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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. [ABSTRACT FROM AUTHOR]

Flow division at bifurcations in the Yangtze Estuary has received ample attention, since it may control the pathways of terrestrial sediments over downstream river branches including the 12.5 m Deepwater Navigation channel. While some efforts have been made to interpret flow division at the bifurcations of the Yangtze Estuary, little attention has been paid to the role of tides. Flow division at estuarine bifurcations is made complicated by tides that propagate from the outlet of the tidal channels into the delta. To quantify the tidal influence on the distribution of river discharge, and more generally, to understand the mechanisms governing the subtidal flow division at the tidally affected bifurcation in the Yangtze River Delta, a two-dimensional hydrodynamic model is employed. In this model, the landward boundary is chosen beyond the tidal limit, where the tidal motion has faded out entirely. The seaward boundary is chosen such that the river discharge does not influence the water level. Subtidal discharges are decomposed using the method of factor separation, to distinguish between the effects of tides, river discharge and river-tide interactions on the subtidal flow division. Results indicate that tides modify the river discharge distribution over distributary channels in the Yangtze River Delta, particularly in the dry season. A significant difference in the subtidal flow division during spring tide and neap tide shows that the tidally averaged flow division over the distributaries in the delta greatly depends on tidal amplitude. By varying the river discharge at the landward boundary and amplitudes and phases of the principal tidal constituents at the seaward boundary of the established model, the sensitivities of the subtidal flow division to the river discharge and tidal amplitude variation were investigated in detail. Generally, the tidal impacts on the subtidal flow division are around 12% to 22%, with river discharge varying from 30,000 m 3 s -1 to 20,000 m 3 s -1 . This effect on the flow distribution can even overwhelm the effects induced by river discharge based on geometry only, when the flow discharge is lowest. Furthermore, the fortnightly tidal cycle plays an important role in enhancing the inequality of the subtidal flow division caused by the M 2 tidal component solely at the tidal bifurcation in the Yangtze River Delta during low flow. [ABSTRACT FROM AUTHOR]