Analysis On Morphological Change And Evolution Of River Regime In The North Passage And North Branch Of The Changjiang River Estuary

In recently years, the geography and landform of Changjiang River estuary haschanged greatly, which exerts great influence on the hydrodynamic characteristics ofChangjiang River estuary, Moreover, the major engineering constructions ofChangjiang River estuary also have great influence on the hydrodynamiccharacteristics of Changjiang River estuary, such as the changes of hydrodynamiccharacteristics due to the construct of Deepwater Navigation can’t be neglected.Recently, the programming of navigation in North Branch is put on the agenda, andthe engineering measures would have certain impact on the hydrodynamiccharacteristic, Therefore, this study analyzes the morphological change and evolutionof river regime in the North Branch and North Passage from the hydrodynamiccharacteristics of Changjiang River estuary, and has great implications to the safety ofharbor and navigation in Shanghai, and even the region of the Changjiang RiverDelta.Firstly, a3D numerical model is set up to simulate the tidal currents in theChangjiang River estuary by Delft3D model, This model has been validated with themeasured data, the computed results of water level, flow velocity magnitude anddirection agree well with the measured data. The morphological change and evolutionof river regime in the North Branch and North Passage is analyzed by this model.For the analysis on the morphological change and evolution of river regime in theNorth Branch, this paper based on the data of water depth and channel cubage in theNorth Branch, the flow spilt ratio of the North Branch and the South Branch, Thereasons for the sediment deposition in the North Branch are comprehensivelyanalyzed, Based on the sediment grain size of riverbed in the North Branch, Douguoren’s formula is applied to calculate the sediment incipient velocity in the NorthBranch, which is compared with the computed bottom velocity fields to analyze theriverbed erosion and sediment deposition in the North Branch. Moreover, based on thehydrology and sediment data, an artificial neural network model is established to study the relationship of channel cubage under0m isobath in North Branch with theflow and sediment discharge at Datong gauging station, and the flow split ratio of theNorth Branch. The network model is trained and tested well; and the computed resultsof BP artificial neural network agree well with that of multiple linear regressions, thecomparison with the observations shows the simulation precise of the model is fairlyhigh，the computed results suggest that raising the flow split ratio and strengtheningthe runoff on the basis of decreasing the tidal prism of North Branch (such asnarrowed the mouth width of North Branch) are the fundamental approaches forcontrolling and preventing the channel from further siltation.For the analysis on the morphological change and evolution of river regime in theNorth Passage，based on the data of water depth in the South Branch and the HengshaPassage, and the flow spilt ratios of the North Channel and the South Channel, theNorth Passage and the South Passage，and the computed results such as the velocityalong the South Passage and North Passage, the flood and ebb flow in the HengshaPassage, and the flow spilt ratio of South Passage and North Passage, the reasons forthe sediment deposition in the Deepwater Navigation Channel of the ChangjiangRiver Estuary are explored, The development of the Nanshatou Passage leads to theincrease of the ebb flow discharge and the southern bank of the South Channelscoured, the scoured sediment is carried by the ebb flow to the northern bank of theSouth Channel, and is silted in the entrance reach of the North Passage, whichdecreases the ebb flow volume entering into the Deepwater Navigation Channel. Thedevelopment of Hengsha Passage causes the increase of sediment and water exchangebetween the North Channel and the North Passage, which weakens the sediment andwater exchange between the North Channel and the North Passage, and enhances thesediment silting in the upper reach of the Deepwater Navigation Channel.The morphological change and evolution of river regime in the North Branch andNorth Passage of the Changjiang River Estuary are analyzed by combining fieldmeasurement data and numerical simulation results, which is significant to providetechnical support for meeting the navigation requirement of3000tons ship in NorthBranch and taking steps for slowing down the sediment deposition in North Passage.