Numerical Simulation Of Saltwater Intrusion In The Qiantang Estuary

As a typical macro-tide estuary,the Qiantang Estuary is characterized in its strong tide,rapid flow and drastic change of river bed.The estuary is the city's important drinking water source which provides 80%fresh water supply to Hangzhou city.However,the saltwater intrusion detorates water sources seriously for many years.It is a necessity to study the saltwater intrasion in the estuary of Qiantang River.Supported by Important National Science and Technology Specific Project?2009ZX07424-001?,this paper investigates the saltwater intrusion and its influencing factors in the Qiantang Estuary.Based on synchronous real-time observation of tide and salinity in multiple stations,this paper analyzes the spatial and temporal variation of saltwater intrusion in the Qiantang Estuary.Temporally,the salinity shows a significant tidal cycle,fortnightly tidal,seasonal and inter-annual variations.In the short time scale of the tidal cycle and the fortnightly tide,the change of salinity mainly depends on the tidal effect,while in the long time scale,it depends on the runoff.Spatially,the salinity decreases along the longitudinal direction,and the maximum gradient occurs near CangQian station;the vertical average salinity in main channel is lower than that in beach generally,but the difference is small;Vertically,the salinity stratification is not obvious,and most of the stratification coefficient is smaller than 0.3,which indicates that the Qiantang Estuary is a typical well-mixed one.An analytical salt intrusion model based on Savenije's steady one-dimensional equation was established for Qiantang Estuary at both high water slack and low water slack,which can predict the salinity distribution along the river and saltwater intrusion length.It was shown that results predicted by the model was in good agreement with observed data.A three-layer salinity prediction model was established in this paper,based on wavelet neural network?WNN?optimized by improved particle swarm optimization?IPSO?.The results showed that the observation data were in excellent agreement with the prediction of IPSO-WNN model at Qibao station.A 3D numerical model for saltwater intrusion was built based on the unstructured model,FVCOM.The model was validated against tidal level,velocity and salinity,and the results demonstrated that the model can effectively reflect the hydrodynamic characteristics and saltwater intrusion processes in Qiantang Estuary.With the simulation results,temporal and spatial variation of Qiantang River estuarine salinity are analyzed,in the dry season strong tide period.On the plane distribution,the salt water moves periodically along with the tide in the channel.Saltwater intrusion is more serious during spring tide,the 0.45 isohaline can reach the position between Zhakou and Wenyan stations,whereas it only reached the position between Cangqian and Qibao stations during neap tide.Vertically,the salinity is well-mixed.The segregation phenomenon is obvious in the high water slack during both spring tide and neap tide,and there is no significant difference in the other time.Based on the saltwater intrusion model,a series of sensitivity tests were performed to analyze the individual impact of runoff,tide,wind velocity,wind directions,and sea level rise on the salinity distribution and saltwater intrusion length.Runoff and tidal range are two main factors that impact on saltwater intrusion.The salinity and extent of saltwater intrusion decrease with the increasing of runoff in upstream,while the salinity and saltwater intrusion length increase when the tidal range enlarged.The saltwater intrusion length shows negative exponential with the runoff,while positive exponential with tidal range.By nonlinear multivariable regression analysis,a logarithmic function was built between the dimensionless parameters and saltwater intrusion length.Wind has some influence on saltwater intrusion in Qiantang Estuary.Under the action of wind stress in the same direction with flood tide,the whole salinity decreases a little during spring tide,while increases during neap tide.Since the practical cases in estuary are complicated,there are some differences in different regions.When the sea level rises,the saltwater intrusion becomes stronger.A 0.3,0.6 and 1.0m sea level rise results in an increase of 1.1,2.2 and 6.0km in saltwater intrusion length during spring tide,while 2.8,5.9 and 9.8km during neap tide.It can also be concluded according to the simulation result that intrusion distance has a negative exponential relationship to runoff,but has an exponential relationship to tidal range.In this study,a proportional relationship between relative intrusion distance expressed by exponential function?exp×?L/L₀??and relative tidal range?H/h?and Froude number of runoff?qr/????has been established by nonlinear regression analysis.With the maximui tidal range of 8.21 m at Ganpu station,the maximum excessive salinity time at Nanxingqiao and Shanhusha water intakes is 10.4?11.5d when the runoff is 50 m3/s,while decreases to 0.2?1.4d when the runoff is 300 m3/s.For the maximum 15-day tidal range of 8.52 and 6.79 m at Ganpu,the minimum runoff required for saline tide resistance are 410 and 250 m3/s respectively for these two water intakes.Under the east wind,the increase of average salinity at these two water intakes are 0.18 and 0.14,and the increase of maximum excessive salinity time are 2.Id andl.9h,respectively.