Filling the void: the effect of riverbank soil pipes on transient hyporheic exchange during a peak flow event.

The hyporheic zone is a transition zone between channel and groundwater which processes nutrients and can benefit water quality. This interface occurs in the riparian zone during bank storage (lung model hyporheic exchange) events during channel stage fluctuations. Recent studies show soil pipes are common in riverbanks and floodplain groundwater, creating highly preferential flow between channel and riparian groundwater that cannot be accurately modeled using conventional Darcy approaches. We are not aware of studies that systematically examine controls on such soil pipe induced riverbank preferential exchange (hyporheic and bank storage). We used MODFLOW with the conduit flow package (CFP) to solve the 2D Boussinesq equation groundwater equation and pipe flow equations to simulate a series of riverbank soil pipes. We examined the effect of soil pipe density (number per meter), 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 conducted particle tracking via MODPATH. We found that soil pipes substantially impact hyporheic exchange as the addition of five soil pipes per meter more than doubled hyporheic exchange volume. Soil pipe length was the most important control; adding one 1.5-meter-long soil pipe caused a 73.4% increase in hyporheic volume, and the hyporheic volume increased exponentially with soil pipe length. 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. The relationship between the soil matrix hydraulic conductivity and percent hyporheic volume added by the addition of one soil pipe generally increased with K but had a local maximum of ~35% at K=10^-5.5 m/s and a local minimum of ~32% at K=10^-4.5 m/s. From particle tracking, the soil pipes repelled particle traces as they traveled away from the stream, but attracted them as they returned to the steam. 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.