Permeability controls on hyporheic flow transitions in salmon redds: from pressure to advection-driven dynamics

<p>Hyporheic exchange represents the interactions between surface and subsurface flows occurring at various geophysical scales. Its importance to the riverine ecological health and the fate of contaminants has long been recognized. Traditionally, the behaviors of hyporheic exchange are explained by the emergence of geomorphological features, such as dune-shaped bedforms, that usually introduce significant pressure differences along the channel bed and, therefore, facilitate exchanges by pumping the flow inward and outward the bed. In addition to this advective mechanism, near-bed turbulence has also been identified as another driver of flow exchange through the turbulent diffusive processes. This study, on the other hand, highlights the decisive control of surface waves on the hyporheic exchange at depth-limited flow conditions, especially for those unbroken standing waves commonly encountered in river riffle areas. It is hypothesized that the presence of surface waves will reshape the distribution of near-bed hydrodynamic pressures, thus altering the properties of advective flows along the channel bed. The validity of this hypothesis is carefully examined through the laboratory experiments using Refractive-Index-Matched (RIM) liquid and solid materials and Particle Tracking Velocimetry (PTV) techniques. This experimental setting helps to simultaneously resolve the surface and subsurface flow patterns to a sufficient detail; the hydrodynamic pressure field can then be derived from the obtained flow velocity fields. Further analysis in a Double-Averaged Navier-Stokes framework indicates that, among different contributing factors, pressure gradient is found to be the most dominant driver of interface exchange. The variations of this driving mechanism, interestingly, can be further decomposed into two parts, namely, the surface wave associated (global-scale) and the bed grain associated (local-scale) components, respectively.</p>

A hybrid coupled model of surface and subsurface flow for surface irrigation

ABSTRACT: Patterns of dry season surface flow in forested headwater channels of southwest Washington were observed during August to September 2001 and July to October 2002. In 2001, 17 channels were sampled once, and the uppermost points of continuous flow (CF) and surface water (SW) were located. In 2002, sampling was replicated three to five times at each of 21 channels. Annual and seasonal data suggested that the location of SW varied less than CF. In most channels, SW remained at or near the channel head year around. The pattern of surface flow between CF and the channel head was used to test alternative hypotheses describing dry season recession patterns: (A) surface flow consistently retreats in a downstream direction, and (B) flow comes from fixed sources along the channel, thus surface flow retreats up-channel towards these sources. The dominant surface flow spatial pattern in streams less than 30 percent slope was increased intermittency without a clear pattern of retreat, and thus inconsistent with either hypothesis. High gradient channels (< 30 percent slope) exhibited a combination of increased intermittency, and extensive upward retreats of surface water consistent with Hypothesis B. Differences between 2001 and 2002 suggest late summer flows in small headwater basins were controlled by spring precipitation, rather than the typically greater winter precipitation.

A model for the subsurface flow and associated oxygen transfer induced by surface flow and slip velocity is presented in a gravel bed river. The shear velocity (U *) and hydraulic conductivity (K) were used to characterize the surface flow and subsurface flow, respectively. The subsurface flow field was described by using the Darcy and the Brinkman equations. The interaction between surface and subsurface flow can be represented by the slip velocity via the shear velocity (U *) and hydraulic conductivity (K). The effect of turbulence above the permeable surface on the subsurface flow can also be described by the Reynolds stress in terms of eddy viscosity (νt ). The model was found to substantially reproduce the experimental time-averaged subsurface velocity profiles. It was shown that turbulence penetrating through permeable sediments is limited to a thinner layer within a few centimetres from the sediment/water interface. This result can be useful to evaluate the effect of turbulence over the permeable surface on oxygen penetration into the sediment.

Overlooked dark dissolved organic matter reveals the migration and transformation of carbon exports in surface and subsurface flows from cropland during rainfall.

