Abundance and dimensions of naturally occurring macropores along stream channels and the effects of artificially constructed large macropores on transient storage

Abstract

Macropores are connected void spaces in the subsurface and can act as preferential flow paths for groundwater transport, but their dimensions and distribution patterns have not been well characterized in stream banks and hyporheic sediments. We ran field surveys in 5 streams of varying size, bed slope, and watershed land use in the Appalachian Mountains of southwestern Virginia. Natural surface-connected macropores were nearly ubiquitous. Macropores in this region arise by a variety of mechanisms, including soil piping, tree root decay, erosion around hardened structures, macroinvertebrate burrows, and invertebrate burrows. Macropore openings were slightly wider than tall, with median cross-sectional widths and heights of 3.5 cm and 3.0 cm, respectively. The median and maximum macropore lengths to first bend were 15.0 cm and 120.5 cm into the bank, respectively. True (tortuous) macropore lengths probably are often greater, and methods to map macropores are needed. Median interspacing of macropores across all streams was 0.38 m, but mean interspacing was 1.12 m, indicating that macropores were clustered in space. Macropores were inundated at different times because of differences in their heights on the stream bank, channel geometry, and stream stage. HEC-RAS modeling of channel hydraulics indicated that only 1 to 32% of macropores were inundated by channel water at base flow, whereas up to 97% were inundated during the largest storms. We ran conservative-tracer injection experiments at base flow in a 30-m reach of a small tributary to 1 study stream. We calculated transient-storage parameters for 2 treatments: 1 without macropores and 1 with artificially created macropores. Our results suggest that constructed macropores weakly increased transient storage (channel:storage zone cross-sectional area [A s /A] by 15.0% and fraction of median travel time spent in the transient-storage zone [F med ] by 31.0%) and may affect surface-water-groundwater interactions. Many macropore openings were above typical baseflow water levels, so macropores may enhance bank storage, a possibility that bears further research.

The effect of macropores on bi-directional hydrologic exchange between a stream channel and riparian groundwater

“…While this study only explores the impact of macropores at the scale of bank storage, larger scale impacts may also be important. For instance, at the reach scale, macropores have been shown to have a measureable effect on baseflow transient storage (Menichino et al, 2015).…”

“…By contrast, fewer studies have explored macropores for their capacity to facilitate bidirectional exchange between streams and riparian zone/floodplain groundwater. Nevertheless, an increasing number of studies suggest that such macropores are common (Bohlke et al, 2007;Gormally et al, 2011;Jones and Cottrell, 2007;Menichino et al, 2015;Newman and Keim, 2013). Some studies have explored the sediment scale impacts of macropores created by bioturbation (Mermillod-Blondin and Rosenberg, 2006;Mermillod-Blondin et al, 2004;Nogaro et al, 2006), however the bank storage effects of macropores remain poorly understood.…”

“…Similarly, macropores may 7 influence solute transport between stream channels and riparian groundwater zones (Angier and McCarty, 2008;Gormally et al, 2011;Newman and Keim, 2013). Macropores have been shown to exist in streams primarily between baseflow and bankfull water levels (Menichino et al, 2015). This suggests that most macropores will be inundated during storm events and may contribute to bank storage.…”

Groundwater–surface-water interactions: current research directions

“…However, heterogeneity in hydraulic conductivity may be too great to be reasonably captured by spatial averaging. Menichino et al (2015) surveyed macropores along the banks of 2 ndand 3 rd -order streams. Large macropores were common in all surveyed streams and modeling with the Hydrologic Engineering Centers River Analysis System showed they would be inundated frequently by high flows.…”

Beheervoorstel voor de dijkvegetaties langs de Zeeschelde, Durme en Rupel

“…In deze grondpijpen of macroporiën kunnen grondwaterstromingen zich concentreren en aanleiding geven tot ondergrondse erosie (piping) (Ghestem et al 2011;Leslie et al 2014). Niet enkel planten maar ook vertebraten (mol, konijn, bever, bruine rat) of invertebraten (krab, regenworm) kunnen grondpijpen of macroporiën vormen (overzicht in Menichino et al 2015). De wetenschappelijke literatuur hieromtrent focust zich echter voornamelijk op hellingen en rivieroevers (Fox & Wilson 2010).…”