Clogging Layer Research Articles

Modelling hyporheic processes for regulated rivers under transient hydrological and hydrogeological conditions

Abstract. Understanding the effects of major hydrogeological controls on hyporheic exchange and bank storage is essential for river water management, groundwater abstraction, restoration and ecosystem sustainability. Analytical models cannot adequately represent complex settings with, for example, transient boundary conditions, varying geometry of surface water–groundwater interface, unsaturated and overland flow, etc. To understand the influence of parameters such as (1) sloping river banks, (2) varying hydraulic conductivity of the riverbed and (3) different river discharge wave scenarios on hyporheic exchange characteristics such as (a) bank storage, (b) return flows and (c) residence time, a 2-D hydrogeological conceptual model and, subsequently, an adequate numerical model were developed. The numerical model was calibrated against observations in the aquifer adjacent to the hydropower-regulated Lule River, northern Sweden, which has predominantly diurnal discharge fluctuations during summer and long-lasting discharge peaks during autumn and winter. Modelling results revealed that bank storage increased with river wave amplitude, wave duration and smaller slope of the river bank, while maximum exchange flux decreased with wave duration. When a homogeneous clogging layer covered the entire river–aquifer interface, hydraulic conductivity positively affected bank storage. The presence of a clogging layer with hydraulic conductivity < 0.001 m d−1 significantly reduced the exchange flows and virtually eliminated bank storage. The bank storage return/fill time ratio was positively related to wave amplitude and the hydraulic conductivity of the interface and negatively to wave duration and bank slope. Discharge oscillations with short duration and small amplitude decreased bank storage and, therefore, the hyporheic exchange, which has implications for solute fluxes, redox conditions and the potential of riverbeds as fish-spawning locations. Based on these results, river regulation strategies can be improved by considering the effect of certain wave event configurations on hyporheic exchange to ensure harmonious hydrogeochemical functioning of the river–aquifer interfaces and related ecosystems.

Influence of surface water level fluctuation and riverbed sediment deposits on groundwater regime

Abstract The spatial and temporal patterns of surface water (SW) - groundwater (GW) exchange are significantly affected by riverbed silting, clogging or erosion processes, by altering the thickness and hydraulic conductivity of riverbed sediments. The duration of SW-GW exchange is controlled by the drainage and infiltration resistance of river bottom sediments (e.g. Andrássy et al., 2012). Generally, these two parameters primarily depend on the hydraulic conductivity and on the thickness of clogged layer. In this study the flow processes between GW and SW were modeled by model TRIWACO for different infiltration resistance and drainage resistance of riverbed sediments. The model area is situated on the Rye Island, which is a lowland area with very low slope. In this area a channel network was built up, where the flow conditions are controlled by water-gates. Because of the low slope and the system of water gates built on the channels, the riverbeds are influenced by intensive clogging processes. First, the applicability of model TRIWACO in the study area was tested by modelling the response of GW on SW level fluctuation. It was simulated, how the regulation of water level and flow direction in the channels influence the GW level, especially in extreme hydrological conditions (drought/flood), and if the GW flow direction and GW level change as it was expected. Next, the influence of channel network silting up on GW-SW interaction was modeled. The thickness of riverbed sediments was measured and their hydraulic conductivity from disturbed sediment samples was evaluated. The assessed hydraulic conductivity was used to calculate the infiltration resistance and the drainage resistance of riverbed sediments in the study area. Then, the GW level and flow direction was simulated for different infiltration resistance and drainage resistance of sediments.

Description and Application of A Model of Seepage under A Weir Including Mechanical Clogging

Abstract The paper discusses seepage flow under a damming structure (a weir) in view of mechanical clogging in a thin layer at the upstream site. It was assumed that in this layer flow may be treated as one-dimensional (perpendicular to the layer), while elsewhere flow was modelled as two-dimensional. The solution in both zones was obtained in the discrete form using the finite element method and the Euler method. The effect of the clogging layer on seepage flow was modelled using the third kind boundary condition. Seepage parameters in the clogging layer were estimated based on laboratory tests conducted by Skolasińska [2006]. Typical problem was taken to provide simulation and indicate how clogging affects the seepage rate and other parameters of the flow. Results showed that clogging at the upstream site has a significant effect on the distribution of seepage velocity and hydraulic gradients. The flow underneath the structure decreases with time, but these changes are relatively slow.

