Spatial variability of phosphorus adsorption in surface sediment at channel confluences: Field and laboratory experimental evidence

Field surveys and a laboratory experiment have been conducted to characterize the spatial variability of phosphorus (P) adsorption in surface sediment at channel confluences with a large tributary discharge. The results show that the grain size of surface sediment and the concentration of soluble reactive phosphorus in overlying water are the most important factors affecting the spatial variability of P in surface sediment. Their differences in the two combining flows can be reduced dramatically by the mixing layer, and the mixing distance can be largely decreased by the distortion of the shear layer and the complex bed morphology as they can increase the transverse mixing of the two combining flows. The influence of hydrodynamics is highlighted especially when other factors were comparable in the two tributaries. The downwelling flow shall improve the hyporheic exchange and induce more P to be absorbed onto the bed sediment at channel confluences. The horizontal flow velocity shall be a more appropriate indicator of the influence of hydrodynamics on P enrichment. Thus, the region of flow separation is beneficial to P adsorption, while the region of shear layer and maximum flow velocity are adverse to P enrichment. All these results help to better understand and predict the spatial variability of P adsorption in surface sediment at channel confluences.