Engineered hyporheic zones: design and applications in stream health restoration – a review Open Access

Intense anthropogenic activities due to rapid growth in population and urbanization have destroyed the natural self-purification capacity of urban rivers (Baca et al. 2005; Komínková 2012). Such unplanned increase in urban population have not only caused high demand for potable water but also severely polluted the available water resources (Karn & Harada 2001) and as the rate of land use and climate change escalates, the competition for ecological services of rivers further intensifies (Boano et al. 2014). These effects are much more prominent in developing countries, where the average rate of urbanization is much higher (Karn & Harada 2001), topped with unplanned growth and amplified population densities due to human migration (McMichael 2000; Capps et al. 2016). About 90% of sewage was directly discharged into rivers in developing countries (UN-Water 2008) and due to its economic, political and social conditions, it would take at least 20 years to build all necessary infrastructure to control river pollution (Baca et al. 2005) and yet, even today, it seems a distant reality. Thus, keeping in view the economic reality of developing countries, there is an urgent requirement for alternative low cost treatment technologies to address this growing problem of urban river degradation.

The Hyporheic Zone (HZ) has been at the heart of discussion while talking about stream restorations for several years now (Boulton 2007; Hester & Doyle 2008; Robertson & Merkley 2009; Boulton et al. 2010; Hester & Gooseff 2010; Ward et al. 2011; Lawrence et al. 2013; Herzog et al. 2016, 2018). It can be attributed to the established contributions offered by the zone to an assortment of ecosystem services and downstream water quality (Findlay 1995; Brunke & Gonser 1997; Krause et al. 2011). Orghidan (1959) was the first to recognise this transition area between ground and surface water as a distinct zone of invertebrate assemblage and called it the ‘Hyporheische Biotop’, which meant ‘under + flow’ biotope (Boulton et al. 2010). The distinct invertebrates found there came to be widely known as ‘hyporheos’ (Brunke & Gonser 1997) and the space as the hyporheic zone. Boulton et al. (2010) discussed the initial attempts to delineate HZ was based on distribution of surface and subsurface invertebrates, followed by delineation based on temperature as tracer (White & Sully 1987) and solute as tracer (Triska et al. 1989). Harvey et al. (1996) defined hyporheic exchange as movement of water between channel and subsurface at small scales of centimetre to meter. Triska et al. (1989) suggested that at least 10% constituent of surface water, with a maximum of 98%, defined the hydrological boundary of the zone and this holds true even for modern day modelling studies (Lautz & Siegel 2006). Boano et al. (2014) quoted that Vaux (1968) may have been the first to develop a physically based model to study hyporheic flows. The authors account that individual works of Bencala & Walters (1983) and Thibodeaux & Boyle (1987) shed new information on bidirectional exchange flows and its subsequent modelling by Harvey & Bencala (1993) and Elliott & Brooks (1997a, 1997b) laid the groundwork for appreciating hyporheic flows and their interactions with stream and sediments.

Although HZ studies have been ongoing for over seven decades (Lewandowski et al. 2019), there is still no clear single definition available to cover all aspects of this zone. The multi-disciplinary nature and the spatiotemporal dynamism of the zone (Gooseff 2010; Ward 2016) make it difficult to give it a static definition (Brunke & Gonser 1997) and a discipline-specific definition would not convey the same concise meaning across disciplines (White 1993). In addition, the definitions varied not only among different disciplines but also within the same discipline (Gooseff 2010; Ward 2016; Lewandowski et al. 2019). Gooseff (2010) and Ward (2016) did a comprehensive discussion and comparison of HZ definitions from different disciplines. Gooseff (2010) purposed a concept of residence time to define HZ, for example, a ‘24-hour HZ’ meant that it took 24 hours for a parcel of water to travel from surface to subsurface, mix and return back. Ward (2016) defined it as a location in streambeds and banks meeting certain criteria. Gooseff (2010) definition would promote unification among interdisciplinary studies by allowing an organized approach, linking different scales of studies and timescales of exchange to timescales of processes. Ward (2016) and Lewandowski et al. (2019) defined HZ as any location in the subsurface saturated with flows originating and ending in the surface, whose residence time was in scale of relevance to the processes and the phenomenon occurred repeatedly. Lewandowski et al. (2019) believed that such criteria would not only be flexible and encompass all the existing definitions, but also enable other researchers to define HZ at different scales.

