Fifty years of hyporheic zone research have shown the important role played by the hyporheic zone as an interface between groundwater and surface waters. However, it is only in the last two decades that what began as an empirical science has become a mechanistic science devoted to modeling studies of the complex fluid dynamical and biogeochemical mechanisms occurring in the hyporheic zone. These efforts have led to the picture of surface-subsurface water interactions as regulators of the form and function of fluvial ecosystems. Rather than being isolated systems, surface water bodies continuously interact with the subsurface. Exploration of hyporheic zone processes has led to a new appreciation of their wide reaching consequences for water quality and stream ecology. Modern research aims toward a unified approach, in which processes occurring in the hyporheic zone are key elements for the appreciation, management, and restoration of the whole river environment. In this unifying context, this review summarizes results from modeling studies and field observations about flow and transport processes in the hyporheic zone and describes the theories proposed in hydrology and fluid dynamics developed to quantitatively model and predict the hyporheic transport of water, heat, and dissolved and suspended compounds from sediment grain scale up to the watershed scale. The implications of these processes for stream biogeochemistry and ecology are also discussed.

Hyporheic flow and transport processes: Mechanisms, models, and biogeochemical implications

A theoretical analysis of the exchange of nonsorbing solutes between a permeable streambed and the overlying water of a stream or river is presented in this paper. In a companion paper [Elliott and Brooks, this issue] the results of experimental studies of such exchange are presented. The analysis focuses on water movements into, within, and out of the bed which result from the presence of bed forms (ripples and dunes) and highlights the mechanisms of “pumping” and “turnover.” Pumping is the movement of fluid through the bed induced by steady dynamic pressure variations over bed forms. Turnover occurs as moving bed forms trap and release interstitial fluid. The detailed analysis was used to generate the distribution of the residence time of solute within the bed, which can be used to calculate the net mass exchange due to pumping and turnover.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96WR02784

Transfer of nonsorbing solutes to a streambed with bed forms: Theory

[1] Advection through hyporheic zones (HZ) consisting of heterogeneous channel bend streambed deposits and their equivalent homogenous medium was investigated using finite difference groundwater flow and transport simulations and forward particle tracking. The top prescribed head boundary was varied in order to mimic various stream channel head distributions resulting from the presence of bed forms and channel curvature. Flux calculations show that heterogeneity causes significant additional HZ flux compared to an equivalent homogenous medium. However, the major cause of HZ flux is a spatially periodic (sinusoidal) head distribution along the boundary, representing the effect of bed forms. The additional influence of heterogeneity on the total channel-bed exchange and the overall HZ geometry are increased when boundary head sinusoidal fluctuation is more subdued. We present dimensionless numbers that summarize these relationships. Heterogeneity's influence is further magnified by considering the effect of channel curvature on boundary heads. The simulations illustrate the dynamic influence of heterogeneity on the hyporheic zone since the various head boundaries employed in our modeling efforts are a proxy for different surface water conditions and bed form states that may occur during a single flood. Furthermore, we show that residence times (total tracking times) of particles originating from the streambed follow a lognormal distribution. In the presence of heterogeneity, residence times can decrease or they can increase compared to residence times for homogeneous conditions depending on the relative positions of the heterogeneities and the bed forms. Hence streambed heterogeneity and stream curvature, factors often neglected in previous modeling efforts, combine with bed form configuration to dynamically determine HZ geometry, fluxes, and residence time distributions.

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Impact of Heterogeneity, Bed Forms, and Stream Curvature on Subchannel Hyporheic Exchange

Experiments were performed to determine the mechanisms and rates of solute exchange between a flowing stream and porous streambed. Mass balance, flow visualisation, and depth profiles of concentration were used to investigate the exchange of an inert tracer in a laboratory flume. The measured mass transfer was compared to the exchange predicted by models of exchange related to bed forms, which are presented in a companion paper. For the initial stages with stationary bed forms or slowly moving bed forms the net exchange was predicted satisfactorily and was dominated by bed form-induced interstitial flows (pumping). In the initial stages with rapidly moving bed forms the exchange was dominated by scour/deposition as bed forms propagated (turnover). At later times the models of bed form-related exchange significantly underpredicted the measured exchange. The additional exchange, and the exchange to a flat bed, may be related to bed inhomogeneity or irregular variations in pressure at the bed surface which are not related to bed forms. Such effects are likely to be more pronounced in the field situation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96WR02783

Transfer of nonsorbing solutes to a streambed with bed forms: Laboratory experiments

[1] Temporary storage of solutes in streams is often controlled by flow-induced uptake in hyporheic zones. This phenomenon accounts for the tails that are generally observed following the passage o ...

/pdf/effect-of-flow-induced-exchange-in-hyporheic-zones-on-vp9uot9wqg.pdf

Effect of flow‐induced exchange in hyporheic zones on longitudinal transport of solutes in streams and rivers

A one‐dimensional Fickian diffusion model for entrainment of interstitial soil water by overland flow is modified to account for early non‐Fickian mass transfer observed during laboratory runoff experiments. Modification involves specifying a time‐dependent diffusion coefficient that decreases with time and asymptotically approaches the constant Fickian value with time. Results show that this asymptotic diffusion coefficient varies with the square of the quantity V*K, where V* is shear velocity and k is media permeability. Modifying the model entails the development of type curves for a range of convective time scales, or times of non‐Fickian mass transfer. A graphical curvematching technique using dimensionless cumulative mass loss data provides a means of estimating the entrainment, or mass transfer, coefficient as a function of the convective time scale. This procedure shows that the entrainment coefficient varies linearly with V*k, Reasonable prediction of mass loss results using the asymptotic diffus...

Modified Fickian Model for Solute Uptake by Runoff

The entrainment of soil pore water by overland flow is examined. In a series of laboratory experiments water was passed at various velocities and depths over a soil bed. The soil was saturated with sodium bromide solution prior to each experiment. Runoff water was sampled at the end of the flume and analyzed for bromide concentration. From these data, mass loss rate and cumulative mass loss curves are developed. A Fickian diffusion model is formulated to describe mass transfer from the soil interstices to the overland flow. A procedure to determine the coefficient of diffusion from experimental data is developed and implemented. In general, laboratory results exhibit typical Fickian behavior for large time. The diffusion coefficient varies with velocity, depth of flow, soil surface roughness, and soil condition. Early runoff data, however, exhibit distinct non‐Fickian behavior.

Pore Water Uptake by Agricultural Runoff

This paper examines frictional resistance for steady free‐surface flow over a porous bed of uniformly‐sized glass beads. This process is important in mass transport problems, such as non‐point source pollution, in which flow extracts chemicals from porous boundaries. Laboratory data show that superposed shear flow interacts intimately with the permeable boundary. Observed reduced frictional resistance for laminar flow supports the Beavers and Joseph's 1967 postulation of interfacial velocity slip. Turbulent flow, however, produced significant momentum transfer across the bed interface, which acted to increase frictional resistance and, thereby, counteract the reduction of frictional resistance caused by velocity slip, A predominance of either mechanism during turbulent flow depends upon media permeability and, possibly, the scale of turbulent eddies. As permeability increases, lateral momentum exchange begins to dominate overall flow resistance, because small scale eddies are able to penetrate the larger ...

C. P. Richardson

Papers

Modified Fickian Model for Solute Uptake by Runoff

Pore Water Uptake by Agricultural Runoff

Friction and Free‐Surface Flow over Porous Media

Closure of "Pore Water Uptake by Agricultural Runoff"