Multi-scale preferential flow processes in an urban streambed under variable hydraulic conditions
Spatially preferential flow processes occur at nested scales at the sediment-water interface (SWI), due in part to sediment heterogeneities, which may be enhanced in flashy urban streams with heavy road sand influence. However, several factors, including the flow-rate dependence of preferential hyporheic flow and discrete groundwater discharge zones are commonly overlooked in reach-scale models of groundwater/surface water exchange. Using a series of controlled-head tracer-injection experiments coupled with cm-scale geophysics within the highly reactive upper 30 cm of the hyporheic zone of an urban stream, we quantified the flow dependence of local less-mobile porosity volume, mass-transfer rate coefficient, and the resulting local residence time in the less-mobile pore space at three controlled downward fluid fluxes (0.8, 2, and 3 m/d). Experiments were performed in two adjacent streambed locations, representing different sediment bulk vertical permeability. Less-mobile porosity parameters were generally substantial and similar between the two streambed locations; though a more competent, thin, organic layer at ∼15 cm depth in one location strongly impacted tracer loading, flushing dynamics, and local residence times. Increased downward flux led to (1) a decrease in less-mobile porosity residence time in all experiments, and (2) an increase in less-mobile porosity fraction for most experiments. Additionally, at the larger stream reach-scale, surface electrodes for electrical resistivity measurement were installed along 22 m of the wetted stream channel. These surface electrode measurements were collected during a natural storm flow event, which revealed widespread, short-term, flushing (e.g. <3 h) of the hyporheic zone with stream water, followed by longer-term (e.g. >60 h) flushing of the SWI with riparian zone groundwater. Flow dependence of preferential hyporheic zone flowpaths, like in the controlled tracer experiments, was also observed in these reach-scale electrical resistivity tomography measurements. Our findings reveal that the spatial and temporal dependence of preferential flow processes create highly dynamic SWI conditions that will affect the physical and coupled biogeochemical functions of the SWI in urbanized, sand-impacted streams.
Additional publication details
|Publication Subtype||Journal Article|
|Title||Multi-scale preferential flow processes in an urban streambed under variable hydraulic conditions|
|Series title||Journal of Hydrology|
|Contributing office(s)||WMA - Earth System Processes Division|