Keywords

1 Introduction

Daijiazhou reach in the middle reaches of the Yangtze River is selected as a typical bifurcated reach for our study. The total length of Daijiazhou reach is about 34 km (see Fig. 1), and it is a slightly curved bifurcated reach. The bar of Daijiazhou in the river divides the channel into two branches, and the left branch is a circular channel while the right branch is a straight one (Liu 2015). The outline of the head of bar of Dai Jiazhou’s is a long upward extending “wedge” shape, which bends on the plane and gradually becomes higher along the longitudinal direction.

The analysis shows that the head of Daijiazhou is constantly changing, moving left or right, or lifting up or falling down (Cai and Li 2011). Its instability is a direct reflection of the instability of channel conditions of Daijiazhou reach in dry season. The head of Daijiazhou is constantly changing under the action of distinct flow and sediment conditions (Fig. 2), resulting in the alternation of main channels in dry season of two branches.

The straight channel of right branch is the main navigable channel of the reach. The study shows that most straight channels will develop into single channels whose channel conditions are good when the head of Daijiazhou extends upward while the multiple transitions in deep pool in the straight reach is easy to form whose channel conditions are poor when the position of the head of Daijiazhou is lower. Obviously, taking engineering measures to stabilize the head of Daijiazhou and promoting its siltation to extend upward is very important to improve the channel conditions of straight channel. However, the project layout of the head of sandbar has a direct impact on the effect of the regulation project. Therefore, it is necessary to study the rule of flow distribution of diversion area at the head of sandbar in bifurcated reach to direct the project layout of the head of bar to be reasonable (Liu 2020).

Fig. 1.
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Sketch of the Daijiazhou reach

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Fig. 2.
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The variation of of topographic longitudinal profile of the head of the Daijiazhou sandbar

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2 The Diversion Point and Confluence Point of Distributed Reach

For the bifurcated reach, there are generally diversion points in diversion area at the head of bar and confluence points in confluence area at the tail of bar. The study shows that the position of the diversion points of Daijiazhou reach move upstream in dry season and moved downstream in flood season (Fig. 3), and there is little difference between the locations of confluence points at various discharge. The reason is that the position of water edge varies greatly under distinct discharge because of flat terrain at the head of Daijiazhou. The position of diversion point moves up and down due to that the larger the discharge is the greater the flow momentum is and it has a large variation range. The tail of Daijiazhou is steep, and the position of water edge under different discharge has little difference, while the position of the flow dynamic axis under all levels of discharge has little displacement here. Therefore, the change of the position of confluence point is small.

Fig. 3.
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The location of diversion and confluence point under various flows

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3 Flow Distribution Characteristics in Diversion Area at the Head of Sandbar

The flow distribution characteristics in diversion area at the head of sandbar are analyzed through numerical simulation calculation and physical model experiment, and the rules of flow movement near the head is revealed comprehensively. The change of flow direction at the head of Daijiazhou is obviously subject to the amount and direction of incoming flow, the terrain at the head of sandbar, the topographic change of two branches, and the comparison relationship of resistance between the two branches (Fig. 4).

  1. (1)

    The flow at the head of Daijiazhou is obviously left biased given the discharge less than 20000 m3/s, and a less discharge leads to a greater angle to the left. This is the phenomenon that the incoming flow under the basic straight river regime is affected by the downstream curved river regime and the flow carrying capacity of the straight channel is limited. This kind of deflection may be closely related to the landform created by the straight channel during the flood period and the difficulty of forming a well connected deep groove in the falling water period. And the deflected flow will have a certain oblique scouring effect on the submerged bar at the head of Daijiazhou. In addition, surface flow deflects more frequently due to that the deflected flow is affected by the topography of the head of Daijiazhou. Therefore, more sediment from the upstream may enter the straight channel, which has negative effect on low-flow scouring of the shoals in straight channel (Fig. 5).

  2. (2)

    The upstream flow of Daijiazhou head has no obvious deflection angle given the discharge between 20000 m3/s and 30000 m3/s. This is because the stagnation point at the head of Daijiazhou enters the bend due to the rise of water level when the discharge is large, and the diversion area is already located in the upper reach of the bend. The horizontal distribution of flow matches the flow capacity of the two branches to achieve the balance. Therefore, the direction of flow at the head of sandbar shall not biase to one side obviously.

  3. (3)

    The flow at the head of Daijiazhou tends to the right given the discharge between 30000 m3/s and 50000 m3/s. With the further increase of the mount of flow and the further downward movement of the stagnation point at the head of Daijiazhou, the wetted area of the straight channel increases with the increase of water level, and it is obviously better than circular channel. Therefore, the increase of the discharge capacity of straight channel is greater than that of circular channel, and the cross-sectional transverse flow velocity distribution in the diversion area is more affected by the curved river regime, which is also limited to the discharge capacity of circular channel to the right.

  4. (4)

    The water overflows Xinzhou and the flood flow is straight given the discharge greater than or equal to 50000 m3/s, and there is no obvious biase to one side near the head of bar.

