Characteristics Of Runoff And Sediment And Channel Evolution In The Middle And Lower Reaches Of Zuli River

As a first-class tributary of the Yellow River upper reaches,Zuli River is a typical river with too much sediment.Serious soil erosion in the Zuli basin not only restricts the economic development,but also affects people's life and production in the lower reaches.Discussion on the sediment and mastery the rule of runoff and sediment change could provide a reference for scientific decision-making and a basis for ecological protection and construction planning of the river basin.In this paper,mathematical statistical indexes such as nonuniform coefficient,complete adjustment coefficient,concentration degree and concentration period are used to analysis characteristics of annual distribution and interannual variation of runoff and sediment based of the data series of Guochengyi hydrologic station and Jingyuan hydrologic station in the middle and lower reaches of Zuli River from 1957 to 2014.R/S analysis,cumulative anomaly curve,Morlet wavelet analysis and Mann Kendall test are used to analyze the continuity,trend,periodicity and mutation of runoff and sediment.The correlation between runoff and sediment in different time scales is analyzed.The paper analyzes the evolution trend of the main channel in the middle and lower reaches of the Zuli River by using the basic draft section and current meter of the hydrological station to measure the change of the flow section shape,including the overflow area,the mean depth and the mean bed level.And the continuity and strength of each index are analyzed.The main conclusions are as following:(1)The annual distribution process of runoff and sediment in the middle and lower reaches of Zuli River is the same,which is single peak type.The distribution of runoff and sediment is uneven in the year,which is concentrated in the flood season.The nonuniformity of runoff is stable before 1995,but that decreased after 1995.The nonuniformity of the downstream runoff is generally less than that of the middle reaches,and the extreme values of the downstream runoff and sediment are also less than that of the middle reaches.(2)The annual runoff and sediment are shown a decreasing trend year by year,and will continue to decline in the future.The decreasing rate of runoff and sediment in the lower reaches is slightly faster than that in the middle reaches.Runoff and sediment are in relative wet season in 1960 s and 1970 s,and in dry season after 1980 s.Sediment decreased obviously after 2000.The annual runoff changes alternately from wet to dry,with frequent interannual changes.(3)The annual cycle of runoff and sediment in the middle and lower reaches of Zuli River mainly includes period of about 30 years and 5 years.In spring and autumn the period is about 20 years and 8 years.Runoff in summer has main period of about 5 years and sediment has period of about 30 years and 5 years.Runoff in winter has a period of 30,20 and 15 years and sediment has a long period of about 10 years and a short period of less than 5 years.(4)The abrupt time of runoff in the middle and lower reaches of Zuli River is from 1998 to 2007 on an annual scale,which occurs around 2000.The abrupt time of runoff in spring is from 1991 to 1974,while abrupt time of sediment is mainly from 1998 to 2005.The abrupt time of runoff and sediment in summer is from 2000 to 2007.The abrupt time of runoff in winter is mainly in 1973,1982 and 2008.The sediment has a abrupt time of 1973,1982 and 2008 in winter.The mutation time of sediment is mainly in 1980 s and 2000 s.Under the same time scale and at the same hydrological station,the abrupt time of runoff should be earlier than the sediment.Under the same time scale,the abrupt time of runoff and sediment of the downstream is earlier than that of the middle reaches.(5)The runoff and sediment have a significant positive correlation.On a monthly scale,the correlation is relatively strong from May to September,which is the strongest in July and relatively weak from November to February.On a seasonal scale,the correlation is shown as summer > autumn > spring > winter,which is stronger in flood season than in non flood season.Under the same time scale,the correlation of runoff and sediment in the same hydrological station increases with the increase of runoff.Under the same time scale,the correlation of runoff and sediment in the lower reaches is relatively weaker than that in the middle reaches.(6)The Guochengyi station channel section is V-shaped,which of the Jingyuan station is U-shaped.In the future,the overflow area and the mean depth in the middle reaches of Zuli River will decrease,and the mean bed level will slowly rise.And the overflow area at the lower reaches and the mean bed level will decrease,and the mean depth will rise slowly.