| The sediment yield of the Yellow River decreased to 2.57 million tons in 2000~2015 from 1.6 billion tons before the 1970 s,especially in 2010~2015,which was only 1.65 million tons.However,it doesn't mean the soil erosion is under controlled,because the whole basin was treated as a ?black box? and sediment discharge monitored at outlet of the basin couldn't explain exactly the process and degree of soil erosion at multiple scales within the entire basin.In the process of sediment erosion,sediment transport can only represent part of sediment erosion in the basin.Channels can effectively transport runoff and sediment from slope to rivers,which is related to erosion and sediment yield on the Loess Plateau and sediment transport in the Yellow River.However,what is the process and degree of sediment transport and its connecitivity in the channel? And how to affect the sediment yield in watershed and sediment transport in rivers? What's the relationship between sediment connectivity and channel morphology and rainfall condition? Therefore,study on the sediment connectivity in channel and its response to channel morphology and rainfall in the hill and gully region on the Loess Plateau was conducted,by selecting the main channel of Fangta small watershed in Ansai county and taking the ?7·26? rainstorm of Qingshuigou and Shejiagou small watershed in Zizhou county as the typical case.Based on the morphological characteristics of channel obtained by GPS and GIS,the monitoring of rainfall,erosion,sediment yield in slopes and slope-gully catchments and sediment deposition in channel,the characteristics of channel morphology and sediment generating rainfall at different scales,channel sediment connectivity and its responses to channel morphology and sediment generating rainfall were analyzed.The main results were as follows:1.The number of level 1 channels in Fangta small watershed was the largest and 2.25 times than the total number of other channels.Channel gradient of branches was much larger than that of the main channel,the gradient of main channel was less than 10% and the branch channel can be reach 80%.The ratio of width to length of the channel of the 3 branches in the upper and middle reaches decreases from upstream to downstream,while the channel ratio of width to depth was in the order of middle reaches> lower reaches> upper reaches.The ration of width to length and ratio of width to depth of channel in the lower reaches of Fangta small watershed were decreases from upstream to downstream,and the ratio of width to length was much smaller and the ratio of width to depth was bigger than that of the 3 branches in the upper and middle reaches.2.From April to September in 2018,there were 20 rainfall events in Fangta small watershed,with a total rainfall amount of 264.2mm and an average rainfall of 13.16 mm each event,the duration of rainfall events ranged from 3 minutes to 19.92 hours.The number of rainfall events generating sediment yield on different types of slope was bare land slope > natural restoration slope > artificial restoration slope.As for the number of rainfall events generating sediment yield in slope-gully,the order was shallow gully slope with farm and orchard land > short slope and long gully with shrubs > long slope and short gully with arbors and shrubs> short slope and long gully with arbors and shrubs> long slope and short gully with arbors.The rainfall events generating sediment yield in channel was in the order of middle reaches> upper reaches>lower reaches.3.From upstream to downstream,with channel width increasing,sediment connectivity became stronger.Check dam decreased the sediment connectivity of the channel,and drainage channels in the dam land were the main connecting path.The damaged check dams increased the sediment connectivity in the channel.The types of slope-gully catchment on both sides of channel also affect gully sediment connectivity.The sediment connectivity between slope-gully catchment and channel was week when the slope-gully catchment was farmland and orchards or has short slope and long gully.The sediment connectivity between slope-gully catchment and channel was strong when the slope-gully catchment with long slope and short gully.4.Antecedent rainfall has a significant effect on channel sediment connectivity.When the total rainfall amount between two channel sedimentation events was more than 100 mm and the current rainfall was more than 60 mm,the channel sediment connectivity was strong.When the total rainfall between two channel sedimentation events ranged from 40 mm to 100 mm,and the current duration of rainfall was more than 3.9h,or total rainfall between two channel sedimentation events was less than 20 mm and the current maximum rainfall intensity of 5min was more than 0.5mm/min,the channel sediment connectivity from upstream to downstream decreased first and then increased.When the total rainfall between two channel sedimentation events ranged from 20 mm to 40 mm and the current maximum rainfall intensity of 5min was more than 0.4mm/min,the sediment connectivity in the middle of the channel was weak,while sediment connectivity was strong in the upper and lower of the channel.5.Under the condition of extreme rainstorm,the type and distribution of check dam were the main factors affecting sediment connectivity.The sediment connectivity of check dam was in the connected state,and the sediment connectivity was strong when the body of check dam destroyed and relatively weak when the dam connected by water release works.Parallel dam system and hybrid dam systems is beneficial to block the flood and weakened the sediment connectivity.At the same time,check dam should be built with water release works and spillways to ensure the safety of flood control and storage.When build a single check dam,it was necessary to avoid the location with large channel gradient and the channel with the shape coefficient of the dam-controlled basin close to 1,in order to reduce the impact of upstream inflow to check dam and increase the security of check dam. |
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