Recent geomorphological evolution of downstream channel and delta of Luanhe River

Abstract

Big dams built across Luanhe River since the 1980s caused great decrease of freshwater and sediment discharges.After Taolinkou Reservoir was built in 1998,the average freshwater decreased to 596×106 m3/a,and the sediment discharges reduced to 24.3×103 t/a.The period of zero flow has prolonged in recent years.Based on remote sensing images from 1979 to 2007,the hydrological changes of the Luanhe River have greatly influenced the development of downstream channel and delta.In a few years after the construction of Panjiakou and Daheiting reservoirs,the downstream channel was cut deeply and eroded widely.But since the 1990s,huge water transfer and irrigation water from the Luanhe River has made the flow in the lower Luanhe River decrease greatly.According to the remote sensing images of 1991,2000 and 2004,the width of the lower channel became obviously narrower due to the reduction of the river flow.The main channel was 3 to 5 km wide around Macheng in 1979,while the width decreased to less than 100 m in 2004.Although the curving cut-off took place in 1991-2000,the migration of the channel has been limited within the old channel.After 2000,the zero flow happened frequently and the curving cut-off almost ceased.Most point bars inside the Luanhe artificial levee have been reclaimed for farmlands and woodlands.In recent years,the frequent zero flow has deteriorated the river water quality.Based on the remote sensing images of 1979,1991,2000 and 2007,the main branch of the Luanhe Estuary in 1979 was abandoned in 1984 and changed its path to Xiaotaiwan.The new main branch kept the single path from then on.Because of the great decrease of the peak flood flow,the width of the estuarine branch became narrower from 1979 to 2007 and the width has decreased to less than 100 m recently.Since the Daheiting and Panjiakou reservoirs were built in 1984,the Luanhe Delta front has stopped growing because most sediment was trapped by the dams.Two segments of the Luanhe Delta were eroded severely.One segment was located around Puochuanmen and the average erosion rate of the sand spit was 29 m/a in recent 28 years.The other was the south flank of the Luanhe Delta,and the average recession rate was 30 m/a in 1979-1991,24 m/a in 1991-2000,and 27 m/a in 2000-2007,respectively.The severity of recent deltaic erosion was related to the history of the abandoned river branches.