Variation in reach-scale hydraulic conductivity of streambeds
Introduction
Hyporheic zones (HZs) are the saturated sediments beneath and adjacent to river channels through which surface water exchanges and mixes with groundwater (White, 1993, Boulton et al., 2010). The HZ is a unique ecotone that supports a variety of hydrological, ecological and biogeochemical processes essential to river ecosystem function (Gibert et al., 1990, Boulton et al., 2010). By regulating the transfer of heat and mass across the sediment–water interface, the HZs play a critical role in temperature buffering (Arrigoni et al., 2008) and biogeochemical cycling (Mulholland and Webster, 2010). They are also permanent habitats for many microbes and invertebrates (Brunke and Gonser, 1999), provide refugia for surface invertebrates or fish (Dole-Olivier, 2011, Kawanishi et al., 2013), and are used by some fish for spawning (Geist et al., 2002). The occurrence and magnitude of processes occurring in HZs largely depend upon the hydrological flux between surface and ground waters (Findlay, 1995, Fischer et al., 2005).
Most laboratory-, field-, and model-based research of hyporheic zone processes has been at the scale of a short river reach (up to several meander wavelengths) or smaller, but efforts to scale up this research to an entire river catchment are very rare (Kiel and Cardenas, 2014). Such efforts will require an understanding of catchment-scale variations in the hyporheic flow regimes including hyporheic flux, residence time, and geometry of flow paths. These are largely determined by variations in pressure at the sediment–water interface and hyporheic zone/groundwater boundary, by bed mobility, and by the variable hydraulic conductivity of porous boundary material (Blaschke et al., 2003). In turn, all these factors vary with river hydrology, channel morphology, and associated fluvial processes (Malard et al., 2002, Tonina and Buffington, 2009).
Although measurements of streambed conductivity have been reported from a broad range of stream types, few empirical studies link spatial (between sites) and temporal (with time) variations in streambed hydraulic conductivity to flow, catchment characteristics, and other geomorphic drivers. Point measurements of streambed hydraulic conductivity found in the literature vary between 10−10 and 10−2 m/s (Calver, 2001), and reach-average values are between 10−5 and 10−3 m/s (Genereux et al., 2008, Song et al., 2009, Chen, 2010, Cheng et al., 2010, Min et al., 2012, Taylor et al., 2013). This upper limit on reach-average values is an order of magnitude lower than might be expected for a uniform gravel [e.g., the Hazen formula (Hazen, 1892) estimates hydraulic conductivity of 0.04 m/s for particle size diameters of 2 mm]. This is because streambed sediments generally have a broad distribution of particle sizes and because hydraulic conductivity is largely determined by the smaller size fractions (Alyamani and Sen, 1993, Song et al., 2009, Descloux et al., 2010). Consequently, variation in hydraulic conductivity between reaches is likely the result of processes controlling presence of fine sediments in the streambed rather than the coarse fraction. Further, point-scale measurements vary considerably within a reach. In some rivers, sections of streambed may be effectively impermeable but the streambed is rarely impermeable throughout the river channel. The lowest reported value of 10−10 m/s, is five orders of magnitude smaller than the lowest reported reach-average value.
In this study we model spatial and temporal variations in hydraulic conductivity to support advances in our understanding of hyporheic processes and their ecological consequences at the catchment scale. After describing a conceptual model of streambed hydraulic connectivity, we use field data collected in 119 surveys of 83 stream reaches across continental France (Datry et al., 2014) to fit and cross-validate an empirical model of reach-scale conductivity as a function of candidate geomorphic and hydraulic controls.
Section snippets
Conceptual model of streambed hydraulic conductivity
Multiple processes likely influence the presence of fine sediments within the streambed and hence its hydraulic conductivity (Fig. 1). These processes drive fine sediment supply, retention on and within the streambed, and fine sediment removal. Fine sediment is supplied from scour of the upstream streambed or banks, and from erosion within the catchment (Wood and Armitage, 1997). Worldwide, land clearance, logging, and mining have increased catchment fine sediment supply whilst sediment
Study reaches
Between February 2010 and October 2011, 153 field surveys of reach hydraulic conductivity were made across 100 stream sites in France. This field program was part of a study to assess use of sediment hydraulic conductivity as a measure of streambed clogging (Datry et al., 2014). Of these sites, 18 were chosen according to their clogging conditions (9 clogged and 9 unclogged sites, as judged by local water managers). The other 82 sites were selected randomly across nine regions in France (Fig. 3
Results
Hydraulic conductivity varied up to a maximum value of 5.6 × 10− 4 m/s across the 119 reaches (Fig. 4). The lower detection limit using this equipment is uncertain, but for our purposes we consider 1.0 × 10− 6 m/s to be the lower bound for this method of estimating reach-average values. The distribution of values was skewed toward lower values, and 9% of values were recorded at or below this lower detection limit.
Three comparisons were made between observed reach hydraulic conductivity and modelled
Discussion
The upper limit for the range in kreach values reported in this study (5.6 × 10−4 m/s) is consistent with published values including ranges of: 1.2 × 10−4 to 7.4 × 10−4 (Chen, 2010); 2.0 × 10−4 to 5.5 × 10−4 (Cheng et al., 2010); 0.2 × 10−4 to 1.3 × 10−4 (Genereux et al., 2008); and 1.3 × 10−4 to 6.6 × 10−4 (Song et al., 2009) (all units in m/s). Despite the dominance of coarse-bed rivers in our study, this upper limit is more than two orders of magnitude lower than hydraulic conductivity expected for well-sorted
Conclusions
Streambed hydraulic conductivity can vary over several orders of magnitude potentially exerting a strong control on spatial and temporal variation in hyporheic flow regimes, including hyporheic flux and residence times. Hydraulic conductivity, even of coarse bed streams, is characteristic of sand and finer sediments indicating that processes of streambed clogging are critical. This empirical study found that hydraulic conductivity depends primarily on reach geometry (increases with bedform
Acknowledgements
The authors are grateful for the thoughtful comments of two anonymous reviewers and the considerable patience and editorial input provided by the Editor-in-Chief Prof. Richard Marston.
Stewardson and Grant acknowledge the support of the Australian Research Council (ARC DP130103619) and the US National Science Foundation Partnerships for International Research and Education (OISE-1243543). Stewardson undertook this research primarily while on study leave and hosted by IRSTEA in Lyon, France.
The
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2021, Advances in Water ResourcesCitation Excerpt :So, this criterion was considered in selecting the thickness of the porous domain. Also, the porosity and hydraulic conductivity were considered as 0.3 and 5.6E-4 m/s, respectively, based on 119 reach measurements in French rivers by (Stewardson et al., 2016). Also, for validation of subsurface model, the emergence locations of injected dye for laboratory and field numerical simulations were compared.
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Host institution for corresponding author during research for this paper.