Hydrodynamic improvement of a goose-head pattern braided reach in lower Yangtze River *

Wen-hong Dai , Wei Ding

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing 210098, China

2. College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China

3. National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, China

Abstract: The Three Gorges Reservoir (TGR) began operation in 2003 with alterations in the natural hydrological regime and with severe environmental impacts. Baguazhou Reach, a goose-head pattern of braided channels in the lower Yangtze River has emerged due to a series of changes in the water and sediment conditions caused by the TGR. This paper makes simulations of the hydrodynamic characteristics under the condition of clear water releasing from the TGR and analyzes the features of deformation within the branches of the reach, as well as predicts the future development of the hydrodynamic improvement and the additional fluvial processes. The results show that the decline of the tributary is mainly due to the weakening of the inflow energy and the channel resistance. The particular protective measures can effectively improve the hydrodynamic performance and the present situation but, they all result in a deflection of the main water flow and increase the local velocity, and over the long term, leading to the deposition in the junction areas, where a special protection is required.

Key words: Braided channel, hydrodynamic, enhancement

Introduction

Currently almost all major rivers in the world are regulated by the construction of water reservoirs[1].There are more than 40 000 large dams in the world and most of them have been built in the last 45 years[2].The construction of water reservoirs provides the flood control and the hydropower and can be used for the agricultural irrigation. At the same time, dams can change the natural hydrological regimes with severe environmental impacts[3-4]. For example, the Aswan High Dam in Egypt completely modified the flow regime in the Nile River, affecting the seasonal flow and the sediment transport of rivers as far as 1 000 km downstream[5]. Among the dams in the world, more than half are in China[6-7]. The largest dam, the Three Gorges Dam, was constructed in the third largest river in the world. The operation of the Three Gorges Reservoir (TGR) starting in 2003 also posed great challenges to the hydrological regime in the middle and lower Yangtze River and attracted attention for studying its effects[8]. The middle and lower Yangtze River basins with a long deep-water shoreline are the homes of over 3.6×108people and are one of the most economically developed regions in China. Changes of the river regime may significantly influence the operation of wharves along the shoreline and the daily supply of water. Thus, a study of the impact of the TGR on the flow regime downstream is very important.

Even before the TGR was built, the sedimentladen flow in the middle and lower Yangtze River had been highly affected by hydropower projects, with a decrease of the annual discharge of sediments by about 60%[9]. The middle and lower reaches had turned from areas of disposing the sediments to ones eroding the sediments. Xu and Milliman proposed to put more pressure on the lower reaches of the Yangtze River already suffering from erosion[9], Straightbraided channels, slightly bent-braided channels and goose-head pattern braided channels along the middle and lower Yangtze River, are the most common river morphologies. Most channels of the Yangtze River from Chenglinji in the mid-Yangtze River to Nanjing in lower Yangtze River are braided channels, of which 40% are goose-head pattern braided channels[10],including the Baguazhou, Yaojian, Luxikou, Longling and Luohuzhou watercourses. The most special type of the braided channels is the goose-head pattern braided channel, shaped like a goose-head, with a left-of-centre asymmetric distribution and a curvature of the tributary being much larger than the main channel. The inlets of goose-head pattern braided channels are usually characterized by multi-branchstate changes, volatile beaches and frequent channel changes[11], and the bifurcation feature seems to be influenced by the upstream conditions[12]. After the operation of the TGR, Gao et al.[13]analyzed the characteristics of the flow regime changes in the post-TGR period and found that with the TGR, the annual minimum flows are increased and the annual maximum flows are reduced downstream. Zhang[14]found that the sediment load started to decrease sharply after 2003 and the maximum sediment tended to be impacted mainly by the hydrodynamics of large discharges. Zhu et al.[15]studied the evolution characteristics and the trend of the branching channels,and found that the declined high water levels and large discharges will lead to worse dynamic conditions of the tributary, the scour of the tributary cannot be guaranteed, and the oversize curvature of the tributary will move the diverging point down. Thus it is urgent to understand the impacts of the TGR and the management plan for the goose-head pattern braided channels in the middle and lower Yangtze River and to determine if some intervention procedure is required. However there is no consensus on what the ideal river pattern should be and which kind of engineering measures are the best. Liu and Pan[16-17]recommended retaining the tributary, while Luo and Li recommend making the goose-head pattern braided channel into a single channel with a slight curve[11,18].Hou et al.[19]found that the extension of the bar head upstream can effectively stabilize the diversion ratio.But there were few studies of the variation of the water and the sediment and the change of the bed form after adopting the diversion dike and the submerged dam.

