Variations In Water And Sediment Process Of Unbranched River Of The Southern Subtropical China

Fluvial water and sediment to the sea are of vital importance to global biochemical cycle,which are the significant components to the land-ocean material and energy exchange.Research on the fluvial water and sediment processes,especially their variations,is the main content of the Future Earth Coasts program.Owning to the combined effects of climate change and anthropogenic activities,it is an indisputable fact that water and sediment discharge of global large rivers have been changed and regulated.However,the current knowledge on water and sediment process for small-medium unbranched rivers in subtropical mountain area is still limited.Small-medium rivers are characterized by wide distribution,high flow velocity,small area,short river length,narrow river width and being highly effected by tropical cyclone.Their water and sediment variations are directly related to the regional ecological,economic and social benefits.Research on the water and sediment variations of unbranched rivers in southern subtropical China can explore the water and sediment transportation processes,river channel variation,coupling relationship between water and sediment variation,as well as the effect of tropical cyclone on riverine sediment variation et al.,which can also provide scientific and technical supports for the river development and management.Nanliu River,the largest unbranched river around the Beibu Bay with a subtropical oceanic monsoon climate,is the study area of this research.Based on the suspended sediment and bed load samples,cross section morphology measurements,sedimentation core sample as well as historical information on regional precipitation,tropical cyclone,water and soil erosion,land use and sand mining,we used a series of methods,including wavelet analysis,Ratio curve of interdecadal monthly water discharge and Suspended Sediment Concentration,empirical orthogonal function,210Pb and 3S dating to systematically analyze the water and sediment variation processes of the Nanliu River and their driving factors.Moreover,the influence of natural process and anthropogenic activities are quantified to further explore the relation mechanism between river channel,estuary and variations in water and sediment.The main conclusions include:There are significant change features in water and sediment of the Nanliu River.Water and sediment flux in the Nanliu River are less than 100 m3/s for most of the year,with water peak in July,August and September,sediment peak in April.There are great difference on the runoff and sediment in flood and dry season,with the contributions of summer half year account for over 90%of the total year.The water and sediment series indicate strong periodicity with a period of 4-6 and 11 years.The maximum monthly sediment lags behind the runoff,being characterized by clockwise"sediment then discharge",which is contrary to the anticlockwise "discharge then sediment" of Changjiang River.Such a hysteresis,however,is decreasing due to the influence of intensive human activities.The change rates of water and sediment become synchronized-The sediment carrying capacity at equal water discharge is decreasing in upper and lower reach of the Nanliu River while the middle reach indicates larger water and sediment variation rates as a result of various controlling factors.In the recent 50 years,annual mean sediment load has significant downward trend while annual mean water discharge has no significant tendency.Variations in water and sediment in the Nanliu River are controlled by multi-factors.Natural forces are the main drivers for water and sediment variations in the Nanliu River while the influence of human activities has intensified recently.Nanliu River is in develop stage with relatively small hypsometric Index.Headward erosion along the river provides basic sediment to the river.The rising of multi-year monthly average temperature accelerates the soil erosion degree,which is conducive to riverine sediment increase.The variation tendency,peak value distribution and periodicity of regional average rainfall are consist with that of water and sediment,indicating the dominate role of climate on water and sediment variations in the Nanliu River.Tropical cyclone is another driving factor of runoff and sediment increase in the Nanliu River,which leads to an increase of 24.38%,43.36%and 10.72%in terrestrial suspended sediment for normal,high and low flow year,respectively.The influence of anthropogenic activities on water and sediment variation is increasing gradually in the recent 50 years.The contribution of soil erosion on riverine sediment flux is as high as 24.76%,roughly equivalent to the contribution of tropical cyclone in normal flow year.Moreover,the sand mining quantity per unit area in the Nanliu River is 1030.82 m3/km2,much larger than the 26.76 m3/km2 of Changjiang River,which dramatically affects the variations in river bed,river bank,riverine runoff and sediment.River channel indicates "erosion in flood season,deposition in dry season and erosion on the whole" following the water and sediment variations.Cross section along the Nanliu River has two typical forms,namely,"U" form and "V" form.The river channel indicates a clear seasonal variation with erosion in flood season and deposition in dry season.During 2001-2012,sediment erosion along the channel are 13.81×104 m3 in flood season,and deposition are 8.33×104 m3 in dry season,suggesting a net erosion of 5.48×104 m3.The channel morphological change has two eigenmodes.The first mode is the main form of morphological change in cross section,showing the periodical oscillation in cross section evolution and the seasonal effect of riverine water and sediment.The second mode shows that extreme events intensify the channel erosion and induce more observable "V" form for local cross section,indicating the significant effect of extreme climates,such as rainstorm,flood,drought and typhoon,which is the second most important factor contributing to the cross section variation.Estuary has no significant response characteristic to riverine water and sediment variation.The soil inflow into the sea is not the main effect factor of resent deposition rate increase in the Nanliu Estuary.Our research show that the average deposition rate in the Nanliu Estuary is 0.83 cm/yr,which is accelerting recently while the fluvial sediment is decreasing.The estuarine sediment compositions are similar to the upper and lower reach,with unobservable double-peaks mode.However,the grain size of estuarine sediment is coarser than the upper and lower reach with good sorting,showing that the upper and lower reach of Nanliu River only contribute part of sediment to the estuary.Marine sediment,tidal current,tidal flat and siltation promotion of mangrove forest are the prime factors lead to the current high deposition in the Nanliu Estuary.Unbranched mountain river(Nanliu River)and small-medium island river(Lanyang Creek)have their own characteristics in water and sediment variations.Nanliu Rive has larger water and sediment flux while Lanyang Creek has strongly sudden in water and sediment variations.The mean annual discharge and sediment load in Nanliu River are respectively 4 and 500 times of Lanyang Creek.The maximum and minimum annual discharge in Nanliu River are over 3 times of Lanyang Creek while the maximum and minimum annual sediment in Nanliu River are over 300 times of Lanyang Creek.Therefore,the hydrological processes in island river are more variable and unexpected than unbranched mountain river.