Rainfall patterns and land cover are two important factors that affect the runoff generation process. To determine the surface and subsurface flows associated with different rainfall patterns on sloping Ferralsols under different land cover types, observational data related to surface and subsurface flows from 5 m × 15 m plots were collected from 2010 to 2012. The experiment was conducted to assess three land cover types (grass, litter cover and bare land) in the Jiangxi Provincial Soil and Water Conservation Ecological Park. During the study period, 114 natural rainfall events produced subsurface flow and were divided into four groups using k-means clustering according to rainfall duration, rainfall depth and maximum 30-min rainfall intensity. The results showed that the total runoff and surface flow values were highest for bare land under all four rainfall patterns and lowest for the covered plots. However, covered plots generated higher subsurface flow values than bare land. Moreover, the surface and subsurface flows associated with the three land cover types differed significantly under different rainfall patterns. Rainfall patterns with low intensities and long durations created more subsurface flow in the grass and litter cover types, whereas rainfall patterns with high intensities and short durations resulted in greater surface flow over bare land. Rainfall pattern I had the highest surface and subsurface flow values for the grass cover and litter cover types. The highest surface flow value and lowest subsurface flow value for bare land occurred under rainfall pattern IV. Rainfall pattern II generated the highest subsurface flow value for bare land. Therefore, grass or litter cover are able to convert more surface flow into subsurface flow under different rainfall patterns. The rainfall patterns studied had greater effects on subsurface flow than on total runoff and surface flow for covered surfaces, as well as a greater effect on surface flows associated with bare land.

The hyporheic exchange (HE) is defined as the interaction between stream water and groundwater, where the stream water passes back and forth between the active channel and subsurface flow paths (Runkel, 1998). The zone of this exchange is characterized by saturated pore spaces, under sand or gravel stream beds (Hancock, 2002) and near the adjacent banks of the stream (Ryan, 2010), that contain some amount of water from the main stream channel (Boulton, Welty, & Larson, 2010). An indication of channel complexity within streams (Grimm, 2005), this transitional zone exposes surface water solutes to alternating oxic and anoxic conditions as they are mixed within the groundwater flow system (Lautz & Siegel, 2007). This plays a crucial role in stream ecosystem functioning with regard to physical characteristics (e.g. stream temperature), biogeochemical processes, (Ryan, 2010), and nutrient cyclings, which ultimately control the water quality of the stream (Lautz & Siegel, 2007). HE is driven by many physical attributes. Parent lithology of watersheds affects HE by controlling sediment porosity and hydraulic conductivity (Morrice, Valett, Dahm, & Campana, 1997). In addition, bed topography (i.e.dunes and ripples in sediments) drives HE through changes in local hydraulics (Harvey & Bencala 1993). Stream slope, morphology, and bed form (e.g. pool-step sequence, channel sinuosity, etc) also influence HE (Wondzell, 2005). As a result, extent of HE depends on a wide variety of factors including, but not limited to, sedimentporosity and hydraulic conductivity, channel morphology, as well as strength of groundwater upwelling, discharge, and size of the channel (Hancock, 2002). Size of HE in the vertical and lateral directions can range anywhere from a few centimeters to tens of meters depending on all of the above stated factors (Kasahara & Hill,2000). From a hydrological perspective, the hyporheic zone also has the potential to reduce peak discharge during storm events (Kasahara & Hill, 2002), a quality of particular importance in urban areas with extensive impervioussurfaces prone to storm runoff. The ecological significance of HE is even greater than just hydrologic flux. HE brings oxygen and other substrates in and flushes wastes out (Hancock, 2002). This process lends itself to a buffer having the potential to impede pollutant transport within both surface water flow paths as well as groundwater (Hester & Gooseff, 2010). HE increases the contact time of stream water with chemically reactive sediments and microbial communities, which creates hot spots for biogeochemical processes (Findlay, 1995).Given the permeable nature of hyporheic sediments, as nutrients flow through the hyporheic zone, it is reactively filtered (Trimmer et al., 2012). Hyporheic zones also provide a means for nitrogen (N) and other nutrients to be temporarily stored before they are lost downstream (Reidy & Clinton, 2004). This longer residence time for materials and reactive filtration allows for nutrient processing and plays a crucial role in regulating theconcentrations and forms of N exported downstream (Ensign & Doyle, 2006).

Despite the growing interest in hyporheic exchange and the associated stream ecosystem processes, few studies consider restoration of hyporheic exchange as a design goal. Here we study the design of three types of subsurface structures for hyporheic restoration after conceptual designs published over 40 years ago. Vaux's designs involve modifying the subsurface with low or high hydraulic conductivity material placed at the streambed or adjacent to a confining layer below the stream. In this preliminary analysis of subsurface structure design we use two‐dimensional groundwater flow modeling of structures to simulate structure performance in plane bed streams for ranges of structure geometric design and hydraulic conductivities. Structure performance is evaluated on the basis of total streambed flux, physical extent of hyporheic flow paths created, and residence time distributions along flow paths modified by the structures. High hydraulic conductivity structures bend flow paths toward and through the structures themselves; performance is controlled by the structure hydraulic conductivity. Results show low hydraulic conductivity structure performance is insensitive to the structure material; hyporheic exchange is created by deflecting flow paths away from the structure itself. Time scales of simulated exchange are great enough to promote nitrification, denitrification, respiration, and thermal buffering in the subsurface, though these processes will also be controlled by site‐specific chemical and biological factors. General design recommendations for specific restoration objectives are presented. Results of this study can be extrapolated to further understand the interaction of natural subsurface heterogeneities (e.g., clay and gravel deposits and bedrock knickpoints) and flow fields in creating hyporheic flow paths.