Flow regulation effects on the hydrogeochemistry of the hyporheic zone in boreal rivers

River–aquifer interfaces are essential for ecosystem functioning in terms of nutrient exchange and biological habitat, but are greatly threatened world-wide. This study examined geochemical aspects of river–aquifer interaction in one regulated and one unregulated boreal river in Northern Sweden to determine whether the geochemical functioning of the hyporheic zone is affected by hydrological alterations, e.g. regulated river discharge and river–aquifer connectivity. In the unregulated Kalix River, the hyporheic pore water was well-oxygenated with orthogonal fluxes (≈0.6–0.7md−1) and acted as a sink for Fe, Mn, Al, NH4, and Ca, with fractional losses of 95%, 92%, 45%, 31%, and 15%, respectively. A corresponding elevation in the concentrations of these elements in the hyporheic sediment was observed, with higher saturation indices of Fe-, Mn-, and Al-bearing secondary minerals in hyporheic waters. In the regulated Lule River, hydraulic connectivity at the river–aquifer interface was altered by the presence of a clogging layer (0.04md−1). In addition, the river discharge oscillated daily, severely reducing exchange flows across the riverbed (<0.01md−1). As a result, the hyporheic pore water was suboxic, with elevated concentrations of filtered Fe and Mn (fractional increases of ≈3700% and ≈2500%, respectively) and other solutes (NH4, Si, S, Ca). A conceptual model revealed functional differences between geochemical features of the hyporheic zone of regulated and unregulated rivers. Overall, the results showed that hyporheic processes are altered along regulated rivers, with resulting impacts on the geochemistry of riverine, riparian and related marine ecosystems.

Relative rates of solute and pressure propagation into heterogeneous alluvial aquifers following river flow events

Conventional theory for homogeneous aquifers states that pressure propagates more rapidly into aquifers than solutes following river stage rise. We demonstrate through numerical simulations of two-dimensional aquifer slices that the relative timing of pressure and solute responses in alluvial aquifers is a function of subsurface structures. Two generic conceptual models of heterogeneity are investigated, a vertical clogging layer and a horizontal sand string. Independent of the conceptual model, the hydraulic conductivity contrast is the primary controlling variable on the rates of pressure and solute transport from a river to an observation point. Conceptual models are compared using metrics for pressure and solute travel time that represent propagation of 50% change in each variable from river to observation point. While not possible in a homogeneous system, a solute travel time less than a pressure travel time can occur in the presence of both types of heterogeneity, and indicates that heterogeneity is controlling propagation from the river to the aquifer. Less than one order of magnitude contrast in hydraulic conductivities is sufficient to create a travel time ratio less than one. Contrasts of this magnitude are often exceeded in alluvial environments and thus simultaneous measurement of solute and pressure has the potential to constrain estimates of exchange flux in a way not possible with pressure measurements alone. In general, flux estimates derived from solute travel times provide more accurate estimates than those derived from pressure responses in heterogeneous environments. The magnitude of error in estimates derived from pressure responses is proportional to the hydraulic conductivity contrast. Travel times calculated from time series pressure and EC data collected in the Mitchell River in northern Australia are used to demonstrate application of this combined approach.

