Analysis On The Spatio-temporal Change Of The Downstream Channel's Flow And Sediment Regime Around The Impoundment Of The Three Gorges Project

Sediment is the main component of suspended substances in natural water,and it not only affects the optical properties of water body,but also influences the process of river erosion and siltation.If treated unproperly,it could intensify basin flood,thereby affects the ecological environment.The construction of the Three Gorges Dam(TGD)has a large scale and long period,which has played a great benefit in flood protection,power generation,shipping,drought resistance and water supplement.Because of the TGD,s operation mode of "storing clear water and releasing the muddy",the seasonal distribution of the original river runoff and the ecological environmental factors has come to change.At the same time,the sediment in the upstream channel of the TGD has decreased and the unsaturated flow in the downstream channel of the TGD continues to scour the river bed for the purpose of sediment supplement,which cause erosion along the long distance from the top to the bottom fiercely.These will have a far-reaching impact on the ecological environment in the downstream channel of the TGD.Therefore,it is of practical significance to fully understand the suspended sediment concentration(SSC)in the regional water body and its spatiotemporal change rules for river control and habitat protection.Due to the advantages for real-time,continuous,large-scale monitoring on the surface,remote sensing technology has become an important means of monitoring and evaluating water ecological environment at a large scale.In this paper,I focus on the advantages of remote sensing images in the water quality monitoring,combined with MODIS images and sediment stations data in the past 2002-2015 years.By seting up the inversion model of the water and sediment situation in this area,I analyse the content of SSC in the regional water body and its change rules.The main conclusions are obtained as follows:(1)It is feasible to monitor the SSC by random forest model,and the correlation coefficient and prediction accuracy are significantly better than others.The red band is a suitable predictor for SSC in the random forest model,but it can not only be used for forecasting alone.SSC remote sensing prediction requires multivariate co-participation.Based on random forest model,the average root mean square error(RMSE)of the seasonal division is 0.46 mg/L and the average relative root mean square error(rRMSE)is 12.33%,which brings about high effect,meeting the needs of high-precision SSC estimation.(2)The impact of the TGD on the downstream channel's water and sediment situation has begun before the official operation.Which is not sudden,but a long and slow process.After water storage,the spatial distribution pattern of SSC in the downstream channel of the TGD is obvious.During these 14 years,the monthly variation of suspended sediment of each section in the downstream channel of the TGP is basically consistent with the overall change.The lowest average content of suspended sediment found in March and April,and highest in September(except the lower Jingjiang River).SSC differs a lot in the same water phase along the river.The SSC's content from upstream to downstream increased gradually.In general,Section Lower Jingjiang>Upper Jingjiang>Yizhi.In terms of the annual variation of SSC,the average content increased gradually from spring to winter,and the spatial distribution in summer was more uniform.The Lower Jingjiang River area in the winter are comparatively richer in SSC,and we should pay close attention to sediment deposition of this region on the river bend.In the dry season,the SSC is less than that in the flood season,while it decreases with the time of storage in the same hydrological regime.The change of SSC in the downstream channel of the TGD is more obvious.As for the interannual variation of SSC,it is significantly lower than that before impoundment,and the total content of SSC will decrease during a small range after every period of storage.Theoretically speaking,the total suspended sediment level of the TGD's downstream channel from Yichang to Chenglingji will continue to decrease in a certain period of time,and will eventually reach a dynamic equilibrium state.This study not only reveals the TGD's downstream river sediment evolution,understands the regional distribution of sediment and sediment variation process,but also can serve for the TGD's management and flood control in Yangtze River.Which can provide technical support and decision-making basis for the protection of ecological environment to promote the development of coordination reasonable exploitation and regional economy in the middle reaches of the Yangtze River resources.