Response Of The Geomorphodynamics To The Artificial Island Construction In Complicated Bathymetry

The tidal flats and radial sand ridges in the middle of Jiangsu coast are valuable land resources.The deep-water tidal channels between sand ridges are important natural port and channel resources.Dongsha,Gaoni and Tiaozini locate in the core,which is the sensitive area of the geomorphodynamical system.In order to utilize the land resources,Jiangsu Province planned to conduct Tiaozini reclamation merging into mainland and construct offshore artificial islands in Gaoni and Dongsha,increasing the land area of 1 million mu.The bathymetry in the study area is complicated and the hydrodynamic is active,so the artificial island construction will be a complicated systematic project,for which preliminary investigations and research work are necessary.In this paper,the Delft3D hydrodynamic and sediment dynamic models were applied to simulate the tidal current and suspended sediment concentration(SSC)field before and after the construction of the artificial islands,for the further calculation of residual and net sediment transport,based on which the geomorphological change of the whole radial sand ridges area over the time scale of 10 years was simulated.In order to analyze the port resources in the radial sand ridges area and the coast stability condition,typical channels and sand ridges were selected.The results showed that:(1)The tidal flow field in the study area was a significant convergence and divergence pattern.In most areas,the maximum current velocity exceeded 1 m/s,increasing by offshore to the core.The current velocity of flood was slightly larger than the ebb in the coastal area,outer sea opposite.After the construction of artificial islands,the tidal flow field maintained convergence and divergence pattern,but there was a larger change in the velocity.Divides by the line from Jianggang to Maozhusha,the north hydrodynamic decreased,the maximum velocity reduced 0.4 m/s or so,while the south hydrodynamic increased,the maximum velocity increasing 0.3 m/s or so.(2)The SSC field was related to the flow field.Regions of high SSC value were Xiyang,the sea area to the east of Dongsha,Chenjiawu,Kushuiyang,Xiaoyanggang and the sea area around Taiyangsha,mainly in the 400-500 mg/L,maximum up to 1 000 mg/L or more.After the construction of artificial islands,the SSC of the northern area reduced on average 200 mg/L,while the SSC of the southern area increased 200 mg/L.(3)The Euler residual current dominated in the tide-induced residual current,mainly controlled by the bathymetry.The maximum residual current velocity in the channel could reach more than 0.2 m/s,while in the nearshore and offshore area,the velocity was generally less than 0.05 m/s.The residual current in Xiyang,Lanshayang,Tiaoyugang and Huangshayang pointed to Jianggang,the core of the area,and in these channels the water transport was to the radial sand ridges area.In Pingtuyang,Xiaobeicao,Chenjiawu and Caomishu the water transport was offshore.Overall,the sea water flew in from the north and the south,and flew out by the middle.(4)The suspended sediment transport pattern was quite similar with the pattern of the residual current.Sediment from the north and south area was transported into the core area,the transport flux of 0.1?0.2kg/s/m,the largest in Xiyang reaching 0.5kg/s/m or more.In the coastal shallow water sediment transport was divided into two parts,to coastal tidal flats with transport flux of 0.05?0.1 kg/s/m.and through Chenjiawu.Kushuiyang and other channels to sea with transport flux of 0.1?0.4kg/s/m.After the construction of artificial island,the sediment transport from the abandoned Yellow River delta to the south still existed and reduced along,but the transport flux rapidly increased in Xinyanggang area,from less than 0.1 kg/s/m to 0.3 kg/s/m.In the top of Xiyang,the suspended sediment transport flux reduced from 0.6kg/s/m to 0.1 kg/s/m or less.The suspended sediment transport flux in Xiaoyanggang channel had great change,with high transport flux to shore,in favor of coastal reclamation project.(5)Construction of artificial islands changed internal sediment redistribution.The sand ridge growth pattern changed from the area addition to the elevation addition,and in the growth process,the sand ridges and channels swinged laterally.The area with elevation higher than 0 m was 1 483 km2 initially,and it was 3 058 km235 years after the construction of the artificial islands,which was 195 km2 more than under natural conditions.Maozhusha and Waimaozhusha migrated southward at a rate of 65 m/a.35 years after the construction,Xiyang deepened 5?6 m,17 cm per year.Chenjiawu deepened 6 cm per year averagely,with its major axis migrating southward at a rate of 85 m/a.Kushuiyang deepened 22 cm per year averagely,with its major axis migrating southward at a rate of 25 m/a.Between 25 to 35 years,the average erosion or deposit rate was less than 0.1 m/a.and the channel-sand ridge system swung back and forth in a very small range,gradually reaching a steady state.The evolutionary trend that the sand ridge grew high and widened,the channel deepened and narrowed,made slope steepened,and the port construction condition was further optimized.(6)After the construction of artificial islands,the original port channel resources were maintained.The channel advantage of Kushuiyang which was to be developed got further increased.And potential port and channel resources appeared in Chenjiawu and Huangshayang.