stayriver courseWater flow andriverbedDeformation of riverbed in vertical and horizontal directions under boundary interaction.[1]There are two understandings of the term riverbed evolution: one is the formation and development of various parts of the river channel from the source to the estuary and the valley;The other refers to the change and development of modern alluvial rivers.
Chinese name
Riverbed evolution
Foreign name
river channel process
Classification
Longitudinal deformation and transverse deformation
Erosion and deposition lead to flow directionriverbedThe change of elevation is called longitudinal deformation, and the horizontal deformation perpendicular to the flow is called transverse deformation.In general, these two deformations are intertwined.According to the evolution and development process of river bed, it can be divided into one-way deformation of river bed sheets in a certain direction for a long time. For example, for some rivers, the river bed has been continuously silted and raised for many years;The recurrent deformation of the river bed develops periodically and reciprocally, such as the shoal is scoured in the dry season and silted in the flood season.In addition, there are some local deformations.Unbalanced sediment transport is the fundamental cause of riverbed evolution.When the amount of sediment coming from the upstream is greater than the sediment carrying capacity of the river, the current is unable to take away all the sediment coming from the upstream, resulting in siltation and rising of the riverbed.When the amount of sediment from the upstream is less than the sediment carrying capacity of the current in the river section, scouring will occur and the riverbed will fall.Under certain conditions, when the riverbed is silted, the siltation speed will gradually decrease until the siltation stops. When the riverbed is scoured, the scouring speed will gradually decrease until the scouring stops.This phenomenon is called the automatic adjustment of river bed and water flow.
classification
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The scouring and silting changes of the riverbed under natural conditions or after human interference.In terms of its manifestation, riverbed evolution has longitudinal deformation and transverse deformation.The longitudinal deformation is the scouring and silting change of the longitudinal section and cross section of the riverbed.The transverse deformation is the swing of the river on the plane.As far as the development direction of riverbed evolution is concerned, there are one-way deformation and reciprocating deformation.One way deformation is the development of the river bed in the direction of scouring or silting in a long period of time.Reciprocating deformation is the alternation of scouring and silting of riverbed in a certain period.As far as the influence scope of riverbed evolution is concerned, there are general deformation and local deformation of long river section.Various forms of riverbed deformation are often intricately intertwined.
Formation process
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Riverbed evolution
River bed evolution is a process of continuous interaction between water flow and river bed. In this process, sediment movement is the link.Any river section has a certain sediment carrying capacity under specific flow conditions.When the amount of sediment coming from the upstream is compatible with the sediment carrying capacity of the flow, the flow is in the balance state of sediment transport and the riverbed is relatively stable;If the amount of sediment coming from the upstream is not suitable for the sediment carrying capacity of the flow, and the sediment transport of the flow is not balanced, the riverbed will have corresponding scouring and silting changes.In turn, changes in the riverbed will change the flow conditions, resulting in changes in the sediment carrying capacity of the water flow. The trend of change is to try to adapt the amount of sediment coming from the upstream to the sediment carrying capacity of the water flow, so that the riverbed can maintain a relative balance. This process is called the automatic adjustment of the river.
According to the shape and evolution characteristics of rivers, rivers are often divided into three types: straight, curved and braided.In the branching river type, a new type is often divided into wandering type (see figure). ①Straight river channel: the plane shape is straight or slightly curved, and there are cross shoals on both banks. The longitudinal profile of the river channel is alternated with shoals and troughs. Its evolution is characterized by periodic erosion and siltation of shoals and deep troughs. The shoals continue to move downstream, and the deep troughs, shoals and thalwegs continue to change positions. ②Curved river: also known as meandering river.Its plane shape is curved, the two curves are connected by straight sections, and the longitudinal profile of the shoal is alternating. The concave bank of the curve is a deep groove, and the transition section is a shoal.Its evolution characteristics are that the concave bank of the bend is collapsing constantly, the convex bank beach is silting up constantly, and the curvature is getting larger and larger. When it develops to a certain extent, it can be bent or even straightened naturally, the old river silts up to form a oxbow lake, and the new river continues to develop towards the bend. ③Branched river channel: the plane shape is relatively straight, wide and shallow. There is one or more sandbanks in the river center, and the water flow is divided into more than two branches. Its evolution characteristics are that the beach is constantly changing, and the branches rise and fall alternately.In this type of river, the river reach with more branches, weaker stability and rapid changes in erosion and deposition is often called wandering river channel.There are also various transition types among the above river types.
