| Suspended sediment concentration(SSC)is an important indicator that reflects the transport and resuspension of fine particles in the water and links from sedimentary dynamic process to geomorphic evolution.The transport and deposition of suspended sediment affect the channel's natural evolution,safety and defence of artificial building,and the management of circumjacent ecological environment.Due to the collective impacts of river discharge,tides,wind-driven waves,and currents,suspended sediment concentration tends to be highly variable and exhibit random characteristic in the estuarine areas.In this paper,we developed multiple regression models in order to predict the surface SSC which was affected by the changes of water discharge,tides,wind-driven waves,and salinity.And we also made quantitative appraisal on the contribution of each factor to surface SSC in the different parts of the Changjiang Estuary.The models were developed on the basis of an extensive set of multi-factor continuous observation data that collected from three long-term Gauging Stations during a 36-months monitoring period.At the same time,we discussed the change laws on the vertical of SSC and calculated the vertical distribution empirical formula which based on the surface SSC.The complex dynamic.factors and topographic features of Changjiang Estuary and the effects of regional climate make runoff,tidal range,salinity and SSC showed obvious spatial and seasonal variations.Controlled by the moonsoon,changjiang estuary had southeaster in summer and northwester in winter.The runoff was highly seasonal,commonly with its minimum in dry season while the maximum was in flood season,the tendency of tidal range variation was opposite to that of runoff.Temporally,the suspended sediment concentrations,salinity and flow velocity showed a flood-ebb tidal cycle,neap-spring tidal cycle and seasonal fluctuations,and it would be different from region to region.Spatially,suspended.sediment concentrations and salinity increased gradually from the surface to the bottom,while flow velocity showed the opposite tendency.According to the wavelet analysis and the long-term variation and short-term sudden change of SSC,we found that surface SSC variation involved regular and stochastic process.With seasonal and spring-neap variations,surface SSC changes were dependent on regular change factors(e.g.,the cyclicity change of runoff and tides),while the short-term(few hours or days)rapidly increasing of surface SSC depended more on sudden power factors(e.g.,wind,human disturbances,etc.),which played a key role in sediment concentration changes.The statistical results showed that water discharge,square of tidal range,square of wind velocity,and salinity were the best explanatory variables for modeling.We selected the best explanatory variables to build a multiple regression model for each station:the surface SSC of XGS mainly depended on water discharge,tidal range and wind velocity,the effect of salinity was not obvious;the surface SSC of SGS mainly depended on salinity,tidal range and wind velocity,water discharge had weak effect on it;the surface SSC of HGS influenced by runoff,tidal current and flocculation,therefore its influencing factors that included water discharge,salinity,tidal range and wind velocity were more complex.Then based on the random theory we built a statistical-features model between surface SSC and each factor through a multiple regression equation,the models were verified as highly accurate(83.3%).These models would be an effective method for surface SSC analysis and forecasting.And the vertical distribution empirical formula could be applied to the estimate of the vertical concentrations.The weights of influencing factors of SSC were different between each station:for XGS,the weights of water discharge,tidal range and wind velocity were 45.6%?40.0%and 14.4%,water discharge has the greatest impact on surface SSC,followed by tidal range and wind velocity;for HGS,the weights of water discharge,tidal range,salinity and wind velocity were 17g.9?47.2%?13.5%and 21.4%,the effect of tidal range was the most obvious one,followed by wind velocity,water discharge and salinity in succession;for SGS,the weights of tidal range,salinity and wind velocity were 43.4%?26.5%and 30.1%,the biggest influencing factor for surface SSC was also tidal range,and the effect of wind velocity was still higher than salinity.By contrasting with the value of the contribution of the same factor to surface SSC between the upper,middle and lower reach of Changjiang Estuary,we learned the different effect of them under the different physiognomy environment conditions.Regarding the contribution of different factors to average surface SSC,tidal range at Sheshan resulted in the maximum surface SSC change.The contribution of water discharge at XGS was same as that at HGS due to the similar surface suspended sediment carried by runoff,and the surface SSC carried by runoff was negligible at SGS where it was mainly controlled by tides;the contribution of tidal range and wind velocity increased gradually from Xuliujing to Sheshan.However,for the maximum surface SSC,the biggest surface SSC change was contributed by wind velocity at Sheshan.It could be seen that under the fair-weather conditions,tides seemed to be more important than waves in controlling variations in SSC.However,during episodic storms period,the surface SSC contributed by wind velocity could reached more than 1 kg/m3,wind velocity wind-driven waves seemed to be the most important factor leading to the change of SSCThe calculating results of models illustrated that in recent decades surface SSC had declined in the upper reach of Changjiang Estuary because the transfer of suspended sediment from Changjiang river to the sea had decreased as a result of human activities.However,there was no significant downtrend for the surface SSC in the middle and lower reach of Changjiang Estuary. |
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