The anthropogenic deterioration of streams and rivers has resulted in degradation of the HZ (Boulton 2007; Lawrence et al. 2013). Hancock (2002) reviewed the effect of human activities on ecosystem functions of the HZ and listed its potential direct or indirect involvement on HZ processes impairment. Active HZ management could provide an excellent and rather unexplored opportunity to improve water quality and support the biodiversity of urban rivers (Lawrence et al. 2013). Bischel et al. (2013) states that in order to restore the ecology and aesthetics of urban streams, municipal wastewater effluents and urban runoff need to be envisioned as resources. Engineering of such zones has been paralleled to constructed wetlands on the basis of enhancements provided to the water quality and habitat (Lawrence et al. 2013); additionally it negates requirements like space, skilled personnel and infrastructure (Peter et al. 2019), which would have been a major limitation in any low-income developing country. Moreover, low-income countries have a wide range of climatic conditions, creating a broad range of scenarios (Capps et al. 2016), some of which may be favourable for hyporheic reactions, as the microbial activity and dissolved oxygen in water is controlled by water temperatures (Zarnetske et al. 2011) which subsequently depend upon the ambient air temperature (Lawrence et al. 2013). Hence, engineered HZ explorations could be the answer to the growing problem of urban river pollution in developing countries, which until now has been limited to developed countries.

Stream restoration measures adopted to protect streams from further deterioration have also had an influence on the hyporheic exchanges underneath the structures (Kasahara & Wondzell 2003). Several key review papers have been published on HZ processes, functions and significance (refer Lewandowski et al. 2019) but as per our knowledge, no review has been done on the induced hyporheic exchanges due to engineered systems. This paper attempts to assimilate literature pertaining to applications of engineered HZ formations in inducing Hyporheic Exchange Flows (HEF) in stream channels, to bring about water quality and ecological changes. The main objective of this paper is (1) to summarize the developments in Engineered Hyporheic Zone (EHZ) studies over the years; (2) to investigate the effectiveness of different structures and setups adopted in inducing HEF; (3) to identify the limitations and research gaps; and (4) to suggest future scope of EHZ applications. The literature examined are in the form of EHZ divided into sub headings surface and subsurface feature modifications followed by its discussion, limitations, future scope and conclusion.

Several authors have previously suggested inclusion of engineered elements into stream restoration projects (Hancock 2002; Boulton et al. 2010; Lawrence et al. 2013) and it is gradually gaining importance in the research field with a sharp rise in the number of papers published in the last fifteen years. (Cardenas 2015; Ward 2016). Although feature modification for hyporheic profits was being researched under different names like engineered elements, engineered flows, and hyporheic engineering (Gooseff 2010; Lawrence et al. 2013), the exact term ‘Engineered Hyporheic Zone’ (EHZ) was only recently mentioned (Herzog et al. 2018; Lewandowski et al. 2019; Peter et al. 2019). For the purpose of this review, this paper defines an Engineered Hyporheic Zone (EHZ) as any space where hyporheic processes occur due to modification of the HZ itself or the channel (surface or subsurface) or floodplains, intentionally or unintentionally, similar to the definition of hyporheic processes given by Ward (2016).

Bedform features like dunes, riffle-pools and gravel bars, in-stream geomorphic structures like cross vanes, dams, log jams and meanders and sudden change in streambed permeability due to buried structures (Figure 1) all influence hyporheic exchanges in stream channels (Kasahara & Hill 2006, 2007; Lautz & Siegel 2006; Hester & Doyle 2008; Ward et al. 2011). Broadly, these exchanges can be categorized as lateral or longitudinal, depending upon the direction of HEF induced by the structure (Kasahara & Hill 2007), while the structures can be labeled as surface or subsurface feature modifications based on their placement on the streambed (Ward et al. 2011). In this paper, the literature is reviewed under the surface and subsurface feature modification. Although numerous literature is available on induced hyporheic exchanges due to engineered elements, this review is focused on specific papers where the main objective of the engineered elements was enhanced hyporheic exchanges and pollutant removal.

In-stream geomorphic features like riffle-steps, gravel bars, meanders, debris/log dams and partially spanning structures like rock vanes or J-hooks, which are commonly used in channel protection and restoration (Kasahara & Hill 2006; Hester & Doyle 2008; Hester et al. 2018) are all categorized under surface feature modifications (Ward et al. 2011). These restoration features have been considered as the primary physical drivers of hyporheic exchanges (Brunke & Gonser 1997; Elliott & Brooks 1997a) and numerous studies have documented their influences on HEF and its ecological aspects (Harvey & Bencala 1993; Kasahara & Wondzell 2003; Storey et al. 2003; Gooseff et al. 2006; Tonina & Buffington 2007). Crispell & Endreny (2009) reported that Kasahara & Hill (2006) was the first to explore the effect of in-channel restoration on hyporheic exchanges. Surface feature modifications can be considered as an indirect method of inducing HEF, as they are usually not induced directly due to the structures, but often by the secondary bedforms created by them (Gordon et al. 2013).