The impact of floods on infiltration rates in a disconnected stream

[1] A few studies suggest that infiltration rates within streambeds increase during the flood season due to an increase in the stream stage and the remove of the clogged streambed. However, some studies suggest that a new clogging layer will quickly form after an older one has been eroded, and that an increase in water depth will compress the clogging layer, making it less permeable during a flood event. The purpose of this work was to understand the impact of floods on infiltration rates within a disconnected stream. We utilized pressure data and daily streambed infiltration rates determined from diurnal temperature time series within a streambed over a period of 167 days for five flood events. Our data did not support the theory that floods linearly increase the infiltration rate. Since the streambed was clogged very quickly with a large load of suspended particles and compaction of the clogged layer, infiltration rates were also low during the flooding season. However, due to an increase in the wet perimeter within the stream during flooding periods, the total recharge amount to the aquifer was increased.

When Can Inverted Water Tables Occur Beneath Streams?

Decline in regional water tables (RWT) can cause losing streams to disconnect from underlying aquifers. When this occurs, an inverted water table (IWT) will develop beneath the stream, and an unsaturated zone will be present between the IWT and the RWT. The IWT marks the base of the saturated zone beneath the stream. Although a few prior studies have suggested the likelihood of an IWT without a clogging layer, most of them have assumed that a low-permeability streambed is required to reduce infiltration from surface water to groundwater, and that the IWT only occurs at the bottom of the low-permeability layer. In this study, we use numerical simulations to show that the development of an IWT beneath an unclogged stream is theoretically possible under steady-state conditions. For a stream width of 1 m above a homogeneous and isotropic sand aquifer with a 47 m deep RWT (measured in an observation point 20 m away from the center of the stream), an IWT will occur provided that the stream depth is less than a critical value of 4.1 m. This critical stream depth is the maximum water depth in the stream to maintain the occurrence of an IWT. The critical stream depth decreases with stream width. For a stream width of 6 m, the critical stream depth is only 1 mm. Thus while theoretically possible, an IWT is unlikely to occur at steady state without a clogging layer, unless a stream is very narrow or shallow and the RWT is very deep.

Role and characteristics of problematic biofilms within the removal and mobility of trace metals in a pilot-scale membrane bioreactor

The characteristics of problematic biofilms (i.e., fouling and clogging layers) were studied with regards to the removal and fate of trace metals (contents well under 100μg/L) during the long-term operation of a pilot-scale membrane bioreactor for the treatment of real wastewaters from a large industrial area.Results showed that clogging layer was more effective than suspended activated sludge in the biosorption of As>Zn>Ni>Cd>Sb>Fe>Se due to the synergic effects of extracellular polymeric compounds and metal-resistant bacteria. In fact the selective microbial speciation of the phylum of Bacteroidetes, which is highly resistant to heavy metals, was observed in the clogging sludge in spite of the very low concentration of dissolved metals in the bioreactor.Compared to the suspended activated sludge, the clogging layer enhanced the biosorption of very toxic substances such as As, Cd and Ni. In fact, the metal contents were respectively: 7.9–7.4 vs. 690–840μgAs/kgTS; 1.5–2.2 vs. 149–219μgCd/kgTS; 58.8–71.7 vs. 227–298μgNi/kgTS. Then, the potential desorption of metals during the membrane acid cleanings was estimated as relevant as 10–15% of the metals associated to the clogging sludge. The combined effects of pH and the selected microbial community, and the minor effect of the redox potential, let us conclude on the major importance of bio-sorption/desorption mechanisms with respect to bio-precipitation/dissolution.

Hyporheic water exchange in a large hydropower-regulated boreal river – directions and rates

Widespread river regulation is known to modify river-aquifer interactions, influencing entire watersheds, but knowledge of the hyporheic flowpath along regulated rivers is limited. This study measured the hydraulic conductivity of the river bed and the aquifer, water levels and seepage fluxes in the heavily regulated Lule River in Northern Sweden, with the aim of characterising water exchange across the river-aquifer interface. While pristine rivers in the area are gaining, the Lule River was recharging the aquifer during 10% of the time. Daily river level fluctuations (typically ±0.25 m) directed ~3% of the total orthogonal flux across the river bed towards the aquifer, while during ~2% of the time the orthogonal fluxes were negligible (≤10–4 m d–1). A clogging layer on the river bed, most likely formed due to the modified river discharge, restricted river-aquifer exchange. The hyporheic zone had higher electrical conductivity than the river and the aquifer and electrical conductivity occasionally decreased following rising river water levels, with 3–5 hours' delay. Overall, hydropower regulation has severely altered the hydrological regime of the hyporheic zone in the Lule River.

