Numerical simulation of overbank hyporheic transport and biogeochemical reactions in a compound channel

Abstract

Transverse hyporheic transport and biogeochemical reactions in a compound channel remain poorly understood. As floodplains are submerged, solutes in the surface water enter the hyporheic zone of the compound channel and react with the solutes upwelling below the floodplain. Aerobic respiration (AR) and denitrification (DN) processes in the hyporheic zone of the compound channel still need to be clarified. In this paper, a 3D hydrodynamic model coupled to a 2D groundwater flow and biogeochemical model was applied to investigate such processes. The model results were verified using laboratory experimental measurements. The effects of bank slope angle, ambient groundwater flow, and ambient groundwater solute concentration on aerobic and anaerobic biogeochemical processes were studied. The denitrification zone was found below the interface between the main channel and the floodplain.The lower bank slope angle favours nitrogen removal. Small losing groundwater discharges promote aerobic respiration and denitrification reactions. As the ambient groundwater oxygen concentration rises, O 2 consumption increases while N removal decreases. As the NO 3 concentration in ambient groundwater increases, NO 3 consumption increases and proportionally NO 3 consumption decreases. These findings not only provide a better understanding of biogeochemical reactions in a compound channel but can also be applied to river restoration to efficiently remove nitrate by modifying bank slope angles.

Effect of bank slope and ambient groundwater discharge on hyporheic transport and biogeochemical reactions in a compound channel

“…In previous numerical studies (Trauth et al, 2014), it was discovered that the expansion of the MRTs led to a progressive improvement in the NO 3 − removal efficiency. Xiao et al (2022) found O 2 removal efficiencies first increased and then decreased with MRTs, and he considered that water packages will complete biogeochemical processes in deep groundwater outside the domain, but the MRT obtained by the model is calculated from the water column in the model area, which might substantially underestimate the true MRT of the water column. Additionally, RT distribution is equally crucial, and an extremely large MRT does not ensure that all responses will progress properly.…”

“…The mesh independence analysis information for the surface water model and groundwater and the validation of surface water model are available in Xiao et al (2022).…”

“…ciency Xiao et al (2022). found O 2 removal efficiencies first increased and then decreased with MRTs, and he considered that water packages will complete biogeochemical processes in deep groundwater outside the domain, but the MRT obtained by the model is calculated from the water column in the model area, which might substantially underestimate the true MRT of the water column.…”

“…Therefore, denitrification capacity in the hyporheic zone of natural compound rivers should be further evaluated, and the effects of ambient groundwater flow need to be attended to. The results of the present laboratory study might be extended to compound channels on the large-scale considering similitude laws.Hyporheic zone biogeochemical responses at laboratory and actual scales were compared byXiao et al (2022), who found that the larger the scale, the more effectively solutes responded. Overbank flow brought on by flood events might result in sediment motion in the main channel-floodplain system(Asselman & Middelkoop, 1995).…”