River System Responses To Flow &Sediment Regime Variation And Its Function Assessment

Global experience of river development history show that river function degradation were commonly during the course of water resources exploitation, due to the absent knowledge of river system response to human disturbance on natural water-sediment regimes. Two reasons underlie the present situation: one for insufficient recognition of the linkages and interaction between water, sediment, and river morphology, which arise unexpected phenomena or negative effects in river system; another for the partial understanding of environmental, social-economical impact of river alternation, certain riverine function is improved at the cost of health degradation of the whole system. Lots of facts indicate that river system functions were often notably affected before human recognition of influence of dam and other constructs. As restoration of riverine ecosystem is very difficult or even impossible, protection measures are more needed than remediation.River system is typically a complex open system, in which the variation of input variables of water and sediment will introduce adjustment of various aspects of the whole system. Processes of"impact and response"are all over the adjustment: changes of some features of flow &sediment regime induce some corresponding consequences of hydrological, hydraulic, morphologic or ecological aspect, thereby cause the change of related riverine functions. River system interaction with flow &sediment alternation covers problems of large variety, which range over vast time and space scales. Previous researches were comprehensive and intensive carried out in many fields, but most of them were from the view of different interests, and have different temporal and spatial scales. Furthermore, because most researches were specialized in single field and associate with single aspect of the system, the cause-effect linkages and hierarchy structure were unclear, as is not fit for trend prediction and holistic evaluation at riverine system scale.Researches in this paper try to find the linkage between variation of flow &sediment regime and riverine system adjustment phenomena, which is fundamental works for prediction of riverine functions, dynamic process of river system and its effects on flood control and ecosystem are considered. Main work in this paper can be divided into four parts: firstly, analysis of the changes of flow &sediment regime due to dam alternation and feature of downstream sediment transportation; secondly, river morphology change owing to flow &sediment regime variation, such as channel pattern and shoals; thirdly, analysis of the flood and ecosystem situation in the new riverine environment; fourthly, preliminary discussion of methodology of riverine functions evaluation and protection. Most work is with the background of downstream of the TGP (Three George Project), some key issues are shown as following:(1) The characteristics of interaction between flow &sediment regime and river system is clarified. With a review of the previous research about river system and its functions, it is concluded that establishment of linkages between variation of flow &sediment regime and riverine system adjustment phenomena is necessary and significative.(2) In the paper, a description of downstream flow &sediment regime from dams is given, which is likely to affect hydrological, hydraulic, geomorphologic or ecologic aspects of rivers.In view of the absent knowledge of sediment transportation character downstream dams, we made a statistical comparison with field data to show that the transportation capacity of down stream cannot exceed the value of pre-dam period under natural situation. This rule is observed in the data of both full size sediment and each grain group, which has little relation with the type of dam operation and downstream channel morphological adjustment.According to the common phenomenon of sediment transportation downstream dams, both feasibility of flow &sediment regulation in the Xiaolangdi reservoir on Huanghe River and rationality of degradation prediction downstream the TGP is discussed. Result shows that, the attempts to increase sediment transport capacity to exceed its natural value need more consideration, and the erosion process downstream the TGP is improbable.(3) Some variables and their interaction with river channel pattern process is analyzed, including range of flow variation, rising/falling rate of flood peak, load amount, sediment particle feature and etc, the role and function of flow &sediment regime and boundary condition is also clarified. With the field data of Danjiangkou dam, the relation between flow alternation and corresponding channel adjustment is classified, based on which the probable channel response downstream the TGP is outlined. Down reach of the Jingjiang river will undergo a bed erosion process immediately after the dam closure, with the gradually discharge increment, bank erosion will dominate the process, and the planform tend to sinuous. The bar-braided reach downstream Chenglingji is unlikely to experience any notable change for the unaltered flow &sediment regime.(4) Previous researches about shoals were often case studies, and there has been no comprehensive analysis of all types. In this paper, we try to summarize the key factor in the evolution of shoals among flow &sediment regimes and local channel morphology; the result indicates the latter plays the pivotal role. The difference between sediment carrying capacity and arriving sediment amount is the main cause of shoals, thus the exponent of sediment carrying capacity changing with discharge variation can be taken as index to indicate the level of shoal state.Using the data of Danjiangkou dam as reference, relation between the change of flow &sediment regime and shoals adjustment in different type of reach section is concluded, based on which, the probable trend of shoals adjustment downstream the TGP is predicted.(5) Flood situation downstream dams both in temporal and special scope are discussed with the background of the middle reach of the Yangtze River. Multi approaches are adopted to design the flood composition of typical flood type of this region. With the result of this part of work, flood situation change owing to the influence of dam regulation and channel bed degradation (or aggradation) is estimated separately. During the early 20 years of reservoir operation, the tense flood control situation will not be much released.Reviewed the simulation methodologies of flood &sediment transportation in regional scale, a mathematical model to simulate sediment transport in complex river network is introduced, in which sediment transport equations can be solved by gradation method with junctions group divided. Measures to solve sediment diversion at junction node are also discussed.(6) In the paper, we illustrated how altering flow &sediment regime affect downstream riverine ecosystem and its temporal and special scope. After an analysis of the concept of environmental flow and its basic factors, the principle and application of each methodology is compared.(7) It is prospected about how to make multi attributes evaluation of riverine functions with the information of all aspects, and how to limit the flow &sediment regime alternation to meet the goal of river management.Discussion in this paper is most of qualitative, and the aim is to give a description of flow &sediment regime alteration and the consequent river system adjustment of multi aspects, which will serve as foundation for the prediction of river function change under flow regulation.