Tracers have been commonly used in field studies of hyporheic exchanges (HE). Kasahara & Hill (2006, 2007) studied the effect of longitudinal-constructed riffle, step, and lateral- gravel bar and re-meandered stream restoration measures on HEF and hyporheic zone chemistry, in a lowland stream near Toronto, Ontario, Canada. Using conservative and non-conservative tracers, nitrate removal rate of over 90% was observed in the restored reaches, but it was not even one percent of the daily stream load. Longitudinal HEF were larger than lateral HEF for the same length of stretch and problem of progressive clogging reduced its performance. The authors suggested that while such streambed elements were successful in inducing hyporheic flux and biogeochemical reactions, it would require large stretches of engineered interventions to bring about noticeable changes in surface water chemistry. Zarnetske et al. (2011) conducted similar stream tracer studies on a third-order stream in western Oregon, USA, to investigate the HZ induced denitrification dynamics in gravel bars. Aerobic metabolic processes dominated short residence times, while longer timescales were required for denitrification to occur. The daily and seasonal changes in hydraulic, temperature, and chemical composition of water influenced the residence time threshold between processes. The authors state that sampling procedures might affect results due to mixing of oxic water while drawing samples. Gordon et al. (2013) used thermal tracers and chemical data to characterize the effect of cross-vane restoration structures on HEF and biogeochemistry of three lowland degraded streams in and around central New York, USA. They reported that low magnitude flux and biogeochemical cycling developed around the structures, which did not contribute to whole-stream system chemistry. The authors reported that secondary pools and riffles created due to cross-vane structures was more effective in inducing HEF and emphasized the importance of generating such secondary bedforms for more efficient exchanges. Doughty et al. (2020) explored the effect of channel spanning logjams on HEF in Little Beaver Creek, a third order tributary of Cache la Poudre River, northern Colorado, USA. The study was first to present the use of electrical resistivity in characterizing the increased HEF and complex pathways in the system. The extent and magnitude of HEF increased with the increase in discharge rates. Lack of field data and a suitable control channel limited appropriate interpretation of results.

Flume studies have been useful in explorations of feature-based influences on hyporheic exchanges (HE). Studies conducted by Mutz et al. (2007), Tonina & Buffington (2007), Zhou & Endreny (2013) and Dudunake et al. (2020) have investigated the effect of engineered elements like a log jam, gravel pool-riffle, boulders and cobble vanes bedform sequence on induced HEF. While Mutz et al. (2007) reported that introduction of wood tripled flow resistance and increased the vertical water flux, mixing depth and sediment pore water volume, Tonina & Buffington (2007) observed that the exchanges induced by bedforms was controlled by discharge and topographic submergence of the structures. Mutz et al. (2007) stated that the study was nontransferable to real streams, whereas 2D modelling gave poor performance in a gravel bed setting for Tonina & Buffington (2007). Zhou & Endreny (2013) investigated the impact of pool riffle bedform with and without channel spanning cobbles and observed that cobble structures created relatively higher vertical flux, but with reduced mixing depths. The authors attributed that the low bed pressure created downstream of the structure counter-balanced the downwelling forces, thus reducing the penetration depth. Dudunake et al. (2020) tried to comprehend the effect of morphological changes induced by boulders on HE and observed increased hyporheic residence time and downwelling flux rate at local and reach scales. The authors reported that the induced morphological changes had greater effect on hyporheic flows than boulders alone, concreting the influence of secondary bedform presented by Gordon et al. (2013). Model limitations and simplification were reported in both Zhou & Endreny (2013) and Dudunake et al. (2020) studies.

