and habitat creation. However previous literature is divided on the extent and conditions of these benefits. This dissertation has explored the dominance of hydraulic conductivity (K) on hyporheic hydraulics and implications to hyporheic zone functions through a series of modeling studies and field experiments.

Computational Fluid Dynamics (CFD) software was used to model the effect of varying K on weir-induced hyporheic exchange hydraulics and heat transport. Fundamental shifts in hydraulics and temperature dynamics occurred at threshold K's. Surface water began noticeably sinking into the bed above a threshold of K=10-3 m/s and inertial forces caused deviation from Darcy's Law. The heat transport model indicated net downstream surface water cooling from weir-induced exchange was maximized by maximizing K (flow-limited function) and thermal heterogeneity increased with K, particularly above K=10-5 m/s. Results suggest that using CFD to predict surface water-groundwater interaction may be important to accurately predict hyporheic hydraulics and functions dependent on flow-rate or residence time.

The importance of macropores to hyporheic transport through meander bends was explored. Transport velocities, hydraulic head gradients, and solute transport rates through the meander bend were increased by macropores. Results indicate that macropores can dictate solute or pollutant transport through meander bends and in the hyporheic zone, which in turn may influence biogeochemical cycling and pollutant attenuation.

Surface-connected macropores along streams were studied as hydrologically important subsurface heterogeneities for surface water-groundwater interaction. Macropores were common geomorphic features in the Appalachian province of southwestern Virginia, and were inundated during storm events over a one-year period. Banks with macropores experienced increased hydraulic head fluctuations, temperature fluctuations, and K. Macropores increased bank storage rates and solute transport between the channel and riparian groundwater zones, which in turn may influence biogeochemical cycling, pollutant attenuation, and hyporheic habitat. Macropores may be important to hyporheic flow and solute transport in a wide range of conditions and may broaden the portion of the landscape in which hyporheic exchange is important. Future work is needed to further assess the impacts of macropores on hyporheic functions and explore new methods to map and quantify macropore geometries and inter-connectivity.

Surface water, Groundwater, Hydraulic conductivity, Preferential flow paths, Stream functions