Evolution Processes And Stability Of The Bifurcated Channels In The Estuary Under Human Control

Bifurcation is a main geomorphic process of natural rivers.Bifurcation channels in estuaries are key nodes that distribute water,sediment,nutrients,and carbon from upstream sources to coastal waters and wetlands,playing a central role in the theory of deltaic geomorphology and the application of river governance engineering.Disproportionate water and sediment often occur at estuarine bifurcation nodes,which can potentially lead to river avulsion.Therefore,the estuarine bifurcation channels are unstable and can easily cause flood disaster,threatening water resources supply and navigation safety.In addition,the backwater effect is easily generated by runoff and tidal support,which makes the mechanism of discharge and sediment distribution in the bifurcation node more complex.The stronger backwater effect increased the risk of urban flood and navigation safetyThe original modes of discharge and sediment transport in estuarine bifurcation channels have been changed by variations of upstream runoff and sediment in the past several decades due primarily to dam construction and soil conservation practices,the continuous enhancement of artificial control projects such as artificial bifurcation,deep-water channel,estuarine dike,fish mouth shaped construction,and dredging,etc.Accordingly,the geomorphological dynamics and stability of estuarine bifurcation channels in response to artificial controls is a crucial scientific problem for maintaining water resource,controling flood and ensuring navigation safety in river delta regions around the world.This dissertation presents a further research on artificial bifurcation channel of the Atchafalaya Estuary which has weak tides and the typical bifurcation channel of the Yangtze Estuary which has moderate tides.Major findings from my dissertation can summarized as follows.1.The backwater effect of the estuarine bifurcation channel of the Atchafalaya River is weakened under artificial controlThe data of historical and recent water(tide)level of hydrological stations along the upper and lower reaches of the Atchafalaya Estuary were collected and analyzed.Hydraulic properties such as,hydraulic gradient,stream power,shear stress,and backwater length under low,medium,and high flow conditions were quantified to analyze the backwater effect.Hydraulic gradient of the Atchafalaya was analyzed for three river flow conditions:low(1800±100 m~3/s),medium(10000±1000 m~3/s),and high flow(18000±1000 m~3/s)conditions,based on the discharge recorded at the Simmesport hydrology station.In1977,the backwater lengths the Atchafalaya River natural mainstream channel(ARMC)were 489 km、436 km,and 188 km,under the low,medium and high flow conditions,respectively.Under these three flow conditions,the backwater lengths of the Wax Lake Outlet channel(WLOC)were 295 km、420 km,and 225 km,respectively.During 2010-2012,the backwater lengths of the ARMC were 519 km,451 km,and 195 km under the low,medium and high flow conditions.In the same year,the backwater lengths of the WLOC were 253 km,318 km,and 191 km.Simultaneously,the unit stream power of Morgan City hydrology station decreased by 5.2 W/m~2、0.9 W/m~2,and 0.1 W/m~2,respectively,and the shear stress decreased by 0.8 N/m~2,0.6 N/m~2,and 0.4 N/m~2,respectively.On the other hand,the unit stream power of Calumet hydrology station increased by 5.4 W/m~2,0.6 W/m~2,and 0.1 W/m~2,respectively,and the shear stres increases by 1.6 N/m~2,0.7 N/m~2 and 0.2 N/m~2,respectively.During 1962-2018,the river stage of Krotz Springs hydrological station and Butte La Rose hydrological station decreased by 3.28 m and 3.05 m,respectively.The river stage of Morgan City hydrology station and Calumet hydrology station increased by 0.75 m and 0.69 m,respectively.Consequently,after the completion of the Old River Control Project and the Wax Lake sub-channel Control Project at the bifurcation node in 1963,both the stream power and shear stress of the WLOC are enhanced,but the backwater effect of the estuarine bifurcation channel of the Atchafalaya had been weakened.2.The artificially bifurcated channel of the Atchafalaya has been continuously eroded and scoured.The natural main channel of the Atchafalaya substantially aggraded following the artificial bifurcation.On the other hand,there is a significant erosion in the South Channel and upper reaches of the South Passage of the Yangtze Estuary while the lower reaches of the South Passage experienced substantial aggradation.The data of sediment of Atchafalaya River and Yangtze Estuary are collected,as well as their micro-geomorphological,historical bathymetric and hydrological data.The calculation among median grain size,bed morphology,width,and bed elevation is done.Further,the decadal changes in the riverbed volume were determined,and