Few studies have attempted to provide a process-based understanding of in-channel restoration structures on HEF (Hester & Doyle 2008; Crispell & Endreny 2009; Wade et al. 2020). Crispell & Endreny (2009) conducted field experiments on a third order stream, Greene County, NY, USA, using thermal sensors to study the influence of J-hook and cross-vane structures. They observed that with increase in discharge, HEF changed around the structure and streambed slope was the controlling factor. Hester & Doyle (2008) chose a simplified hypothetical stream to comprehend the impact of weirs, steps and lateral structures and concluded that structure size, background groundwater discharge and hydraulic conductivity (HC) were the primary factors controlling HEF, while channel slope and baseflow discharge was relatively less important. Among the structures, weirs were most effective followed by steps and then lateral structures in inducing HEF. Both the studies were influenced by the work of Rosgen (2001), who had assessed different structures in case of stream stabilization and river restoration and both the studies were limited in rigorous evaluation of features due to 1D modelling. Wade et al. (2020) researched Beaver Dam Analogues (BDA), an artificially constructed permeable structure to promote hydraulic head differentials to induce exchanges, in a third order Red Canyon Creek near Lander, Wyoming, USA. The authors observed higher vertical fluxes, zones of spatially varying nitrate production and anaerobic reduction. The authors also reported that exchanges were enhanced only after a certain structure height was crossed and that it had little influence on surface water chemistry.

Modelling studies were valuable to deduce the complex hyporheic processes induced by streambed features. Boano et al. (2010) applied numerical simulations to investigate the influence of river sinuosity in spatial distribution of chemicals involved in redox reactions and their temporal variation due to meander evolution. A steady state supply of single dissolved organic compound was assumed to be supplied from stream to HZ. The authors reported that the neck and apex region of the meander had slowest and quickest hyporheic flowpaths, which also determined the limits of reaction timescale. Azinheira et al. (2014) and Hester et al. (2016) modelled a restored stream stretch of Blacksburg, Virginia, USA to analyze the performance of in-stream structures and inset floodplains in solute retention and nitrate removal under different flow conditions. Azinheira et al. (2014) observed that in-stream structure retained solutes during the summer baseflow scenario while the floodplains retained it during stormflow conditions. Hyporheic residence time in an in-stream structure was three to five times larger than in the floodplain and performed better in the case of nitrate removal, but was not enough to influence water quality via biochemical reactions. The reaction was transport limited as the rate of induced HEF was low, whereas nitrate removal in floodplains was reaction limited; that is, nitrogen uptake limited (Hester et al. 2016). Neither of the structures was engaged throughout the year nor could they be engaged simultaneously, resulting in limited solute retention and nitrate removal. Yang et al. (2018) used a reactive transport model to quantify the effects of dam-induced hydrodynamics on biogeochemical transformations of chromate (Cr) in Columbia River in Washington State, USA. The authors reported that flow direction reversals caused by the dam influenced the rate and extent of pollutant transformation depending upon dissolved and particulate organic carbon supplied by the stream. Monofy & Boano (2021) prepared a synthetic coupled surface-ground water model of Maruia River, New Zealand, to quantify the influences of streamflow, groundwater and sediment anisotropy on HZ characteristics due to fully developed alternate bars. They observed the existence of two HZs, shallow and deep, influenced by surface water and groundwater variations respectively, and sediment anisotropy further enhanced it. The authors also proposed a predictive model to predict HZ flux, residence time and depths based on bar submergence, surrounding groundwater and anisotropy of sediments. Summary of the literature on surface modification features is presented in Table 1.

While surface features were more intended for stream restoration than creating hyporheic flux (Herzog et al. 2016), subsurface features were typically engineered to enhance hyporheic exchanges (Ward et al. 2011). These modifications are related to the applied changes in the streambed subsurface like alteration of HC of streambed sediments, or introduction of subsurface structures or direct modifications of the HZ (Ward et al. 2011; Herzog et al. 2018; Peter et al. 2019) resulting in HEF enhancements. Vaux (1968) was first to purpose introduction of engineered structures with varying HC in the shallow subsurface to enhance hyporheic exchanges (Ward et al. 2011). Robertson & Merkley (2009) reported few reviews of initial studies on streambed modification for nitrate removal from agricultural drainages; it included use of tiled drains, constructed wetlands, end-to-pipe bioreactors and denitrification walls.

