| The Modaomen estuary is located in the mouth of the Xijiang River,and the transport of water and sand is the first of the eight gates of the Pearl River.Hydrodynamic,sediment dynamics and sedimentation have formed the unique topography and geomorphology of the Modaomen estuary,as well as the complex salt tide movement.The frequent movement of salt tide seriously affects human production and life,and also has a certain impact on environmental ecology.It is of great significance to fully understand the influence factors of salt tide movement to take appropriate measures to ensure fresh water supply and the restoration of ecological environment.So far,some domestic studies have paid more attention to the dominant factors of Modaomen estuary and neglected the influence of topography,or have not discussed the influence of topography in detail when analyzing the law of salt water intrusion in the Modaomen estuary.There is little analysis of the salt water intrusion characteristics in different topographic features of Modaomen waterway combined with numerical model and observation data.But this kind of research is indispensable in the understanding and mastery of the law of salt water intrusion in the Modaomen estuary.Based on the analysis of observation and numerical simulation,the influence of topography on the salt water intrusion is further explained in order to reveal the influence of the topography on salt water intrusion,the change of the dominant relationship when salt water intrudes in different reaches and the characteristics of salt water borderline response in different discharge when the different factors act together.So as to enrich the understanding of the movement law of salt tide in the Modaomen waterway and provide reference for the engineering for suppressing salt water intrusion.First of all,the fortnight ship-mounted observations were carried out for three times in the Modaomen waterway during the dry season of 2017-2018,and the hydrological and meteorological data of the waterway during this period were collected.Secondly,analyzing the features of longitudinal and plane topography of different river reaches of the waterway.Thirdly,according to the topographic features of different river reaches,the correlations of salt borderline and influencing factors was analyzed.Fourthly,using the hydrodynamic model software Delft3 D establish the 3D hydrodynamic model of the Modaomen estuary and verify with the observation data.Fifthly,the distribution of salinity and velocity,the sensitivity of salt borderline to discharge and the change of salt flux at different reaches of different topographic characteristics are analyzed.Finally,the variations of density flow intensity and salinity mixing and stratification in different river reaches are analyzed according to the calculation results.The main conclusions are as follows:(1)The topography has a significant effect on the correlations of salt borderline and runoff,tidal range,average sea level and north wind speed.The water depth of the downstream reach of Zhupaisha is relatively smaller than the upstream reach.With Zhupaish as the dividing line,the downstream reach which is the bifurcating reaches has some middle bars which divide the channel into two deep channels,east thalweg and west thalweg,and the water depth of the upstream reach of Zhupaisha is large and the river width is narrow,which is a single narrow deep channel.Due to the modulation of the bifurcating reaches,the correlation coefficients in the downstream with middle bars in the channel are smaller than that in the whole observed section.The landward limit of saltwater in the upstream with a narrow and deep single channel has higher correlation with runoff discharge and tidal range,meanwhile the salt water is easy to accelerate the intrusion.It is also found that although the runoff is usually the most relevant dominant factor,when combined action of the other factors such as tide,wind and topography become prominent,its influence on the saltwater movement may be weakened and lose the dominating role.(2)The variation tendencies of salinity and velocity distribution in different reaches are different.When salt borderline moves in the bifurcating reaches,the intrusion distances of salt water in east thalweg and west thalweg are different.The retreating velocity in the reaches which from east channel near Zhuhaidaqiao to east channel near Denglongshan to west channel near Lianshiwan to west channel near Zhupaisha is large.And the intrusion velocity in the reaches which from west channel near Zhuhaidaqiao to west channel near Denglongshan to east channel near Lianshiwan to east channel near Zhupaisha is large.When salt water intrudes in the upstream deep single channel,the salt borderline is more sensitive to discharge changes.The upstream slope and the downstream slope of the deep channel will suppress intrusion and retreating of bottom salt water,respectively.(3)There are differences between the upstream reach which is a single narrow deep channel and the downstream reach which is the bifurcating reach in terms of variation of density flow intensity,salinity mixing and stratification.The response of salt borderline to discharge and tidal range is different when it is located in different reaches.The density flow intensity of upstream slope in Shenwan Port is higher than that of downstream slope due to the blocking effect of upstream slope in upstream reach.After the salt water intrudes to the single narrow deep channel,the increase of the stratification degree due to the upstream slope and the increase of salt transport to the land during the low tide period prompted the salt water to continue intrude.The bed undulation of the upstream reach in the Modaomen waterway has little effect on the salt water of the surface layer with small salinity and has much effect on the salt water of the bottom layer with large salinity,and the density flow intensity of the surface water will be reduced by the whole slope topography. |
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