erosion/deposition patterns in these channels were assessed.During 1977-2006,the average elevation of the lower Atchafalaya River,the Atchafalaya River natural mainstream channel,and the Wax Lake outlet channel narrowed by approximately 2 m,140 m,and 20 m,respectively.The average bed elevation in the Lower Atchafalaya River degraded by 1.47 m with a total riverbed erosion of 2.75×10~7 m~3.the Atchafalaya River natural mainstream channel aggraded by0.96 m with a total riverbed deposition of 6.39×10~7 m~3.The Wax Lake outlet channel degraded by 1.47 m with a total riverbed erosion of 1.19×10~7 m~3.Hinging on these data,as the Old River Control Project of the first bifurcation channel completed,the lower Atchafalaya river and the artificially bifurcated Wax Lake Outlet have generally been scoured.However,the natural main channel of Morgan City has experienced substantial aggradation.During 2002-2018,the South Channel of the Yangtze Estuary degraded with a total riverbed erosion of 11.53×10~7 m~3.The upper reaches of the North Passage has characterized a significant decrease in the back silation volume.And the median grain size of the surface bed sediments in the South Channel,the South Passage and the North Passage of the Yangtze Estuary increased by 0.065 mm,0.068 mm,and 0.037 mm,respectively,in the flood season from 2010 to 2012.It increased by 0.056 mm,0.014mm and 0.008 mm,respectively,in the dry season of the same period.Similarly,the average length of subaqueous dunes increased by 1.35 m,5.21 m,and 3.34 m,respectively during 2015-2018.Meanwhile,the average of dunes increased by 0.06 m,0.08m and 0.11 m,respectively.This significant trend of enhanced erosion of riverbed,coarsening mean grain size of bed sediments and increase in the spatial scale of bedforms in a short term is mainly attributed to the combination effects of Three Girges Dam,the deep-water channel regulation project and the north-south passage fish mouth shaped construction,large number of dredging and Qingcaosha Freshwater Reservior construction.3.The stability of the estuarine bifurcation channles is enhanced due to a potential trend of equilibrium between erosion and deposition in the channels under artificial control.This dissertation evaluated the stability of the estuarine bifurcation channel by collecting the measured or calculated ebb flow/sediment diversion ratio and calculating the empirical discriminant coefficient of stability.During 1977-2006,while the water discharge and sediment flux ratio of the Wax Lake outlet channel(WLOC)increased by 10%and 8%,respectively,the erosion rate of the WLOC decreased by 67%.The siltation rate of the Atchafalaya River natural mainstream channel decreased by 68%.The empirical coefficient K of the estuarine bifurcation channles stability increased from 1.2 to 2.4.As can be seen,since the completion of the Old River Control Project and the Wax Lake sub-channel Control Project at the bifurcation node,the siltation/erosion rate of Wax Lake outlet channel and Atchafalaya River natural mainstream channel has been decreasing,developing in the dynamic balance of stable bifurcation channel development.During 1998-2018,the trend of ebb flow diversion ratio and ebb sediment diversion ratio for the South Passage kept consistent,with an expansion at first and then stability.The ebb flow diversion ratio of the South Passage grows from 45.9%to 51.1%and then it leveled off at about 57.0%.The ebb sediment diversion ratio of the South Passage increased from 40.0%to 48.7%,and remained comparatively unchanged at about 63.9%.The degradation rate of the South Passage dropped by 90%while the aggradation rate of the North Passage keeps the same percentage.The empirical coefficient K of the South and North Passage stability increased from 1.21(1998-2001)to 1.65(2002-2018).Thus,as the completion of the North-South fish mouth shaped construction in 2001,the South Passage and North Passage of the Yangtze Estuary have developed toward a dynamic balance of scouring and silting.This may have helped reduce the disproportionate water discharge and sediment flux between the bifurcated channels,contributing to enhancing the stability of the North and South Passages.To summarize,although the properties of the two largest estuaries are different,the changes of riverbed deformation under artificial control share some similarities.The type of estuaries has contributed to the difference in channel changes.These estuarine bifurcation channels built on the world’s two major rivers that are under artificial control are developing consistently.The results from this dissertation research expand the current knowledge of estuarine bifurcation theory and shed a light both the safe operation of estuarine bifurcation diversion projects and the sustainable utilization of resources in the delta region,scientifically and practically.