Hyporheic Exchange in a Straight Pool-Riffle Stream With Floodplain

Abstract

Hyporheic exchange (HE) is the exchange between surface water, including suspended and dissolved matter, and pore water in the sediment surrounding a stream. It is essential to physicochemical and biological processes in both the stream and the surrounding sediment. This study systematically investigated the effects of the stream characteristics (i.e., stream discharge, bedform amplitude and wavelength, stream bed slope and groundwater flow) on the characteristics of HE (i.e., flowrate, scale, and residence time) in a straight pool-riffle stream with floodplain using laboratory experiments and numerical simulations. The HE in the stream bank and streambed and its controlling factors were evaluated using numerical models. The results show that the HE flowrate and volume in the stream banks (i.e., in the floodplain) and streambed are significant and comparable at a high discharge. The discharge, bedform amplitude and bed slope directly affect the flow state in the pool, and the flow state determines the patterns of the effects of these three factors on the HE flowrate. The HE flowrate shows a linear relationship with discharge and bed slope when the surface flow in the pool remains subcritical. If the flow state in the pool changes from subcritical to supercritical conditions as discharge or bed slope increase, the HE flowrate becomes non-monotonic with discharge and it first increases and then decreases with increasing bed slope. The flowrate, scale, and flux-weighted mean residence time of HE linearly increase with bedform wavelength and exponentially decrease with both gaining and losing groundwater flow, independent of discharge.