Connectivity Mechanism And Prediction & Evaluation Model Of Alluvial River System

Water system connectivity is a significant indicator of river function.Climate change and human activities will cause the deterioration of water system connectivity.Based on the theories of sediment movement,riverbed evolution,ecological environment,and slope stability,this paper explored the connectivity mechanism and developed prediction&evaluation model of alluvial river systems,by method of field investigation,data analysis,theoretical research,fuzzy mathematics and graph theory.The main results were stated as follows:The connotation of water system connectivity was improved and analysis mode of water system was proposed.Based on the basic elements of river system,the connotation of river system connectivity was systematically improved from the aspects of river boundary,flow,sediment and ecology.Then,combined with the theory of sediment movement and river bed evolution,the analysis models of channel longitudinal connectivity,lateral connectivity between beach land and main trough,and the connectivity of diversion-confluence reach were proposed,according to the basic connectivity units and structure of river system,the types of connectivity of river system are summarized.The mechanism and main influencing factors of water system connectivity were discussed.Based on the characteristics of water and sediment transport in the connected channel,this paper deduced the hydrogeometric relation of the longitudinal connected channel,discussed the morphological and stability changes of the connected channel,and revealed connecting mechanism.According to the principle of flow-sediment exchange and the evolution characteristics of the riverbed,the lateral connection model of beach and channel was summarized and the connection mechanism of the connection mode between the distributary and confluence reaches was analyzed.On the study about characteristics of erosion,expansion and siltation of the connected channel,the riverbank collapse-retreat mode and the calculation method of riverbank collapse-retreat rate were proposed.Furthermore,the mechanism of impacts of climate change and human activities on river system connectivity was analyzed.The index system and prediction&evaluation model of water system connectivity were constructed.The connectivity index system was established based on the connotation and types of water system connectivity,which could reflect the boundary,flow,sediment,ecology of the river system.Combined with the mechanism of channel connectivity and the control equation of channel connectivity index,the functional connectivity index of river system was predicted.Using analytic hierarchy process(AHP)and graph theory,the prediction and evaluation model of river connectivity was developed and the evaluation criteria of river connectivity were proposed.This could provide basis for comprehensive evaluation of river connectivity of typical river systems.The response relationship of river connectivity to flow and sediment variation were discussed.In view of flow-sediment variation in the lower reaches of the Yellow River and Jingjiang River,the response relationship between the connectivity index of typical rivers and the sediment inflow was derived according to the hydraulic geometry and functional connectivity index of rivers.Then,the influence of water and sediment variation on the connectivity of river system was analyzed.The connectivity change of typical river systems was systematically evaluated and associated suggestions were proposed.The results showed the connectivity of the lower reaches of the Yellow River decreased at first and then increases,as a "V"shape.The lower reaches of the Yellow River had lowest connectivity from 1991 to 2003,and it gradually improved after 2003.Flow-sediment variation,reservoir regulation and water diversion were the main factors affecting connectivity,especially the regulated operation of sand and water in Xiaolangdi Reservoir made great contribution to the improving of river connectivity.Generally,the connectivity of Jingjiang River system was on a high level and tended to increase.The connectivity of the main stream was gradually increased,but the connectivity of the Three Outlets was decreased,and other units did not change obviously.The main factors affecting the connectivity of Jingjiang River system included reservoir construction,river regulation,river bending,reclamation and other activities.Especially,the Three Gorges Reservoir impoundment was the key factor affecting the connectivity of Jingjiang River system.