FLUMES, FINITE-ELEMENTS, FIELD OBSERVATIONS, FOURIER-SERIES AND FRACTALS: FUNDAMENTAL LINKAGES IN HYPORHEIC STUDIES

Physical and biogeochemical processes occurring in hyporheic zones (HZs) are complex. Despite advances in our understanding of these processes, several fundamental questions remain. Addressing these questions requires inter- and multi-disciplinary approaches. This talk will review links amongst and between previous studies and recent modeling studies that integrate results from flume experiments through fractal behavior in stream chemistry.

Flume experiments provide crucial data for validating high-fidelity finite-element computational fluid dynamics (CFD) models that couple turbulent open channel flow with Darcy flow through streambed sediments. Validated CFD models allow for testing various hypotheses to explain field observations and can provide a mechanistic basis for integrated observations, such as those from in-stream tracer experiments.

First, we illustrate the application of coupled turbulent open channel flow, groundwater flow, and heat transport models in explaining field observations of differential heat transport through gravel beds in pool-riffle systems. Second, we present an explanation for power-law residence time distributions (PL-RTDs) of solutes through the HZ. PL-RTDs in HZs are hypothesized to be responsible for short-time fractal scaling in time-series observations of stream chemistry. Our coupled CFD-solute transport simulations result in PL-RTDs, thereby providing a mechanistic basis for fractal stream chemistry. Heavy tails in the breakthrough curves of solutes transported through the HZ appear to be driven by stagnation zones within the sediments.

Results from past flume experiments have been used as boundary conditions for 2D and 3D numerical and analytical groundwater flow models that simulate hyporheic exchange. Lastly, we illustrate how results from CFD simulations considering multiple bedform geometry could allow for refinement of exact Fourier-series solution for 3D hyporheic flow as well as for prediction of fluxes through the HZ.