The Erosion-deposition Law Of The Yangtze Estuary Under Dam-induced Seasonal Runoff Flattening

Estuaries,with aboundant resources,are located at the transitions of rivers and the sea;their erosion and deposition concern the defense against deluge disasters,the development of shipping industries,the living environment of aquatic organisms,etc.,in estuarine regions.Over the past decades,many reservoirs have been constructed on rivers,having intensely altered the fluvial water and sediment discharges and then produced significant impacts on the erosion-deposition of estuaries.Existing studies have mainly focused on the effects of the dam-induced changes of the total yearly fluvial water and sediment fluxes on the erosion-deposition of estuaries;however,the research on the influences of the dam-induced seasonal variability of runoff discharge is insufficient.Meanwhile,since the understanding on the trends of the total quantities of fluvial and marine water and sediment discharges that into the estuaries is immature,the achievements on the erosion-deposition trends of estuaries need to be supplemented.This study considers Yangtze Estuary and divides it into three sections,namely,the reaches downstream of Xuliujing,Jiangyin-Xuliujing and Datong-Jiangyin.Data of this study mainly include the following ones from 1950 to 2014:water discharge and sediment load time series of the main hydrological gauge stations along the Yangtze River,tributaries and lakes;flow velocity data of the cross-sections and ebb partition ratio data of the braided branches downstream of Datong;tidal level data,tidal range data,tidal discharge data of tidal gauge stations and sea area;sediment load data of sea area;morphological data of the Yangtze Estuary.Through a systemic organization of the data,the impacts of dam-induced seasonal variability of runoff discharge on the erosion-deposition of the Yangtze Estuary are analyzed,and the erosion-deposition trend of the Yangtze Estuary is predicted based on the trends of the fluvial and marine water and sediment discharges.First,this study reveals the processes and trends of the fluvial water and sediment discharges of Yangtze River under the influences of their constituents.The analysis on the trends of the total yearly water discharge of Yichang,Hanjiang River,Dongting Lake and Poyang Lake and the yearly precipitation between Yichang and Datong indicates that the total yearly water discharge of Datong has changed little over the past decades under the inter-action of the constituents.The analysis on the contribution proportions of the seasonal variations of the constituents in that of Datong indicates that the dam-induced flattening of the seasonal runoff variability at Yichang(accounts for?50%in Datong for the total yearly water discharge)is the main cause for the runoff flattening at Datong,and the dam-induced runoff flattening at Hangjiang River has also made some contributions;however,the seasonal variabilities of the runoff discharge at Dongting Lake and Poyang Lake and the seasonal precipitation between Yi chang and Datong have shown some negative effects.The maximal positive and negative contribution proportions of Yichang,Hanjiang River and the two lakes are 152.7%,118.3%and-54.2%,respectively.The flattening of seasonal runoff variability at Datong performs as the decreases of the multi-year averaged duration days of the high(>50000 m3/s)and low(<10000 m3/s)discharges,and the increase of that of the higher-low discharge(10000-20000 m3/s).The upper cascade reservoirs help the Three Gorges Reservoir flatten the runoff discharge,and the operation rule of the additional reservoirs in the future is similar to that of current ones,therefore,the runoff flattening at Datong will proceed.Because the total regulating capacity of the constructed upper cascade reservoirs only accounts for?6%in the yearly total water discharge at Datong,the scale and number of the additional reservoirs in the future are limited and the runoff flattening helps maintain the general water balance over the year,the total yearly water discharge at Datong will not change obviously in the future.The comparison of the sediment contribution proportions of Yichang,tributaries,lakes and river-bed supply in Datong before and after the impoundment of the Three Gorges Reservoir(pre-TGR:116.0%,7.8%,-12.2%,-11.6%;post-TGR:30.5%,10.6%,13.5%,45.4%)shows that the main sediment source of Datong was Yichang before the impoundment and is the river-bed supply now.Combining the changing law of the river-bed supply downstream of reservoirs and existing achievements in reservoir deposition-equilibrium,this study predicts that the yearly sediment load into the sea will not exceed 1.5 ×10 8 t/yr in the long term(>300 yr).Second,this study reveals the erosion-deposition response of the Yangtze Estuary to the dam-induced runoff discharge flattening.The ebb partition ratio and the position of flow axis are taken as the indexes that reflect the water excavating force in channels respectively downstream and upstream of Jiangyin.After an analysis of the relationship between the water excavating force and the erosion-depostion in the braided branches,and the relationship between the seasonal variability of runoff discharge and the water force in the braided branches,the erosion-deposition law of the Yangtze Estuary is obtained.The relationship between the ebb partition ratio,the position of flow axis and the erosion-deposition performs as follows:the increase of the ebb partition ratio in a branch or the extension of the time that the flow axis locates in a branch promotes the erosion or the alleviative deposition in this branch,and leads the downstream movement of