Ward et al. (2011) revisited the original proposals made by Vaux (1968) and prepared numerical models of the conceptual structures to study hyporheic flux and residence time induced by it. The results exhibited that structures with high HC converged flowpaths towards and through the structures while low HC structures deflected them. The authors also suggested suitable structure type, material, length and height to implement based on the restoration, design and residence time objectives. They further quoted that the structural design results were applicable to natural features and the results could be used for design of subsurface component of traditional restoration structures. The study was conducted in 2D space with homogeneous and isotropic sediment condition, which resulted in simplified flowpaths. While Ward et al. (2011) evaluated the structural performance keeping the field conditions like slope and insitu HC constant, Herzog et al. (2016) explored more complex model and its sensitivity to varying insitu HC and slope condition, as they believed that these variables could also be manipulated in case of large restoration projects. A conceptual model of a constructed stream facility was prepared to analyze the modified streambed structures, termed as Biohydrochemical Enhancement Structures for Stormwater Treatment (BEST) for their suitability in varying insitu HC and slope settings, concerning removal of metals, E.coli, nitrogen and phosphorus. The authors observed that combination of high and low HC structures placed together were best suited for pollutant removal and recommended that incorporation of geomedia into restoration structures would further enhance the reaction rates and make the structures more efficient. They reported that the modules were most appropriate for urban streams, which received inputs from stormwater runoff and recycled water, but would not be feasible in streams with high silt loads.

Lab-scale flume experiments have been successfully used to study the bedform influence and sediment characteristics on HEF, bacterial diversity and pollutant transformations (Kunkel & Radke 2008, 2011; Nodler et al. 2014; Li et al. 2015; Jaeger et al. 2019). Flume mesocosms are best suited for investigating micropollutant degradation process due to its small-scale exchanges and short flow paths (Zarnetske et al. 2011; Jaeger et al. 2019; Schaper et al. 2019) and it also bridges the gap between field and batch experiments (Jaeger et al. 2019). Recirculating flumes have been used (Kunkel & Radke 2008; Li et al. 2015) more than one at a time (Jaeger et al. 2019, 2021; Cook et al. 2020) to investigate HC and transformation relations, bacterial diversity and flow influence on degradation half-lives and bedform feature modulations, sediment size influence on biofilm communities. Results showed that bacterial diversity in the sediment and their size had paramount effect on micropollutant degradation (Jaeger et al. 2019; Cook et al. 2020; Betterle et al. 2021) and risk of secondary contamination due to transformed products was high (Li et al. 2015). Higher bacterial diversity was inversely related to hyporheic flux (Betterle et al. 2021) due to biofilm clogging, but could be resolved by larger bedform structures (Cook et al. 2020). Although direct extrapolation of results to real systems is complicated due to simplification in laboratory experiment, flume studies delivered important insights into micropollutant transformation processes and their product formation in the HZ (Li et al. 2015; Jaeger et al. 2019).

Field studies on application of constructed HZ was reported by Robertson & Merkley (2009), Herzog et al. (2018), Peter et al. (2019) and Bakke et al. (2020). Each individual study varied in all aspects from construction to its objective of application. While Robertson & Merkley (2009) constructed an instream bioreactor in a first order agricultural stream in southern Ontario, Canada, for nitrate removal, Herzog et al. (2018) performed outdoor flume studies using smart tracers (resazurin) in a constructed stream facility in Colorado, USA to test BEST structures for induced hyporheic exchange and reactive solute attenuation. Both the studies used woodchips as geomedia for increasing reaction rates and are the only known field experimentation of engineered HZ incorporated with geomedia. The coarse media of woodchips provided high permeability in subsurface allowing for higher flow through rates, even for small hydraulic head drop (Robertson & Merkley 2009). The bioreactor was constructed in an agricultural ditch with variable height outlet pipe to control flow rate. Nitrate removal rates were greater than in constructed wetland and depended directly on flow rate and atmospheric temperature. The BEST structure achieved 54% more hyporheic transient storage and required 55% less length than the control structure to remove 1-log of the reactive tracer, resazurin. Herzog et al. (2018) suggested that the BEST structure was best suited for low-discharge streams in urban catchments and stormwater channels with flow modulation. Although both designs offered definite advantage in induced hyporheic exchanges and the use of geomedia for pollutant removal was an added benefit, problems like colmation reduced the efficiency of the bioreactor over time and would also have affected the performance of BEST structures, if tested.

