Nitrate removal processes in the riverbed during a single‐peak flood event

Abstract The effect of the interplay between unsteady flow and bedform in a flood event on nitrogen cycling in the hyporheic zone (HZ) remains poorly understood. In this study, a reactive transport groundwater model with different flood hydrographs was proposed to investigate the effect of modified hyporheic flow on nitrate dynamics in the HZ, including nitrate source‐sink function, response to the single‐peak flood event and removal efficiency. The results demonstrate that there exists an optimal range of river channel gradients that could enhance the biogeochemical reactions (respiration, nitrification and denitrification) in a flood event. The HZ acts as a nitrate sink especially after the flood event, and its source‐sink function is independent of the unsteady discharge/stage conditions. The nitrate in the HZ has a hysteretic response to peak stage/discharge, and its removal efficiency is decreased by up to 70% compared to steady flow conditions. These findings not only provide a better understanding of nitrogen dynamics under the effect of unsteady channel flow, but also can be applied for river restoration to efficiently remove nitrate in the HZ by modifying river channel gradients.