Alluvial rivers have formed a relatively balanced or quasi balanced state in the long-term development process. There is a certain relationship between the riverbed shape and the incoming water and sediment of the basin and the riverbed boundary conditions, which is called river facies relationship.This relationship can be used to predict the critical state of scouring and silting of river bed after water and sediment flow and boundary conditions change, and can be used as a reference basis for river control project planning.
The construction of reservoirs or other engineering facilities on rivers will change the incoming water and sediment of natural rivers or the boundary conditions of river beds, thus destroying the balance of rivers and causing erosion and siltation of river beds.This kind of change has different impacts on water conservancy project construction and many national economic sectors. It is often necessary to make estimates in advance and formulate corresponding engineering measures to prevent their adverse effects.In order to predict the change process of riverbed erosion and deposition, physical model test and mathematical model calculation, or a combination of the two methods can be used.
Factors affecting riverbed evolution
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The fundamental cause of river bed evolution is the imbalance of sediment transport.If the amount of sediment from the upstream does not match the sediment transport capacity of this reach, the riverbed of this reach will be deformed.When the amount of sediment from the upstream is greater than the sediment transport capacity of the river section, the flow cannot transport all the sediment from the upstream, and some sediment will be deposited in the river section;When the amount of sediment from the upstream is less than the sediment transport capacity of the river section, the amount of sediment cannot meet the requirements of the sediment transport capacity of the river section, which will lead to scouring of the riverbed.The longitudinal deformation of riverbed is caused by the imbalance of longitudinal sediment transport, the transverse deformation is caused by the imbalance of transverse sediment transport, and the local deformation is caused by the imbalance of local sediment transport.The scouring and silting of river bed changes the flow and sediment conditions, which in turn will cause changes in the flow and sediment transport capacity.The siltation of the river bed reduces the section of the river section, increases the gradient, increases the flow rate, and thins the bed sediment, so the sediment transport capacity of the water flow gradually increases. The upstream river section has backwater due to the influence of the siltation of the river section. With the increase of the water depth, the gradient and flow rate will both decrease, and the sediment transport capacity of the water flow will also decrease accordingly.As a result, the amount of sediment from the upstream will gradually decrease, while the sediment transport capacity of the river will gradually increase, so the siltation will gradually stop.On the contrary, the riverbed scouring makes the section of the river section increase and the gradient decrease;However, the section of the upstream reaches decreases and the gradient increases.As a result, the amount of sediment coming from the upstream increases, and the sediment transport capacity of the river section decreases, until the sediment transport balance is reached, and the scouring stops.This is the "automatic adjustment function" of the riverbed and water flow, that is, during the development of scouring and siltation, the riverbed and water flow are adjusted to make the sediment transport capacity of this river reach adapt to the upstream sediment conditions by changing the river width, water depth, gradient and bed sediment composition, so as to promote the erosion and siltation to develop in its stop direction.The deformation of the riverbed caused by the imbalance of sediment transport is absolute, while the stability of the riverbed caused by the balance of sediment transport is relative.On the one hand, the upstream sediment situation is always changing, which will cause the destruction of sediment transport balance, and the movement of large and small sand bodies on the riverbed is always existing, so that the riverbed is constantly changing.The factors affecting the riverbed evolution are very complex, and the main factors can be summarized as follows: water inflow and its change process caused by climate and watershed conditions;Sediment volume, sediment composition and its change process;River bed morphology, geological conditions and human activities.[1]
Research methods of riverbed evolution
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Analyze the measured data of natural rivers and analyze the historical evolution process of rivers;The development trend in the next period can be inferred from the river morphology, flow and sediment characteristics.Applying the basic theory of sediment movement and the basic principle of riverbed evolution, the future riverbed deformation is theoretically calculated.The riverbed evolution is predicted through model test.[1]
