Impacts Of Deep Waterway Project On Morphological Change Within The North Passage Of Changjiang Estuary, China

Changjiang River is the largest river discharging into the Changjiang Estuary and East China Sea, with abundant of freshwater and sediment discharge, and it is medium strength tidal estuary. Due to about half of fine-grained suspended sediment discharge is deposited in the Changjiang Estuary and its submerged delta, bathymetry around the river mouth gradually grows shallower, which potentially limits shipping into and out of Changjiang River. Through many investigations and research of nearly 40 years, the Deep Waterway Project off the Changjiang River was launched in 1998 to improve navigational conditions into and out of the estuary in South Channel and North Passage. Deep Waterway Project has had a marked impact on flow field, salinity field, suspended sediment distribution and the bed's erosion-deposition. So it is needed by scientists and engineers to study the sedimentation process and driving mechanisms impacted by Deep Waterway Project.In this paper, according to the morphological data of longtime scales from January 1998 to February 2008, the characteristics and change rules of bed's erosion-deposition distribution were in-depth and systematic analyzed in North Passage, then the impacts of difference stage phase of Deep Waterway Project on bed's erosion-deposition distribution in North Passage. The results showed that the project has produced significantly spatial characteristics and change rules of bed's erosion-deposition distribution in North Passage along with constructions proceeding. The main conclusions are as follows.The total volume of sedimentation is as high as 3.37×108 m3 from January 1998 to February 2008 in North Passage. The north region of shipping channel in entrance region and the groin-blocked regions experienced deposition, in where the maximum thickness of deposition is 3.9 m in north upper area of shipping channel of the groin-blocked regions. The main-channel mainly experienced erosion, in where the middle region and the south lower region, where nearby area for mud named 10#, were deposition.In the period of PhaseⅠand perfected PhaseⅠ, from January 1998 to February 2002, the upper region of main channel was eroded significantly, the maximum thickness of erosion is about 1.25 m, which effectively improved the condition of shipping channel at sand bar to 8.5 m. The upper area of the groin-blocked regions and the middle area of North Passage experienced deposition, once significantly deposition area appeared northwest-southeast direction in the middle region with thickness of deposition above 2.0 m. The northern of entrance region experienced deposition with averaged thickness of deposition above 0.70 m.In the period of phaseⅡ, from February 2002 to February 2005, the northern of the lower of main channel experienced significant erosion with averaged thickness of erosion about 0.2～1.3 m. The entrance region, the groin-blocked regions and the upper and southern of middle region of main channel experienced deposition, the thickness of deposition is significant bigger in northern of entrance region, the north groin-blocked regions of the upper and the southern of middle-lower area of groin-blocked regions with biggest thickness about 3.0 m.In the period of after PhaseⅡ, the lower of main channel experienced significant erosion with averaged thickness of erosion around 0.70 m. the thickness of deposition decreased in the groin-blocked regions and the entrance region. The southern of middle and lower region of main channel deposited back silting serious, in which the biggest thickness of deposition was 1.26 m in area of between S5 and S6.For the annual change, the results showed the following conclusions. The entrance region and the groins-blocked regions almost were deposition in the period of 10 years, and once peak of deposition appeared in 2004 impacted by the constructions of PhaseⅡwith averaged thickness of deposition about 1.22 m in entrance region and about 1.10 m in the upper and middle of the groins-blocked regions. The annual changes were complex in main channel with significant difference among different regions. The upper and lower regions of main channel were net erosion, and twice peaks of erosion respectively appeared in 2000 and 2004 impacted by the constructions of perfected PhaseⅠand PhaseⅡwith the thickness of erosion in the range of 1.40～1.25 m. The middle of main channel was net deposition, and twice peaks of deposition respectively appeared in 2002 and 2006 impacted by the constructions of PhaseⅡand the mud area named 10#. The thickness of erosion and deposition decreased year by year after the peak of erosion or deposition in the range of 25 to 70 percent. In addition, the bed's erosion-deposition has significant changes with the dry and flood seasons. More fine suspended sediment setting during flood season than dry season in entrance region and the groins-blocked regions, and more fine suspended sediment was deposited than re-suspended during flood season and more sediment was re-suspended than deposited during dry season in main channel. In conclusions, the distribution of bed's erosion-deposition was impacted mainly by the constructions of Deep Waterway Project.In order to study the dynamical mechanism of the impact of Deep Waterway Project on bed's erosion and deposition in North Passage, A 3D nested sediment model with Delft3D international advanced was used to model sediment transport in North Passage of Changjiang Estuary. After calibration and verification with a large amount of practically measured data, the model was used to simulate the flow field and suspended sediment field, five numerical experiments designed respectively with the same external driving force and different depth and constructions at different Phase. Then the simulation results were analyzed for the difference of flow field and suspended sediment concentration between different Phases.The results showed conclusions as follows. Under the influences of constructions of Phase I, hydrodynamics intensify decreased in entrance region and the maximum speed decreased of depth-integrated current velocity averaged from spring to neap tide cycle was 0.32 m/s; hydrodynamics intensify significantly increased in the upper of main channel and the maximum speed increased of the velocity was 0.44 m/s; hydrodynamics intensify significantly decreased in groins-blocked regions, the regions of velocity decreased northwest-southeast direction appeared in middle of main channel, and the variation of velocity was weak. Under the influences of constructions of perfected PhaseⅠ, hydrodynamics intensify increased in entrance region, most significantly increased in middle region of main channel in range of 0.12 to 0.45 m/s, changed non-significant in the upper and lower of main channel and continue to decrease in the groins-blocked regions. Under the influence of constructions of PhaseⅡ, hydrodynamics intensify significantly increased in the lower of main channel, in where the biggest amplification of depth-integrated current velocity averaged from spring to neap tide was about 0.68 m/s, decreased in groins-blocked regions, in where the biggest amplitude was 0.86 m/s, and changed non-significant in entrance region, the upper region and the middle of main channel. During PhaseⅢ, hydrodynamics intensity decreased in entrance and the upper regions and changed non-significant. All the change of hydrodynamics intensity was similar to the change of bed's erosion and deposition under the influence of different phase stages of DWP. Thus it can be seen that the changes of hydrodynamics intensity increased and decreased under the influence of different phase stages of DWP were the main dynamic mechanism of the changes of bed's erosion and deposition in corresponding regions.In addition, the simulation results of suspended sediment concentration confirmed that the effect was remarkable using north and south dikes blocking suspended sediment into North Passage. The simulation results of forecast bed's erosion and deposition showed that the groins-blocked regions were quickly deposition and to achieve the new erosion-deposition balance. The lower of main channel was erosion. But the entrance region and upper region of main channel kept on deposition, it may be an important factor of preventing the long-term development of shipping channel in North Passage...