This study investigated the efficiency of a sequential system involving surface flow and vegetated sub-surface flow constructed wetland (CW) in the treatment of Brewery Wastewater. Six experimental CW (2 surface and 4 subsurface flow) and control (1 surface and 2 vegetated subsurface flow) with 200 mm depth of 19.05 mm diameter granite and 100 mm depth of sharp sand as substrate were used for the experiment. The CWs were planted with locally available macrophytes: water hyacinth (Eichhornia crassipes), Cattail (Typha latifolia) and Vetiver grass (Vetiveria nigritana). The microcosms were irrigated using wastewater from Brewery Effluent (BE), pollution parameters were measured and treatment efficiency was monitored. The pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO) and Biological Oxygen Demand (BOD5) of the BE studied were 6.84, 1189 μs/m, 2998 mg/l, 9.4 mg/l and 1244 mg/l, respectively. Average reductions of 92.53, 48.30 and 67.16 % were observed in the TDS, BOD and Nitrate after treatment. The BOD5 percentage removal was higher in the Subsurface CW than in the Surface flow CW (30.11 and 49.04 % for surface and subsurface flow, respectively). The study showed that CW using surface and subsurface flow constructed wetland with locally available macrophytes is efficient in Brewery wastewater treatment.

Wastewater treatment by constructed wetland is an appropriate technology for tropical developing countries like Indonesia because it is inexpensive, easily maintained, and has environmentally friendly and sustainable characteristics. The aim of the research is to examine the capability of constructed wetlands for treating laboratory wastewater at our Center, to investigate the suitable flow for treatment, namely vertical subsurface or horizontal surface flow, and to study the effect of the seasons. The constructed wetland is composed of three chambered unplanted sedimentation tanks followed by the first and second beds, containing gravel and sand, planted with Typha sp.; the third bed planted with floating plant Lemna sp.; and a clarifier with two chambers. The results showed that the subsurface flow in the dry season removed 95% organic carbon (COD) and total phosphorus (T-P) respectively, and 82% total nitrogen (T-N). In the transition period from the dry season to the rainy season, COD removal efficiency decreased to 73%, T-N increased to 89%, and T-P was almost the same as that in the dry season. In the rainy season COD and T-N removal efficiencies increased again to 95% respectively, while T-P remained unchanged. In the dry season, COD and T-P concentrations in the surface flow showed that the removal efficiencies were a bit lower than those in the subsurface flow. Moreover, T-N removal efficiency was only half as much as that in the subsurface flow. However, in the transition period, COD removal efficiency decreased to 29%, while T-N increased to 74% and T-P was still constant, around 93%. In the rainy season, COD and T-N removal efficiencies increased again to almost 95%. On the other hand, T-P decreased to 76%. The results show that the constructed wetland is capable of treating the laboratory wastewater. The subsurface flow is more suitable for treatment than the surface flow, and the seasonal changes have effects on the removal efficiency.

The combined effects of transient, surface and subsurface flows and infiltration govern the rainfall-induced potential failure of unsaturated soil slopes stabilised by matric suction. In this paper, it is shown how fundamental fluid dynamics can be employed to closely model the above flow effects than the empirical Richard’s approach. The fluid dynamics-based Navier-Stokes formulation also facilitates the integration of the three modes of flow: (1) surface, (2) subsurface and (3) infiltration water flows, into a unified analytical model. Therefore, in contrast to previous studies, this fundamental approach can seamlessly incorporate the effects of continuous interplay between surface and subsurface water flows and the inertial terms of flow on the reduction of matric suction in unsaturated soils. Transient flow effects have been combined with geomechanics of unsaturated soil to demonstrate their impact on the stability of an embankment under a more realistic non-uniform rainfall. The innovative use of the laboratory-evaluated soil properties such as the hydraulic conductivity and porosity to express the drag force in the Navier-Stokes equations facilitates the examination of the sensitivity of slope stability to the relevant soil classification parameters.

