Filling the Void: The Effect of Stream Bank Soil Pipes on Transient Hyporheic Exchange During a Peak Flow Event

Abstract

The hyporheic zone is the ecotone between stream and river channel flow and groundwater that can process nutrients and improve water quality. Transient hyporheic zones occur in the riparian zone (bank storage or “lung model” exchange) during channel stage fluctuations. Recent studies show that soil pipes are widespread in stream banks and beneath floodplains, creating highly preferential flow between channel and riparian groundwater such that the traditional Darcy model of flow does not apply. We used MODFLOW with the conduit flow package to model a series of stream bank soil pipes and examined soil pipe density (number per m), length, diameter, height above baseflow water surface, connectivity, and matrix hydraulic conductivity on transient particle flow paths and total hyporheic exchange volume (i.e., bank storage) over the course of a peak flow (e.g., storm) event. We found that adding five soil pipes per meter more than doubled hyporheic volume. Soil pipe length was the most important control; adding one 1.5‐m‐long soil pipe caused a 73.4% increase in hyporheic volume. The effect of increasing soil pipe diameter on hyporheic volume leveled off at ~1 cm, as flow limitation switched from pipe flow to pipe‐matrix exchange. To validate our approach, we used the model to successfully reproduce trends from field studies. Our results highlight the need to consider soil pipes when modeling, monitoring, or managing bank storage, floodplain connectivity, or hyporheic exchange.

Preferential Flow in Riparian Groundwater: Gateways for Watershed Solute Transport and Implications for Water Quality Management

“…Gravel veins and paleochannels are present in riparian zones, floodplains, and natural levees and can dominate water and solute transport (Bridge, 1985; Cardenas & Zlotnik, 2003; Duval & Hill, 2006; Fox et al, 2011; Heeren et al, 2010, 2014; Johnson et al, 2012; McArthur et al, 2008; Newman & Keim, 2013). Similarly, soil pipes are widespread in streambanks, riparian zones, hyporheic zones, and floodplains (Hester et al, 2020; Jones & Cottrell, 2007; Menichino & Hester, 2015; Menichino et al, 2015), enhancing flow and transport functions (Lotts & Hester, 2020; Menichino et al, 2014, 2015).…”

Subsurface hydrology of tile‐drained headwater catchments: Compatibility of concepts and hydrochemistry

“…The change in dominant flowpaths upward into shallower zones (Seibert et al, 2009) and further inland from the stream (Blume & Van Meerveld, 2015) during stormflow or melt events may result in changes in the chemistry of the stream, including dilution of ‘geogenic’ solutes (Botter et al, 2020). The higher stream levels during events can cause temporary flow reversals in some zones, where streamwater recharges groundwater in the riparian zone as ‘bank storage’ (Lotts & Hester, 2020; Pinder & Sauer, 1971).…”

Effects of Successive Peak Flow Events on Hyporheic Exchange and Residence Times

“…Perturbations in river stage and discharge—alter the pressure distributions at the interface between the water column and the streambed, the main driver for hyporheic exchange, and thus lead to exchange fluxes several orders of magnitude higher than the ones under the base flow conditions (Gomez‐Velez et al., 2017; Singh et al., 2019; Wu et al., 2018). In addition to river stage fluctuations and pressure variations, continuous exchange of water, solutes, and energy between the water column and the hyporheic zone is controlled by interactions between geomorphological settings, channel gradient, hydraulic conductivity, sediment heterogeneity, and spatial variability in heads at the sediment‐water interface (SWI) and preferential flow paths (Gomez‐Velez et al., 2014; Gomez‐Velez & Harvey, 2014; Lotts & Hester, 2020; Marzadri et al., 2016; Menichino & Hester, 2015; Tonina & Buffington, 2011). Interactions between these driving mechanisms determine the hydrodynamics and residence times within the hyporheic zone.…”