The channel contains less sediment because of the Three Gorges Dam construction, which leads to a situation of more sediment being carried away from the Nanjing Reach than what is entering. So, the river bed elevation of the Baguazhou south channel is lowered and thus leads to a lower water level. In turn,more discharge to the flow through the south branch is caused, furthermore, with decreased water level in the south branch as well. This is a problem since the diversion ratio between the south and the north branches is increased with the time. In the long term it will lead to short water supply to the north branch if no hydrodynamic improvement measures are taken in both branches. It is highly imperative to improve the tributary's hydrodynamic performance so as to avoid its declination, and provide good enough hydrodynamic condition to make sure that the hydrodynamic performance will not be impaired due to the siltation.

Considering the occurring processes and mechanisms are still not well understood, it is very reasonable and effective to use numerical model based on the physical process of hydrodynamics and sediment transport to simulate the suspended sediment transport and river channel evolution in braided rivers[20-21]. This paper studies the typical goose-head pattern braided channel called Baguazhou, with a two-dimensional depth-averaged flow and sediment transport model, the Federal Highway Administration's finite element surface-water modelling system(FESWMS), to analyze the hydrodynamic situation and evaluate the impact of a structure on the water flow with consideration of both the hydrodynamic improvement and the sediment transportation.

1. Materials and methods

1.1 Governing equations and study area

In the FESWMS, the finite element method is used to solve the steady-state and time-dependent systems of equations for the two-dimensional depthaveraged surface-water flow and the sediment transport, and the FESWMS can be used to simulate the movement of the water and the sediment in rivers,estuaries, and coastal waters.

The depth-averaged surface-water flows are calculated by integrating the three-dimensional mass and momentum transport equations in the vertical direction from the bed to the water surface, when the vertical velocities and accelerations are neglected. The vertically-integrated mass transport equation or continuity equation is

The equations for the momentum transport in x and y directions, respectively, are as follows:

where β is the isotropic momentum correction coefficient, H is the water depth, zbis the bed elevation,ap is the atmospheric pressure, Ω is the Carioles parameter, τbxandbyτ are the bed shearstresses in x and y directions, respectively,sxτ andsyτ are the surface shear stresses caused by the wind in x and y directions, respectively.

Fig. 1 (Color online) Location of Baguazhou

The discharge-weighted sediment concentration for the ith particle size class, expressed in volume per unit volume, is described by the following transport equation

where Csiis the discharge-weighted sediment concentration for the ith sediment particle size class,Cesis the bed mass flux rate coefficient, Csi*is the discharge-weighted equilibrium concentration for the ith sediment particle size class.

The bed elevation changes are given by

wheresη is the porosity of bed sediment, qsiis the volumetric sediment transport rate in the streamwise direction.

Figure 1 shows the location of the research area,where the upstream boundary is the Fourth Wharf and the downstream boundary is Tongjiaying. The total river length inside the region is approximately 50 km.Six-node triangles and nine-node “Lagrangian”quadrilaterals are used in the mesh to represent the compacted and uneven terrain of the riverbed and the tortuous waterfront.

The topographic data of the studied reach is obtained in situ in July 2010 and July 2011 (the scale is 1:10 000)). The topographic and the hydrologic data are obtained by the Lower Reaches of the Yangtze River Water Resources Survey Bureau and from China National Vertical Datum 1985.

Fig.2 Velocity calibration

1.2 Boundary conditions and modelling calibration

The sediment discharge from the Yangtze River is relatively small. Supplemented by the impoundment of the TGR, the sediment discharge becomes even smaller downstream (i.e., in the Nanjing reach) where it is nearly zero[22].

Fig. 3 Hydrologic measuring layout diagram

Fig.4 Measured and calculated contour graphs

Overally, Fig. 2, Table 1 show that the measured and calculated results agree well (Fig. 3 plots the hydrologic measuring layout diagram), in terms of both the velocity U and the water levels Z . Also,the major contour lines calculated by the model are basically equivalent to the real measured and shaped contour lines, and H is the elevation, as shown in Fig. 4, It is desirable to further study the diversion ratio improvement and the following fluvial processes.