Potential Clogging and Dissolution Effects During Artificial Recharge of Groundwater Using Potable Water

To avoid the water quality deterioration that are caused by artificial recharge (AR) of groundwater, potable drinking water has been used as one of the source water for AR to control the side effects caused by the over-exploitation of groundwater. Chemical clogging problems can still be caused by certain chemical components, especially Fe and Al, and a lower concentration of these elements can cause a notable decrease in hydraulic conductivity at the top layer of the infiltration medium. Some components in AR source water can be obstructed by the clogging layer, leading to a change in water quality. The accumulation of total suspended solids (TSS) at the clogging layer can cause physical clogging and worsen the degree of chemical clogging. Although clogging and the related change in water quality were the dominant issues that affect the infiltration rate and health risks during the AR process, the dissolution of the aquifer matrix should also be taken into account. This dissolution contributed to not only the hydraulic conductivity of the infiltration medium but also the potential change in water quality during the aquifer recharge, storage and recovery processes.

The spatial variability of streambed vertical hydraulic conductivity in an intermittent river, northwestern China

Streambed vertical hydraulic conductivity (K) plays an important role in river water and groundwater interaction. The K at the ten transects (Ts1–Ts10) at the Donghe River (an intermittent river) in the Ejina Basin, northwestern China, was measured to investigate its spatial variation. Based on the sediment characteristics and vertical hydraulic conductivity of the riverbed, the entire riverbed of the Donghe River could be divided arbitrarily into two parts: an upper part (starting at Ts1 and ending at Ts9, without an obvious and continuous clogging layer) and a lower part (the remaining riverbed, with an obvious and continuous clogging layer). In the upper part, although the K varied with depth within the 0–30 cm layer, the variability with depth could be ignored in practice. The arithmetic mean K of the upper part ranged from 12 to 27.6 m/day, three orders of magnitude larger than that of the lower part (0.06 m/day). The change of K along the river cross section was significant, and larger values of K often occurred in the parts of the channels with greater water depth. However, there were no consistent patterns of the variability of K at transects across the river, which was influenced by the variation in streambed characteristics. The results could be useful for the estimation of groundwater recharge from river and groundwater resources evaluation in the Ejina Basin.

Groundwater recharge from overbank floods

Overbank flood recharge is increasingly acknowledged as important for estimations of aquifer sustainable yield. The physics of this process in areas with shallow groundwater, however, is not well understood and typically is not included in river or groundwater models. Modeling of the overbank flood recharge process was undertaken using a fully coupled, surface‐subsurface flow model to compare the volume of infiltration through a floodplain with varying surface sediment, aquifer, and flood parameters. The infiltration volume was found to increase with the conductance of the clogging layer (represented by a thin veneer of sediments across the floodplain and river bed), flood wave height, peak duration, and aquifer transmissivity and to decrease with increasing water table gradient (positive toward the river). The influence of the flood wave and aquifer hydraulic parameters was more pronounced in systems with sand or loam clogging layers. Irregularities in floodplain elevation had a large effect on infiltration volume. A dimensionless analysis of the flood recharge process identified the factors that limited flood infiltration for each of the modeled scenarios: the clogging layer conductance, unsaturated aquifer storage, or aquifer transmissivity. A dimensionless numberF* was used to predict the limiting factor in floodplain systems. An analytical equation was developed to estimate the infiltration volume for catchments where full numerical modeling is not warranted or applicable. Results from the analytical equation compared favorably with recharge modeled using a more complex numerical model.