Peter et al. (2019) designed and implemented the first direct HZ manipulations in an urban stream channel in Seattle, Washington, USA. The authors determined the fate of commonly found urban water contaminants in the EHZ, which they defined as Hyporheic Design Element (HDE). A year later, Bakke et al. (2020) published a similar streambed engineering study in the same channel, focused on the design and performance of the constructed HZ in inducing hyporheic exchanges. The design features incorporated for both studies were use of channel-spanning logs to create a plunge pool, barrier, excavation of HZ and backfill with clean gravel. While the logs and barriers forced the water parcel lower, which increased flowpath lengths/residence times, the backfill increased the hyporheic exchange capacity. Peter et al. (2019) used dye and tracers to determine hyporheic flowpaths and achieved greater than 50% removal of pollutants in flow paths with more than three hours of residence time. Bakke et al. (2020) relied on temperature mapping, tracer studies to determine flux and reported that the vertical flux had increased by 89%, hyporheic volume by three fold and that the engineered element maintained the natural scour and fill during the duration of the study. Summary of the subsurface feature modification studies is given in Table 2.

The reviewed literatures explored various vertical (riffle-step, channel-spanning logjams, boulders, dams, BDAs, weirs and partially channel spanning cross-vanes and J-hook) and lateral (gravel bars, alternate bars and meanders) flux inducing features in the case of EHZ exchanges. Kasahara & Hill (2007) reported that exchange rates induced were similar for both types of features, but vertical flux inducing structures were more efficient in nitrate removal based on their smaller size. Full channel spanning structures induced larger vertical flux and promoted exchanges than partially spanning structures (Hester & Doyle 2008) while partial structures could serve an important role in benthic life sustenance and help in ecosystem management (Crispell & Endreny 2009; Gordon et al. 2013). Although subsurface modifications were not explored as much as its counterpart (Tables 1 and 2) (Hester et al. 2018), yet they held definitive advantages over surface features which included control over residence timescales, minimal effect on geomorphology, aquatic life and surface water temperatures (Ward et al. 2011; Herzog et al. 2016; Hester et al. 2018).

Most of the studies reviewed focused on exploration of HEF induced by individual structures. While such individual feature studies were important for general understanding of hyporheic processes at local scale, it did not reflect their ability on overall water quality improvements (Morén et al. 2017). Moreover, restoration practitioners usually assessed water quality targets at reach or catchment scale (Hester & Gooseff 2011; Morén et al. 2017; Hester et al. 2018), highlighting the need for reach scale multiple feature studies (Morén et al. 2017; Hester et al. 2018). Few studies compared the engineered interventions to natural morphological features (Smidt et al. 2015; Morén et al. 2017; Hester et al. 2018) and reported higher overall flux induced due to engineered structures. Recent studies at bedform scale have highlighted the importance of streambed heterogeneity, bed and width undulations in case of hyporheic response to modification studies (Liu et al. 2020; Movahedi et al. 2021) where the sediment architecture was often more influential than channel morphology in case of highly heterogeneous streambeds (Liu et al. 2020). Advancement in computing capabilities enabled numerical simulation and optimization studies of design structures (rock-vanes, weirs) for stream restoration and enhanced nitrogen removal (Khosronejad et al. 2018; Liu & Chui 2020).

Few authors have addressed the need for long-term assessment of restoration structures (Lewandowski et al. 2019). Drummond et al. (2018) assessed hyporheic exchanges in restored and unrestored reach nine years post restoration. The authors observed restored reaches had greater exchanges of fine particles and was not affected by clogging, rather scouring of fine particles was taking place. Mayer et al. (2021) presented a detailed long-term assessment from 2002-2012 of geomorphic stream restoration features on nitrogen(N) transport and transformation and observed progressive reductions in N content in restored reaches over the years. The authors reported that some engineered structures fared better at regulating N levels while other eroded post restoration. Morley et al. (2021) assessed EHZ aided floodplain restoration on microbial response at three paired stream reaches from 2014 to 2017. The authors observed fall of water temperatures, increased particulate organic matter (POM) and dissolved organic carbon (DOC) concentrations, shifts in microbial compositions and increased hyporheic invertebrate densities and richness.

The selection of appropriate methodology was governed by the attributes being evaluated, type of outcome desired, its spatiotemporal extent and the cost and effort available (Bakke et al. 2020). Numerous approaches (Tables 1 and 2) has been adopted to quantify the effects of modified features on complex physical and biochemical processes (Betterle et al. 2021). Kalbus et al. (2006) and Brunner et al. (2017) provided an overview of different measuring and modelling techniques in determination of HEF. Lewandowski et al. (2019) reported that each methodology adopted had its own advantages and limitations. While field investigation provided the best realistic conditions, it lacked in control of environmental factors. Flume studies could control environmental factors, but efforts required was higher. Numerical models could achieve in-depth assessment of hyporheic processes, isolate environmental factors and predict future outcomes, but were limited by assumptions, accuracy of measured data and system feedback to applied changes (Dudunake et al. 2020). Jaeger et al. (2019) suggested adoption of a multi-feature, multidisciplinary approach of study to reduce inadequacies and maximize research outputs.