the depo-centre in this branch;whereas,the decrease of the ebb partition ratio in a branch or the shortening of the time that the flow axis locates in a branch results in the converse variation.Furthermore,the depo-centre outside the mouth(the position where the deposition rate is the most over the whole Yangtze Estuary)moves southward and northward separately when the ebb partition ratios of the north and south branches downstream of Xuliujing increase.In order to identify the effects of dam-induced runoff discharge flattening on the water excavating force in the braided branches,this study takes the section downstream of Xuliujing as an example,established the regression equation between the ebb partition ratio and runoff discharge and tidal range,and calculated the multi-year variations of the ebb partition ratios of the branches under the changes of the duration days of each runoff discharge level.The results demonstrate the impacts of the dam-induced runoff discharge flattening on the water excavating force in the braided branches.Hence,the impacts of dam-induced runoff discharge flattening on the erosion-deposition of the branches can be acquired.Since the ebb partition ratios of the north and south branches downstream of Xuliujing have been reduced and enlarged respectively under the dam-induced runoff discharge flattening,the shrinkage of the north branches and the development of the south branches are aggravated,the depo-centres in the north and south branches are pused upstream and downstream separately,and the depo-centre outside the mouth is pused northward.The reason for the post-2002 heavy deposition of the submerged delta in the range of 121°48'-122°40,E and 30°49'-31°31' N is the heavy deposition of its northern area under the runoff discharge flattening.The Deepwater Channel Regulation Project accelerates the shrinkage of the North Passage and the development of the South Passage;meanwhile,the deposition and erosion of the North Passage mainly occur in the groin field and the deep channel respectively.In order to improve the shipping function of the North Passage,its yearly ebb partition ratio must be enhanced.The reclamation along the North Branch accelerates its shrinkage and makes the high-runoff-induced erosion mainly occurs in the deep channel of its middle-lower reach;in order to maintain its life,the reclamation amount(especially the amount along its upper-middle reach)should be restricted and the inflow condition of its upper entrance should be improved.For the section between Jiangyin and Xuliujing,the ebb partition ratios of the left branch(including the north waterway,middle waterway and the Shuangjian Shoal)of the Fujiangsha Reach,the Liuhaisha Waterway of the Rugaosha Reach,the west waterway of the Tongzhousha Reach and the west waterway of the Langshansha Reach have been reduced under the yearly water discharge)is the main cause for the runoff flattening at Datong,and the dam-induced runoff flattening at Hangjiang River has also made some contributions;however,the seasonal variabilities of the runoff discharge at Dongting Lake and Poyang Lake and the seasonal precipitation between Yichang and Datong have shown some negative effects.The maximal positive and negative contribution proportions of Yichang,Hanjiang River and the two lakes are 152.7%,118.3%and-54.2%,respectively.The flattening of seasonal runoff variability at Datong performs as the decreases of the multi-year averaged duration days of the high(>50000 m/s)and low(<10000 m/s)discharges,and the increase of that of the higher-low discharge(10000-20000 m3/s).The upper cascade reservoirs help the Three Gorges Reservoir flatten the runoff discharge,and the operation rule of the additional reservoirs in the future is similar to that of current ones,therefore,the runoff flattening at Datong will proceed.Because the total regulating capacity of the constructed upper cascade reservoirs only accounts for?6%in the yearly total water discharge at Datong,the scale and number of the additional reservoirs in the future are limited and the runoff flattening helps maintain the general water balance over the year,the total yearly water discharge at Datong will not change obviously in the future.The comparison of the sediment contribution proportions of Yichang,tributaries,lakes and river-bed supply in Datong before and after the impoundment of the Three Gorges Reservoir(pre-TGR:116.0%,7.8%,-12.2%,-11.6%;post-TGR:30.5%,10.6%,13.5%,45.4%)shows that the main sediment source of Datong was Yichang before the impoundment and is the river-bed supply now.Combining the changing law of the river-bed supply downstream of reservoirs and existing achievements in reservoir deposition-equilibrium,this study predicts that the yearly sediment load into the sea will not exceed 1.5×1 08 t/yr in the long term(>300 yr).Second,this study reveals the erosion-deposition response of the Yangtze Estuary to the dam-induced runoff discharge flattening.The ebb partition ratio and the position of flow axis are taken as the indexes that reflect the water excavating force in channels respectively downstream and upstream of Jiangyin.After an analysis of the relationship between the water excavating force and the erosion-depostion in the braided branches,and the relationship between the seasonal variability of runoff discharge and the water force in the braided branches,the erosion-deposition law of the Yangtze Estuary is obtained.The relationship between the ebb partition ratio,the position of flow axis and the erosion-deposition performs as follows:the increase of the ebb partition ratio in a branch or the extension of the time that the flow axis locates in a branch promotes the erosion