Research On The Evolution And Ecological Effect Of Tender Beaches In The Typical Wide Shoal Wandering Section In The Lower Yellow River

The wide shoal wandering section in the Lower Yellow River is a densely populated and economically developed area,but the Yellow River’s channel migration is unpredictable and its flow is unstable.Stable river flow is the foundation for economic development of the wide shoal wandering section in the Lower Yellow River.Previous research has yielded rich results on river flow,but research on the downstream sandbars of the Low Yellow River,especially the tender beach,is scarce.Therefore,this paper takes the Tiexie-Jiahetan as the study area,based on Landsat satellite remote sensing images,and uses software such as ARCGIS and ENVI to study the evolution of the tender beach and river flow since the operation of the Xiaolangdi Reservoir（2001-2020）.Taking 2021 as a typical year,the annual evolution process of the tender beaches in 2021 was analyzed.In addition,the wide shoal wandering section in the Lower Yellow River is also an important source of water resources in China.The author measured the ammonia nitrogen concentration in the water at Taohuayu and used the MIKE 21 ECO Lab model to simulate the water quality of the Tiexie-Huayuankou section,focusing on ammonia nitrogen（NH₃-N）and simulating the concentration changes of ammonia nitrogen along the river and on the floodplain.This not only provides a deep understanding of the characteristics of the evolution of the water environment in Lower Yellow River but also provides a reference for water resources management and environmental protection.The main research results of this paper are as follows:（1）After extracting the river water boundary in 2001,2007,2014,and 2020using ARCGIS,it was found that the width of the river during the flood season gradually increased since the operation of the Xiaolangdi Reservoir.The growth of the flood season water surface width was more significant in the initial operation period of the Xiaolangdi Reservoir,and the water surface width during the flood season was positively correlated with the flow rate.Through the extraction of the mainstream water body using ENVI,it was found that after 20 years of natural adjustment following the operation of the Xiaolangdi Reservoir,the river flow in the Tie Xie-Jiahetan area has been significantly improved,and the mainstream has basically returned to a stable state.Alternating increase and decrease along the tender beach and Under the control boundaries imposed by the production embankment and other factors,the degree of evolution of the shoal along the reservoir has weakened in recent years compared to the initial stage of reservoir operation.（2）Using ARCGIS,we extracted the water boundary of the river in 2021 and studied the annual variation of the river’s width.We found that mid-channel beach is one of the important factors affecting the width of the water surface.By extracting the main stream water body,we found that the river course in 2021 was unstable and the migration direction of the main stream was variable.River regulation projects have a positive effect on stabilizing the river course.Overall,the variation of the main stream during the flood season is more significant than during the non-flood season,and high-flow floods have a stronger shaping effect on the river course.By extracting the area of the beach using ARCGIS,it was found that the distribution of the left and right bank beaches in the Tiexie-Huayuankou river section is uneven.The average area of the left bank beach is about 529 km²,which is larger than that of the right bank beach area of 275 km².However,the proportion of the area of the right bank tender beach to the total area of the beach is larger.In the process of water erosion and sediment accumulation,the tender beach is constantly changing,but some sections of the river have relatively stable tender beaches.The embankment protection projects along the river also have a certain impact on the evolution of the beach.（3）Based on terrain data and by selecting appropriate parameters,a MIKE hydrodynamic model was established,and validated using measured water levels at various cross-sections,with good validation results.The established MIKE hydrodynamic model was then coupled with a water quality model,suitable state variables were selected,and initial conditions were determined to calibrate and validate the model.（4）Using the validated MIKE 21 ECO Lab model,we simulated the water quality for Class I,Class III,and Class IV water using the daily flow data from the Tiexie cross-section and the daily water level data from the Huayuankou for the periods of January 1-15,2021,April 1-15,2021,May 1-15,2021,and June 15-29,2021.Three points were selected at the Gaozhuang,Shayugou,and Mopanding cross-sections along the river to output the daily concentration of NH₃-N.The results showed a decreasing trend in NH₃-N concentration along the river.If the concentration of ammonia nitrogen（NH₃-N）in the Tiexie section happens to be in the transition zone between Class II and Class III water,the concentration will be further reduced due to the dilution of the boundary,and the water quality of some river sections may be increased to Class II water,and the water quality will be improved.Under high flow conditions,pollutants with high concentration can spread over a wider range,and it is necessary to further reduce the concentration of discharged pollutants.Flood inundation does not reduce the concentration of pollutants in the main channel.