A Study On Hydrodynamics And Sediment Transport Dynamics Between Flats And Channels In Estuary

Hydrodynamics and sediment transport dynamics in estuaries are key processes in land-ocean interaction. Study of hydrodynamics and sediment transport processes in an estuary like the Yangtze has broad implication for understanding of the turbidity maximum mechanisms. For the North Passage (NP) in the Yangtze Estuary, two crucial questions are where does the sediment deposited in the NP come from and how does the sediment deposit therein? To partly address these questions, this study is devoted to studying the flow and sediment transport characteristics in the lower NP and adjacent tidal flats based on extensive hydrological data collected in the dry and flood seasons in 2014. The spatial and temporal variations of current velicity, suspended sediment concentration (SSC), bottom sediment composition are analyzed. We further elaborate the exchanges between near bottom suspended sediment and bed sediment, and the sediment exchange between the South Passage (SP), NP and the flats.The main conclusions are as following:(1) Spatial and temporal variations of current velocities, SSC and bed sediment composition show different patterns between channels and flats.Current velocity is larger in the NP and its mouth than that over the tidal flats.Tidal currents exhibit a reciprocating flow pattern in the NP and over the Jiuduan Shoal but a rotating flow pattern off the NP and over the Hengsha Shoal. Current velocity is overall larger in flood season than dry season, and larger at spring tide than neap tide.The SSC is larger at both spring and neap tides of flood season and at spring tide of dry season, but smaller at neap tide of dry season in the NP than the Jiuduan shoal and Hengsha Shoal.In the NP, SSC is larger in flood season than dry season, whereas over the tidal flat, SSC exhibits variations featured by spring tide of flood season> spring tide of dry season> neap tide of dry season> neap tide of flood season.The bottom sediment is coarser over tidal flats than in the NP. Inside the NP, bottom sediment is coarser in the north side than the south side. Over tidal flats, bottom sediment becomes finer towards the sea with increasing depth. Bottom sediment is also found coarer in the NP and finer over tidal flats in the wet season compared to the dry season.(2) Changes of bottom sediment response to typhoon over the flatBased on historical data, we find that the median diameter (D50) of bottom sediment increased in the post-typhoon period compared to pre-typhoon measurements in July 2000 upon the Jiuduan and Hengsha flats (JDF and HSF). For instance, the D50 of surface sediment in the Jiuduan Shoal increased from pre-typhoon 50 μm to post-typhoon103 μm. After the construction of the jetties along the NP, however, the D50 of surface sediment over the flats decreased after a typhoon in August 2014. The D50 of surface sediment in the Jiuduan Shoal and Hengsha Shoal decreased from 90 μm to 55 μm and from 80 μm to 69 μm, respectively, given the influence of a typhoon. This variation differing from conventional finding, suggests the strong impacts of the jetties on the wave impact on sediment transport processes over the flats.(3) Silt and clay of marine source play an important role in siltation in the turbidity maximum.We improve the method propsed by Liu et al. (2010) in two aspects:(a) considering sediment source both from the river and the sea and (b) different exchange potential for clay, silt and sand. We find higher exchange ratios in the NP than over tidal flats. Given riverine sediment source, the exchange ratios are bigger (0.4～0.6) in the south side of the NP than the north side (<0.4), and smaller in the region off the NP (<0.3). Given marine sediment source, the exchange ratios are smaller in the south side (<0.4) of the NP than the north side (>0.5), and bigger off the NP (>0.5). These results suggest that the silt and clay of marine source and the silt of riverine source possibly contribute most to siltation in the turbidity maximum.(4) The water and sediment transport patterns in the ’SP-JDF-NP-HSF’ system.In the ’SP-JDF-NP’ system, water and sediment transports are southward in the early flood tide, followed by transport from JDF to NP and SP before ebb tide, and transport from south to north in the early edd tide, transport from SP to JDF in the middle and late ebb tide. In the ’NP-HSF’ system, water and sediment transports are from flat to channel in the early flood tide, then from channel to flat before early ebb tide, and water and sediment exchanges terminated in the middle and late ebb tide.The amount of water exchange is larger in flood season than dry season and larger at spring tide than neap tide. The amount of sediment exchange is largest at spring tide of flood season, followed by spring tide of dry season, neap tide of dry season, and neap tide of flood season.In conclusion, this study examined the hydrodynamics and sediment transport characteristics in the lower NP and adjacent tidal flats, and quantified the vertical exchange between near bottom suspended sediment and bed sediment and horizontal sediment exchange between the NP and the flats. This study has broad implication for understanding of sediment source deposited in the NP and the turbidity maximum formation mechanism.