or the alleviative deposition in this branch,and leads the downstream movement of the depo-centre in this branch;whereas,the decrease of the ebb partition ratio in a branch or the shortening of the time that the flow axis locates in a branch results in the converse variation.Furthermore,the depo-centre outside the mouth(the position where the deposition rate is the most over the whole Yangtze Estuary)moves southward and northward separately when the ebb partition ratios of the north and south branches downstream of Xuliujing increase.In order to identify the effects of dam-induced runoff discharge flattening on the water excavating force in the braided branches,this study takes the section downstream of Xuliujing as an example,established the regression equation between the ebb partition ratio and runoff discharge and tidal range,and calculated the multi-year variations of the ebb partition ratios of the branches under the changes of the duration days of each runoff discharge level.The results demonstrate the impacts of the dam-induced runoff discharge flattening on the water excavating force in the braided branches.Hence,the impacts of dam-induced runoff discharge flattening on the erosion-deposition of the branches can be acquired.Since the ebb partition ratios of the north and south branches downstream of Xuliujing have been reduced and enlarged respectively under the dam-induced runoff discharge flattening,the shrinkage of the north branches and the development of the south branches are aggravated,the depo-centres in the north and south branches are pused upstream and downstream separately,and the depo-centre outside the mouth is pused northward.The reason for the post-2002 heavy deposition of the submerged delta in the range of 121°48'-122°40,E and 30°49'-31°31' N is the heavy deposition of its northern area under the runoff discharge flattening.The Deepwater Channel Regulation Project accelerates the shrinkage of the North Passage and the development of the South Passage;meanwhile,the deposition and erosion of the North Passage mainly occur in the groin field and the deep channel respectively.In order to improve the shipping function of the North Passage,its yearly ebb partition ratio must be enhanced.The reclamation along the North Branch accelerates its shrinkage and makes the high-runoff-induced erosion mainly occurs in the deep channel of its middle-lower reach;in order to maintain its life,the reclamation amount(especially the amount along its upper-middle reach)should be restricted and the inflow condition of its upper entrance should be improved.For the section between Jiangyin and Xuliujing,the ebb partition ratios of the left branch(including the north waterway,middle waterway and the Shuangjian Shoal)of the Fujiangsha Reach,the Liuhaisha Waterway of the Rugaosha Reach,the west waterway of the Tongzhousha Reach and the west waterway of the Langshansha Reach have been reduced under the dam-induced runoff discharge flattening,thus has led to the shrinkage of these branches and the upstream movement of the depo-centres in these branches;the situation for another branch of these Reaches is quite the opposite.The trends for the total erosion-deposition quantities of the upper(Jiangyin-Tianshenggang)and lower(Tianshenggang-Xuliujing)reaches present some differences:the upper reach changed from deposition to erosion quickly with the reduction of the fluvial sediment load during 1998-2014;however,the lower one showed a sluggish response to the sediment reduction under the reinforced jacking effect of tide,which was related to the decrease of the wet-season averaged runofF discharge.When the wet-season averaged runoff discharge exceeds 36000 ml/s,the tidal force will be significantly strengthened and will excavate the lower reach.The dam-induced runoff discharge flattening increases the multi-year averaged duration days of the discharge level<36000 m3/s,and then helps facilitate the erosion of the lower reach and the deposition of the upper reach(The reason for the deposition-promotion effect of the upper reach is that the eroded sediment of the lower reach can be brought into the upper reach by the flood tidal discharge).For the section between Datong and Jiangyin,the time that the flow axis locates in the left branch of the Tuqiao Reach,the left and middle branch of the Heishazhou Reach,the left branch of the Wuyu Reach,the left branch of the Baguazhou Reach,the right branch of the Shiyezhou Reach and the right branch of the Hechangzhou Reach has been shorthened under the dam-induced runoff discharge flatteing,and then has led to the shrinkage of these branches and the upstream movement of the depo-centres in these branches(the situation of the depo-centres in the Tuqiao branches is taken as an example here);the situation for another branch of these reaches is quite the opposite.Thirdly,this study reveals the erosion-deposition trend of the Yangtze Estuary under the trends of the seasonal runoff variability and the total sediment quantity.Because the tidal force at the open sea area is stable in the inter-annual scale and the runoff' discharge flattening will proceed with the continuous construction of watershed reservoirs,the erosion-deposition differences among the braided branches will go on.Meanwhile,the analysis on the fluvial sediment load and marine sediment load(including that from the Yellow River delta and the Hangzhou Bay)indicates that the total sediment quantity into the Yangtze Estuary will not exceed the current level;therefore,the total deposition rate of the Yangtze Estuary(including the submerged delta area in the range of 121°48'-122°40,E,30049'-31°3 1'N)will not exceed the current level as well.