Influences of riverbed morphology on patterns and magnitudes of hyporheic water exchange within a natural river confluence

Abstract

River confluences are pivotal junctions that have vital influences on hydraulic characteristics and sediment particle distributions as well as on hyporheic water exchange. However, hyporheic water exchange is poorly understood because of the complicated river morphology within a river confluence. Therefore, the river confluence between the Juehe River and Haohe River located in arid and semi-arid areas of northwestern China was selected to evaluate the hyporheic water exchange processes, and the temperatures of different depth sediment were measured on July 11th and 12th, 2016 by an instrument equipped with five temperature sensors. The patterns and magnitudes of hyporheic water exchange were estimated using a one-dimensional heat transport model coupled with temperature data. Meanwhile, terrain points elevations and sediment particle distributions in this river confluence were collected to determine the confluence riverbed morphology. Furthermore, the river confluence morphology effects on hyporheic water exchange were investigated in this study. The different riverbed morphology and hydrodynamic zone locations were observed, which were controlled by the planform geometry with low river confluence flux momentum during the test period. Meanwhile, the upwelling flow dominated in the test area, and the downwelling occurred at three of the 23 total test points, which was induced by the complicated riverbed morphology and hydraulic characteristics. Additionally, the magnitudes of the upwelling flow were significantly influenced by the sediment heterogeneity subjected to the erosional and depositional processes. Hyporheic water exchange is a vital hydrological process in arid and semi-arid areas, where it significantly impacts the ecological environment of rivers. This study provides a valuable guideline for water quality and quantity management in arid and semi-arid areas.