Fig. 4.
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The diversion at the head of Daijiazhou under distinct levels of flow under the current terrain

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Fig. 5.
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Local flow pattern at the head of Daijiazhou sandbar under low flow (Q = 7310 m3/s)

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4 Diversion Surface and the Principle of Project Layout of Head of Sandbar

Taking Daijiazhou reach of the middle reaches of the Yangtze River as an example, the concept of diversion surface is put forward creatively based on the study of the flow distribution characteristics of diversion surface at the head of sandbar. The diversion surface of the head in bifurcated reach is studied and its layout of regulation project is guided through numerical simulation calculation (Liu et al. 2009).

The diversion surface refers to the curved surface along the vertical line calculated according to the diversion ratio of two branches in the local reach of the head of diversion area (Fig. 4). That is some sections in the reach upper the head of sandbar should be taken and the cumulative discharge between sections taken and the bank shall be calculated respectively. The points on each section where the cumulative discharge is equal to the diversion amount of this branch are connected and the longitudinal surface formed by the connecting lines at distinct vertical positions of the point is the diversion surface. The location of the diversion surface is different under different levels of discharge. For example, there are obvious difference between low-flow and flood locations of diversion surface of Daijiazhou (Fig. 6).

Based on the concept of diversion surface, the layout principle of the regulation works at the head of sandbar in bifurcated reach is put forward combined with the natural flow characteristics at the head of sandbar in bifurcated reach. The regulation works should deviate from the side of low-flow diversion surface as far as possible, such as the left or right side if the regulation works are arranged at the head to increase the low-flow diversion ratio of the straight channel (main branch). The sandbar-head project should be arranged near the flood diversion surface in order to ensure that the regulation project does not affect the flood characteristics of two branches. The sandbar-head project shall be set on the low-flow diversion surface if the regulation project does not significantly change the diversion ratio of the two branches. The above principles can play an important role in the layout of bar-head regulation works in bifurcated reach.

Fig. 6.
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The diagram of the relation between sandbar-head diversion surface and layout of regulation works

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5 The Project Layout in Sandbar-Head Diversion Area

As the diversion area at the head of sandbar is an inundated area, which shows characteristics of single section not bifurcated section. Therefore, the change of single section to bifurcated section will bring special changes in flow conditions. First, the diversion ratio of two branches will change. The sandbar-head project is more manifested in changing the inflow conditions of the branch to make the diversion ratio of the two branches adjusted to a certain extent due to that the diversion ratio is affected by many factors, including the length of the branch (on-way resistance), section profile, local resistance, sand wave resistance and inlet inflow conditions, etc. Second, the sandbar-head project changes the flow velocity on both sides of the project area significantly, and the flow velocity on one side (the side to which the flow at the head is biased before the project) increases because more water replenished along the way through the diversion surface bypasses the project head and enters the artificial branching reach, which shows that the discharge and flow velocity of this reach increases from the lower to upper gradually, while the other side is opposite, that is, the discharge and velocity in this reach decreases from the lower to upper gradually. This kind of phenomenon is more obvious in the early stage of project implementation and will weaken to a certain extent with the continuous development of the influence of sandbar-head project on riverbed deformation.

The fish mouth type dividing dike project is often set in order to stabilize or change the diversion condition of flow and sediment of two branches (Li 2007). Stabilizing the diversion condition of flow and sediment of the two branches or increasing the diversion ratio of the main channel, and narrowing the width of river in dry season (the river width in dry season in diversion area of bifurcated reach is often larger, which will lead to small depth inevitably, and extending upstream with “fish mouth” project can create a sandbar of a certain scale so as to improve channel depth condition in sandbar-head area) are two main functions of “fish mouth” project. The project of sandbar-head diversion area of Daijiazhou reach is arranged on the left side of diversion surface in dry season to increase diversion ratio of main channel in dry season. And the project acts on shaping sandbar pattern, narrowing river width and increasing depth (Fig. 7).

Fig. 7.
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The diagram of fish mouth type dividing dike project at the head of sandbar

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6 Conclusions

  1. (1)

    The study shows that there are diversion points in the diversion area at the head of sandbar in bifurcated reach generally. The terrain of the head of Daijiazhou is flat. The position of flow edge under distinct discharges varies greatly, and the larger the discharge is the greater the flow momentum is. Therefore, the position of diversion point moves up and down with a large change amplitude.

  2. (2)

    The change of flow direction at the head of Daijiazhou is obviously restricted by factors such as the amount and direction of incoming flow, the topography of the head of Daijiazhou of the reach, the topographic change of the two branches and the resistance comparison relationship between the two branches. There are many phenomena such as obvious left deviation, no obvious deviation angle, right deviation of the flow at the head of Daijiazhou.

  3. (3)

    There is a diversion surface in the diversion area at the head of bar in meandering bifurcated reach due to the difference between directions of surface flow and the bottom flow. The concept of diversion surface is put forward through study. The location of the diversion surface varies under distinct discharges. For example, there are obvious differences in the locations of diversion surface of dry season and flood season in Daijiazhou reach.

  4. (4)

    The layout principle of the regulation project at the head of bar of bifurcated reach is put forward based on the concept of diversion surface and combined with the natural flow characteristics at the head of bar of bifurcated reach. The sandbar-head project shall be set near the relatively stable diversion surface if the diversion ratio of the two branches shall not be changed significantly while the purpose can be achieved by shifting the starting point of the project to one side to make the project play its role of changing the diversion ratio of the two branches.