Lewandowski et al. (2019) reported availability of numerous in-stream tracers in flume and field studies of HZs (salt tracers, heat tracer, which includes an active heat pulse and fiber optics-based sensors, radioactive gas tracers and smart tracers). Of these, only a few have been explored in EHZ studies such as salt traces (Kasahara & Hill 2006, 2007), heat sensors (Crispell & Endreny 2009; Gordon et al. 2013; Bakke et al. 2020), smart tracers like resazurin-resorufin for microbial activity determination (Herzog et al. 2018) and fluorescence tracer (Mutz et al. 2007; Drummond et al. 2018). It was reported that temperature-based studies were the most suitable methodology in HZ studies as it was a robust, quick, easy and inexpensive parameter to measure (Kalbus et al. 2006; Lewandowski et al. 2019). High spatiotemporal measurements of temperature using emerging technologies such as advance heat pulse sensors and fiber optics (Lawrence et al. 2013; Lewandowski et al. 2019), further added to its benefits. Although extensively used, tracers could only capture the net solute transport, provide empirical rather than process based estimates and could not indicate exact locations of flux change or the factors influencing it (Lautz & Siegel 2006).

Numerical modelling of groundwater flows addresses the shortcomings of tracer-based studies and could bridge the gap between filed observations and characterization of hyporheic processes (Lautz & Siegel 2006). The extensive improvements in computational capabilities coupled with advancement in fluid dynamics algorithms have broadened the path for refined study of complex hyporheic processes and have proven to be powerful tools in groundwater flow studies (Betterle et al. 2021). Groundwater flow models allow for simulation and quantification of gross effects of hyporheic processes, including area of flux change, residence times, and flux rates (Lautz & Siegel 2006) and recently, the role of microbial diversity (Betterle et al. 2021). A wide range of numerical models is available for study of groundwater flows and their selection processes have been thoroughly reviewed by Kumar (2019). While numerical models have been successful in studying induced exchanges, they neglect the effect of structural heterogeneity, downstream variability and streambed dynamics, known to influence flux rates, residence time and flowpaths. (Cardenas et al. 2004; Ward & Packman 2018; Liu et al. 2020). Limitations like assumption of constant model parameters, spatial homogeneity and stationary bedform dynamics prevent upscaling of modelling results into predictions at larger scale (Ward 2016; Liu & Chui 2018). Further, numerical studies lacked accountability of feedback from engineered structures on streambed morphological changes due to sediment transport (Dudunake et al. 2020).

Laboratory and outdoor flumes have been useful for investigating drivers of hyporheic processes as they enable control over environmental factors, thus reducing variability in the system (Betterle et al. 2021). Flume mesocosms are a valuable tool in linking control of lab experiments to originality of field experiments (Jaeger et al. 2019). However, research in flume studies have rarely considered bed movements (Lewandowski et al. 2019) although their relevance has been long mentioned (Elliott & Brooks 1997a, 1997b). Recent literatures have been trying to address such limits, as Liu & Chui (2018) quantified the effect of streambed heterogeneity and anisotropy on residence time (RT) and observed that it reduced the mean RT. Marttila et al. (2019) reported that the addition of sand (fine sediments) in gravel beds had a negative effect on hyporheic flux. Liu et al. (2020) observed that heterogeneous sediments with high sorptive capacity compressed the mixing zones and Betterle et al. (2021) reported that high bacterial diversity in sediments also reduced the hyporheic flux.

For the hyporheic processes to occur in the HZ, physical, microbial, and biochemical parts are equally important (Ward et al. 2011; Ward 2016). However, the necessary environment to drive the microbial and biochemical processes is provided by the hydrodynamic conditions created by the features (Battin 1999, 2000; Ward et al. 2011) and is the focus area of this review. Although the multidisciplinary understanding of the complex and dynamic surface and subsurface water interactions have improved in recent decades (Conant et al. 2019) but it still lacked, the framework to predict complex interactions, manage and transfer knowledge at reach or larger scale (Ward 2016; Magliozzi et al. 2018; Reeder et al. 2018; Ward & Packman 2018; Conant et al. 2019). Further, the detailed understanding of role of biodiversity (Marmonier et al. 2012), multi-instream structural influence (Hester et al. 2018) and long-term effects of stream restorations (Mayer et al. 2021) on HEF and transformations were limited. Below are some of the major limitations observed in EHZ studies:

• All the studies undertaken were at local, sub-reach or reach scale.

