Poster, AGU Fall Meeting 2016, San Francisco, USA: 2016-12-12 - 2016-12-16
Abstract:
Previous research has identified streambed morphology, sediment properties, channel discharge and groundwater flux as key factors controlling flow and transport characteristics within hyporheic zones (HZs). While the potential dampening of hyporheic exchange flow (HEF) by groundwater up-welling (or down-welling) has been investigated, particularly for ripple bedforms, the impact of time-varying discharge, causing non-stationarity in advective pumping induced HEF, for different types of streambed morphologies remained largely unclear. In this study, we explore the complex interplay among transient driving forces (freshets) and both streambed morphology and groundwater up-welling/down-welling conditions. We simulate HEF driven by a wide range of freshet scenarios in riffle-pool, dune and ripple bedforms under different magnitudes of gaining and losing conditions. To evaluate the effect of the dynamic forcing while modulated by groundwater fluxes, we chose the following metrics: HEF, HZ extent, and the spatial patterns of residence time and oxic/anoxic zones. Our study revealed that HZ emerged, vanished, expanded and contracted during freshet events as a function of freshet duration and symmetry, geomorphology and groundwater up-welling/down-welling conditions. Furthermore, zones of substantially increased residence times or even stagnant water were found to vertically move up and down with the expansions and contractions of HZ, bearing potential impacts for biogeochemical transformations. In addition to substantially affecting HZ size, shape and quantity of HEF, the investigated transient freshet scenarios proved to cause significant changes in hyporheic RTD of relevance for further research into biogeochemical cycling within hyporheic zones and stream-aquifer management at larger scales.