Removal of Pathogens by River Bank Filtration at Haridwar, India

AbstractMeasurements of river water quality at Haridwar, India, taken during 2005–2006 show that the river water does not meet the World Health Organization and Bureau of Indian Standards criteria of drinking water quality, especially with respect to total coliform and fecal coliform. This study investigates the removal of pathogens at a river bank filtration site in Haridwar. Using the quality of river water and the quality of abstracted water from a nearby production well, semi‐empirical models based on the concept of filtration coefficient are developed and tested for their effectiveness in removing pathogens under varying bacteriological quality of source water. A two‐tier model, which includes the effect of clogged layer, is developed to obtain an equivalent filtration coefficient. This coefficient is found to be linearly related with natural logarithm of the concentration of pathogens in the source water. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Modeling effects of floods on streambed hydraulic conductivity and groundwater‐surface water interactions

Flood events can induce temporal changes in streambed elevation and particle‐size composition, which may influence the bed's hydraulic properties and stream‐aquifer fluxes during and after an event. This study combines a set of previously developed modeling approaches to create a synthetic flood event during which bed sediment is entrained and deposited as a function of hydraulic conditions and particle size. One simulated river reach in a state of approximate dynamic equilibrium is chosen to investigate the impacts of size‐selective sediment transport on stream‐aquifer interaction. Along this reach, the preferential entrainment of fine sediment during the flood's rising limb leads to overall bed coarsening, and increases in vertical hydraulic conductivity (Kbv) and downward fluxes of floodwater into the streambed. Progressively finer sediment layers are deposited during the event's falling limb, causing the redevelopment of a colmation (clogging) layer on the bed surface and a decline in overall Kbv by the event's conclusion. This reduction in Kbv leads to prolonged retention of event water in the streambed (after the reach reverts from losing to gaining river conditions) when compared with what is expected if pre‐event Kbv values are used to estimate river‐aquifer exchanges. This process of sequential bed coarsening and fining during a flood event provides a mechanistic explanation for the event size‐and‐duration threshold, inferred in some systems, that must be exceeded for significant amounts of flood recharge to occur. The major consequences of these processes—enhanced infiltration and prolonged floodwater retention—have potentially major implications for groundwater‐surface water interactions, water quality, contaminant transport, and riparian biogeochemistry.

First-principles modeling of frost accumulation on fan-supplied tube-fin evaporators

Abstract The present paper investigates the frost formation on air-supplied tube-fin evaporator coils under typical operating conditions of light commercial refrigerating appliances. To this end, a first-principles simulation model based on air-side mass, energy and momentum balances was put forward to predict the evaporator frosting over time. Experiments were also carried out to gather key data for the model validation exercise. The numerical results were compared with the experimental air-side pressure drop, air-flow rate, cooling capacity, and accumulated frost mass, with all predictions falling within the experimental uncertainty range. The model was then used to investigate the evaporator thermal-hydraulic performance under frosting conditions accounting for the non-linear effect induced by the combination of the frosted evaporator and the fan-supplied air-flow rate. The effects of progressive frost clogging and low conductivity frost layer on the overall thermal resistance were also assessed. It was found that the former is the main cause of cooling capacity reduction under frosting conditions.

Laboratory assessment of factors affecting soil clogging of soil aquifer treatment systems

In this study the effect of soil type, level of pre-treatment, ponding depth, temperature and sunlight on clogging of soil aquifer treatment (SAT) systems was evaluated over an eight week duration in constant temperature and glasshouse environments. Of the two soil types tested, the more permeable sand media clogged more than the loam, but still retained an order of magnitude higher absolute permeability. A 6- to 8-fold difference in hydraulic loading rates was observed between the four source water types tested (one potable water and three recycled waters), with improved water quality resulting in significantly higher infiltration. Infiltration rates for ponding depths of 30 cm and 50 cm were higher than 10 cm, although for 50 cm clogging rates were higher due to greater compaction of the clogging layer. Overall, physical clogging was more significant than other forms of clogging. Microbial clogging becomes increasingly important when the particulate concentrations in the source waters are reduced through pre-treatment and for finer textured soils due to the higher specific surface area of the media. Clogging by gas binding took place in the glasshouse but not in the lab, and mechanical clogging associated with particle rearrangement was evident in the sand media but not in the loam. These results offer insight into the soil, water quality and operating conditions needed to achieve viable SAT systems.

