| The Huangpu River(HPR),running through the center of Shanghai,influences flood control safety,economic and social development of the city.In recent years,the downstream channel of the HPR has been narrowed.As a result,the water level has risen,and variations in the dynamic geomorphology has been seen.The risk of waterlogging has increased,especially in extreme weather conditions such as typhoon-induced storm,heavy rainfall,astronomical tides,and basin flooding.Besides,there has been a rise in the number of navigational accidents along the downstream section of the HPR,the risks of navigation are growing.Given that the dynamic geomorphology of the downstream HPR is influenced by the upstream inflow and its special hydraulic structure at the confluence with the South Channel(SC)of the Yangtze River Estuary(YE),its variation is directly related to human activities in that area.Previous studies have indicated significant impacts of the Taihu Basin project on the hydrodynamic force of the upstream HPR,and the enhanced tidal force of SC is due to projects in the Yangtze River basin and the estuary.However,there are few studies on the characteristics and evolution trends of the hydrodynamic and geomorphic changes at the confluence of the SC and the downstream of the HPR,which has important theoretical and practical significance to the flood control planning and shipping traffic of the HPR.Further research is urgently needed.To address these issues,the confluence section of the HPR and the SC is divided into three sections in this paper:the confluence section of the HPR,the upper confluence section of the SC and the lower confluence section of the SC.Data like the annual maximum tidal levels of Mishidu Station,Huangpu Park Station and Wusong Station from 1979 to 2018 and the runoff of Songpu Bridge section of HPR and Xuliujing station in the downstream of the Yangtze River from 2000 to 2021 were collected.Based on the comparative analysis of the historical chart data of the confluence section,the dynamic deposition,field measurements were conducted for water level,flow velocity and direction,salinity,suspended sediment concentration and particle size,surface sediment particle size,and underwater topography and geomorphology data.Emphasis was put on the analysis of the dynamic sedimentation,channel morphology and the riverbed micro-geomorphologic characteristics of the confluence section of HPR and the SC,exploring its process and influencing factors,subsequently analyzing the response of tidal forces in the underwater topography in confluence section of HPR.Main innovations are as follows:(1)In the past 40 years,the annual maximum tidal levels of Mishidu Station,Huangpu Park Station and Wusong Station in HPR increased by 0.46 m,0.09 m and0.01 m respectively.In the confluence section,the duration of ebb tide is longer than that of flood tide,and the asymmetry of tides intensifies during neap tide.The average tidal flow velocity increases downstream,with the average flow velocity during spring tide being higher than that during ebb tide,while the opposite is observed during neap tide.Both spring and neap tides exhibit ebb tide dominance,resulting in a net seaward sediment discharge.The average suspended sediment concentration increases downstream,with higher concentrations during flood tide compared to ebb tide,and the difference in average suspended sediment concentration between flood and ebb tides increases downstream.During spring tide,due to significant differences in hydraulic strength,bottom sediments are more prone to resuspension,resulting in a large amount of sediment being transported during flood tide,leading to increased suspended sediment concentration in the water.This dominance of sediment transport during spring tide results in a net landward sediment transport.The sediment deposition rate at the mouth of the HPR is much higher than that in the upstream section,making it prone to sedimentation influenced by tidal dynamics.(2)In the confluence section of the HPR,the bed sediments mainly consist of sandy silt and clayey silt,and the concave bank of the riverbed exhibits features such as scour holes,steep slopes,and sand waves as micro landforms.From 2006 to 2014,the confluence section of the HPR experienced an overall accumulation of 1.03×10~7 m~3of sediment.The construction of revetment projects caused the boundary of the tidal flats to shift towards the center of the river,resulting in a reduction in the volume of the river channel.At the mouth of the HPR,a small sediment barrier has formed at the mouth of the Wusong River,and its sediment boundary has migrated 1 km towards the main channel of the SC.The longitudinal cross-section of the river channel has a steeper slope towards the sea compared to the landward slope,exhibiting a positive proportional decline characteristic.From 2010 to 2018,in the upper confluence section of the SC,the construction of the Xinliuhe embankment and submerged dike projects has promoted the development of the Xinliuhe shoal and the central shoal.This also led to significant erosion of the Ruifengsha shoal at the tail end due to water scouring,causing it to nearly disappear,and the main channel of the SC to widen.In the lower confluence section of the SC,there is a net erosion of 0.56×10~8 m~3,indicating noticeable erosion.The ebb flow on the north side is suppressed,and the morphological changes of the riverbed are not significant,maintaining a stable bed form.On the south side,it is influenced by the deep-water channel project,and the main channel on the south side is scoured to a depth of approximately 2-4 m.(3)In the upper confluence section of the SC,the Ruifengsha has shortened and narrowed,while the main channel has widened.The reverse proportional decline of the river channel shape has reduced its water discharge capacity and strengthened the uplifting effect of tributaries.In the lower confluence section of the SC,the deep-water channel project has induced scouring of the river channel,facilitating the upstream movement of tidal currents to the HPR.The intersection angle between the HPR and the SC remains largely unchanged,but the confluence ratio in this section increased from 2000 to 2017,enhancing the uplifting effect of tributaries on the downstream water of the HPR.This has led to the formation and downstream migration of small sediment barriers and further intensified the backwater effect in the downstream of the HPR.The increased runoff of the HPR has intensified erosion of the concave banks and deposition on the convex banks.The concave banks have formed scour holes,increasing turbulence in the water.Moreover,the significant influence of tidal currents on the HPR,coupled with the complex sediment dynamics in this section,has resulted in multiple collisions between ships and waterfront platforms,further reducing the volume of the river channel and exacerbating the rise of water levels during extreme weather events.In summary,tidal currents play a dominant role in shaping the riverbed in the downstream of the HPR,while changes in landforms feedback on the dynamic evolution of this river section,leading to increased flood risks and navigation safety hazards in the central area of Shanghai in the future. |
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