Heterogeneity in hydraulic conductivity (K) and channel morphology both control surface water‐groundwater exchange (hyporheic exchange), which influences stream ecosystem processes and biogeochemical cycles. Here we show that heterogeneity in K is the dominant control on exchange rates, residence times, and patterns in hyporheic zones with abrupt lithologic contrasts. We simulated hyporheic exchange in a representative low‐gradient stream with 300 different bimodal K fields composed of sand and silt. Simulations span five sets of sand‐silt ratios and two sets of low and high K contrasts (1 and 3 orders of magnitude). Heterogeneity can increase interfacial flux by an order of magnitude relative to homogeneous cases, drastically changes the shape of residence time distributions, and tends to decrease median residence times. The positioning of highly permeable sand bodies controls patterns of interfacial flux and flow paths. These results are remarkably different from previous studies of smooth, continuous K fields that indicate only moderate effects on hyporheic exchange. Our results also show that hyporheic residence times are least predictable when sand body connectivity is low. As sand body connectivity increases, the expected residence time distribution (ensemble average for a given sand‐silt ratio) remains approximately constant, but the uncertainty around the expectation decreases. Including strong heterogeneity in hyporheic models is imperative for understanding hyporheic fluxes and solute transport. In streams with strongly heterogeneous sediments, characterizing lithologic structure is more critical for predicting hyporheic exchange metrics than characterizing channel morphology.

The complex surface topography of river substrates controls near‐bed hydraulics and drives the exchange of subsurface and surface flow. In rivers, the topographic structures that are studied are usually formed by the flow, but it is known that many animals also create biogenic bedforms, such as pits and mounds. Here, a large‐eddy simulation model of flow over a pit and a mound is evaluated with flume experiments. The model includes actual bedform topography, and the topographic complexity of the surrounding bed surface. Subsurface grains are organized in a body‐centered cubic packing arrangement. Model evaluation showed strong agreement between experimental and modeling results for velocity (R2 &gt; 0.8) and good agreement for Reynolds stresses (R2 &gt; 0.7), which is comparable to other similar studies. Simulation of the pit shows that the length of the downwelling region is smaller than the upwelling region and that the velocity magnitude is higher in the downwelling region. Simulation of the mound reveals that the flow is forced into the bed upstream of the mound and reemerges near the top of the mound. The recirculation zone is limited at the leeside of the mound. With increasing Reynolds number, the depth of the upwelling region at the leeside of the mound increases. The analysis of shear stress indicates that sediments on the upstream edge of the pit and on the downstream face of the mound are relatively unstable. These results demonstrate the effect of biogenic structures on the near‐bed flow field, hyporheic exchange, and sediment stability.

We report direct numerical simulation (DNS) results of hyporheic exchange for a flat river bed with two different particle roughness textures, at a surface flow friction Reynolds number of 395 and a bed permeability Reynolds number of 2.6. Transit time distributions (TTDs), subsurface flow patterns, and the interfacial volumetric fluxes are discussed. The transit time was quantified using a forward particle tracking method based on pure advection by three‐dimensional, pore‐resolved, time‐mean velocities. Results show that bed roughness induces deep subsurface flow paths that yield a TTD with a power‐law tail. Roughness obstructs the surface flow, creating interfacial pressure variations which induce subsurface flow. Next, the molecular diffusion is accounted for based on a random walk method and is shown to increase transit times regardless of roughness texture. This work demonstrates that particle roughness on a macroscopically flat sediment bed can induce significant hyporheic exchange that is fundamentally similar to that induced by bedforms.

We investigated surface‐subsurface (hyporheic) exchange in two morphologically distinct arctic headwater streams experiencing warming (thawing) sub‐channel conditions. Empirically parameterized and calibrated groundwater flow models were used to assess the influence of sub‐channel thaw on hyporheic exchange. Average thaw depths were at least two‐fold greater under the higher‐energy, alluvial stream than under the low‐energy, peat‐lined stream. Alluvial hyporheic exchange had shorter residence times and longer flowpaths that occurred across greater portions of the thawed sediments. For both reaches, the morphologic (longitudinal bed topography) and hydraulic conditions (surface and groundwater flow properties) set the potential for hyporheic flow. Simulations of deeper thaw, as predicted under a warming arctic climate, only influence hyporheic exchange until a threshold depth. This depth is primarily determined by the hydraulic head gradients imposed by the stream morphology. Therefore, arctic hyporheic exchange extent is likely to be independent of greater sub‐stream thaw depths.