A conceptual approach based on suspended solids to estimate clogging time in constructed wetlands

A conceptual model has been developed by using the parameter of influent suspended solids (SS) concentration to estimate the clogging time in constructed wetlands (CWs). The depth of the clogging layer is estimated through the effective porosity vertically along the CWs. The basis of the model to predict the clogging time lies in the fact that, when clogging occurs, the pore spaces were reduced and thus the infiltration rate was reduced. A group of laboratory scale CWs was employed to develop the model and validate its utilization. Good agreement between the experimental data and the predicted clogging time was obtained. This indicates that the model can be used in different operation conditions for estimating clogging time in CWs.

Observation of bioturbation and hyporheic flux in streambeds

In the Elkhorn River, burrows, tubes, and sediment mounds created by invertebrate bioturbation were observed in the exposed streambed and commonly concentrated on the fine-sediment patches, which consist of silt, clay, and organic matter. These invertebrate activities could loosen the thin layer of clogging sediments and result in an increase of pore size in the sediments, leading to greater vertical hydraulic conductivity of the streambed (Kv). The measurements of the vertical hydraulic gradient across the submerged streambed show that vertical flux in the hyporheic zone can alter directions (upward versus downward) for two locations only a few meters apart. In situ permeameter tests show that streambed Kv in the upper sediment layer is much higher than that in the lower sediment layer, and the calculated Kv in the submerged streambed is consistently greater than that in the clogged sediments around the shorelines of the sand bars. Moreover, a phenomenon of gas bubble release at the water-sediment interface from the subsurface sediments was observed in the groundwater seepage zone where flow velocity is extremely small. The bursting of gas bubbles can potentially break the thin clogging layer of sediments and enhance the vertical hydraulic conductivity of the streambed.

Clogging pattern in vertical-flow constructed wetlands: Insight from a laboratory study

Substrate clogging caused by the accumulation of the particulate solids is the worst operational problem for vertical-flow constructed wetlands (VFCW). In this paper, the effects of particulate solids distribution and their accumulation in the substrate with different gravel sizes were investigated. The results demonstrated that the clogging layer can be considered as two parts: one is the blanket-like deposition layer, and the other is the upper substrate clogging layer. Furthermore, the clogging process shall be partitioned as three stages of puncture phase for the pollutants; the formation of the blanket-like deposition layer; and the formation and compaction phase to the whole clogging layer. With reference to the clogging mechanism, it is believed that the particulate solids (<100 μm) were absorbed firstly by electrostatic forces and van der Waals’ forces. This is followed by the “bridging” made by the accumulated solids which act as a “sieve”, thus further restricting larger particulate solids to flow through.

Variability of streambed vertical hydraulic conductivity with depth along the Elkhorn River, Nebraska, USA

Determination of streambed hydraulic conductivity is of great importance in the analysis of stream-aquifer interactions and stream ecosystems. In this paper, in situ falling-head standpipe permeameter tests were conducted to determine streambed vertical hydraulic conductivity (Kv) of two connected layers of sediments at 60 test locations from eight sites in the Elkhorn River, Nebraska. Our results show that the Kv values for the upper layer of sediments are generally larger than those in their respective lower layer of sediments. The individual Kv value for the upper layer of sediments from all test locations of the eight sites range from 2.7 to 104.9 m/d, and the average value is 26.6 m/d. The individual Kv value ranges from 0.4 to 73.4 m/d and the average value is 16.1 m/d for the lower layer of sediments. We believe that hyporheic processes can enhance larger streambed Kv in the upper layer of sediments. Inflow and outflow through hyporheic zone produce more porous and permeable sediments and further result in an increasing of streambed Kv. Moreover, we observe that bioturbation activities can destroy the clogging layer of the exposed sediments and create pore spaces in the sediments, which also further increases streambed Kv.