• Most of the studies were concentrated on individual structural influence on HEF.

• Simplification like spatial homogeneity, steady hydrologic conditions and stationary bedforms in the case of flume and modelling studies.

• Studies on the influence of induced exchanges on biological and chemical processes is limited.

• EHZ applications are limited to agricultural and urban streams.

• EHZ studies have been limited to local and reach scales, their explorations at channel and catchment scales is still missing (Magliozzi et al. 2018).

• Influence of integrated surface subsurface features and multi structural features on hyporheic exchanges were not explored (Ward et al. 2011; Hester et al. 2018).

• Nitrate removal rates, microbial activity and dissolved oxygen (DO) in water are regulated by stream temperatures, which depend upon ambient air temperatures (Robertson & Merkley 2009; Zarnetske et al. 2011; Lawrence et al. 2013). Thus, atmospheric temperature is an important variable to be considered in flume and field studies.

• Many authors have suggested use of geomedia to enhance reaction rates such as biochar woodchips, recycled industrial materials (Robertson & Merkley 2009; Herzog et al. 2016, 2018; Peter et al. 2019), of which only woodchips has been tested in EHZ studies (Robertson & Merkley 2009; Herzog et al. 2018).

• Accounting for feedbacks, streambed heterogeneity and dynamics in modelling studies (Liu & Chui 2018; Dudunake et al. 2020).

• Exploration of EHZ management practices on aquatic life (Marmonier et al. 2012).

The field of EHZ applications in water quality and habitat improvement is still young (Lawrence et al. 2013) with all the treatment processes in the experimental stage (Bakke et al. 2020). There is still a large gap between the current stage of knowledge and management activities in the case of EHZ applications (Morén et al. 2017; Ward & Packman 2018). Although methodological advances and use of innovative technologies (Lewandowski et al. 2019) in recent decades have vastly reduced this gap, it is still far from being significant to restoration managers (Morén et al. 2017). Limitations like lack of proper design guidelines to quantify hyporheic processes, which reflect water quality improvements at decision-making scales (Morén et al. 2017), hinders its adaptation. Some immediate future scope in EHZ studies can be listed as:

• Studies need to correlate the extent of restoration required with significant water quality benefits desired at reach and higher scales.

• A holistic study of hyporheic processes and impacts which integrates multi-disciplinary knowledge at channel and catchment scales.

• Investigations of integrated design approach of surface and subsurface structures to maximize HEF.

• Standardization of frameworks for conceptualization of surface water and groundwater interactions at each of local, reach, and catchment scales.

• Scope of EHZs can be explored in artificial channels, as low-cost post-effluent treatment units for water reclamation can be a valuable addition to water conservation and management practices.

This review assembled a comprehensive overview of the design and applications of EHZ in stream health improvement. Each individual research reviewed, unique and specific to some predefined objectives, not only imparted some valuable insights on EHZ processes and controls but also underlined some major limitations in its applications. Though subsurface structures had potential benefits with little effect on stream temperature, geomorphology and aquatic life, it was not explored as much as surface features nor was any literature on integrated structures available. Numerous approaches were undertaken to quantify the effects of engineered features, field, laboratory, and modelling, which all had their pros and cons, with thermal monitoring based field studies being most suitable with quick, robust and inexpensive heat sensors. Flumes provided convenience of controlling environmental parameters to isolate controlling factors and numerical models allowed for conceptualization, simulation, optimization and prediction of exchanges at different scales.

Rivers are not isolated systems but interact continuously with surrounding floodplains and underlying aquifers (Magliozzi et al. 2018). Developing an understanding of these interactions and their influences on water quality, quantity, and ecosystem health can provide the basis for land and water management and conservation of the environment (Conant et al. 2019). Under the stress of accelerated climate change and anthropogenic activities (Boano et al. 2014), it becomes imperative to consider integrated approaches for sustainable management of water resources (Wu et al. 2020). HZ research provides such opportunity by drawing researchers from various backgrounds to exchange ideas, knowledge and technologies to maintain and conserve land, water and related resources (Krause et al. 2011). Although there is potential in EHZ applications to make its way to standard restorations practices, yet it is important to note that EHZ is just one part of a comprehensive management strategy for river restoration and its application alone cannot improve the stream health (Lewandowski et al. 2019).

All relevant data are included in the paper or its Supplementary Information.