Intra-tidal division of flow and suspended sediment at the first order junction of the Pearl River Network

Abstract

At tidal junctions, flow, sediment, and, indirectly, flooding risk are distributed over the downstream river branches. To characterize the hydrodynamic pattern related to the transport and distribution of suspended sediment, an intra-tidal field measurement was conducted at the first order tidal junction in the Pearl River Network, Makou-Sanshui (MK-SS), which was the first order junction and the apex of the delta. Two 13h boat-mounted ADCP surveys were implemented and 468 water samples were taken synchronously across three cross sections at wet and dry seasons during the spring tide. Observed spatiotemporal velocities were applied to elucidate the intra-tidal flow pattern in the tidal junction, and the acoustical backscatter intensity collected by ADCP was the inverse of the cross-sectional suspended sediment concentration (SSC). Additionally, vertical profiles of velocity and SSC allowed us to fit the lateral distribution of shear velocity and Rouse number, respectively. From these, we obtained the spatiotemporal variations of settling velocity, which then made it possible to interpret the time lag between flow and suspended sediment. Intra-tidal substance exchange, including flow flux produced by tidal wave and net suspended sediment, was inferred by Stokes transport, integrating axial velocities and the SSC. As a consequence of tidal impact, the flow processes of MK and SS were out of phase by 1–1.5 h. As the connecting channel of MK and SS, Ganggen (GG) experienced more frequent substance exchange, displaying a significant difference in flow pattern. A substantial clockwise secondary flow field was detected during the intra-tidal survey at GG, which was affected by the curved channel. This phenomenon was related to the cross-sectional distribution of SSC. Furthermore, the intra-tidal transport of suspended sediment exhibited an inverse direction during the wet and dry seasons. Sediment during the dry season favoured delivery from MK to SS, while it displayed an opposite trend during the wet season. The results reveal the pattern of intra-tidal partitioning of flow and sediment at tidal junctions.