2. Analysis of current situation

The surface patterns at the south-north-passage bifurcation have a same tendency, as shown in Fig. 5.In both discharge scenarios, the water level is decreased slowly along the horizontal axis, with the water flow gradually reduced from the northern to the southern parts along the cross section C1. This indicates that there is a transverse gradient in the area,with an increased diversion ratio for the southern branch[23]. Due to the deterioration of the inflow boundary conditions, the inflow velocity at the entrance of the northern branch is significantly smaller than that of the southern branch as shown in Fig. 5(a).The bow-shape of the northern branch indicates that the local resistance coefficient is much larger than the corresponding coefficient of the southern branch. The length ratio between the two branches is almost 2.1: 1(21.6 km: 10.5 km). Because of these and the underwater shoals and the bayonets along the northern branch, the frictional resistance in the northern branch is even larger. The hydraulic structures along the northern branch also enlarge the coefficient. As Fig.5(b) shows, because the northern branch is at a right angle to the southern branch, the outflow is impeded because of the backwater effects caused by the stronger current flow in the southern branch. These effects will raise the water level and trigger an additional increase of the flow resistance in the northern branch.

The inflow conditions in the northern branch have not improved over time; instead, the flow-path curvature around the entrance of the northern branch will further increase with the siltation at the Huangjiazhou beach. Therefore, the diversion ratio has a slightly decreasing trend and the navigation and the water conditions will further deteriorate.

Fig. 5 Flow field without engineering measures

3. Improved hydrodynamic performance and dis-cussions

As shown in the previous section, the inflow boundary conditions, the flow domain area resistance coefficients and the outflow boundary conditions are all important factors that affect the flow and the sediment transport and therefore, the shrinkage of the northern branch of Baguazhou. Generally speaking,some relevant protective measures are the most common approaches to improve the hydrodynamic performance of the tributary, such as the shoal cutting,the dredging and the bayonet-enlargement in the northern branch, as well as the diversion dikes[24]and the submerged dams in the southern branch as shown in Fig. 6, in the next chapters, λ is the north branch diversion ratio and d is deposition and erosion depth.

Table 1 Water level calibration

Table 2 Summary and results of shoal cutting, dredging, bayonet-enlargement and submerged dam

Fig. 6 Layout diagram of engineering measures

3.1 Shoal cutting, dredging and bayonet-enlargement

For reducing the local resistance coefficient,thereby, improving the hydrodynamic performance of the tributary, the shoal cutting, the dredging and the bayonet-enlargement are adopted in different excavation elevations as shown in Table 2. The results indicate that the added diversion ratios are less than 1%, while the amplification of the diversion ratio is reduced gradually as the excavation elevation is decreased. These protective measures have a beneficial effect for attracting the inflow and improving the flow in the outlet, but the improved effect of the hydrodynamic performance is not significant, because most of them are located at the recirculation area, the flow velocity is generally reduced and the intensity of the back silting will be large. If there is no other method to provide an enough improvement of the hydrodynamic performance in the north branch, it is expected that the effect of the hydrodynamic improvement will soon disappear due to the siltation.

3.2 Diversion dike

The diversion dike is used at the entrance with different lengths and deflection angles and the dike at the exit is set along the sand ridge line with different lengths. Figure 7 shows the results of the length and angle sensitivity analysis of the influence of the diversion ratio at the entrance of the northern branch.Only when the angle is increased to 115°, the diversion dike can work and no matter how long the dike is, the northern branch diversion ratio keeps at a fixed value at the angle of 115°, The more the diversion dike has closed off the southern branch, the more significant the improved effect will be. In addition, the diversion ratio increases as the length increases. The diversion dike at the exit also increases the diversion ratio of discharge in the northern branch.The analyzing result is in conformity with the result of Yu[22]. In view of the technical and financial constraints on the building of such a dike, a 550 m long and 145 angled diversion dike is a practical and feasible option for the hydrodynamic improvement.

Fig. 7 Length and angle sensitivity analysis of north branch diversion ratio

Figure 8(a) shows how the diverting point changes because of the significant effect of the diversion dike at the entrance of the northern branch.It has a significant impact on the flow field around the diversion dike. As shown in Fig. 8(a), when the diversion dike at the entrance is added, the northern inflow velocity generally decreases because the transverse flow is cut off, but combined with the shoal cutting and the dredging, the northern inflow velocity increases slightly. The water area on the right side of the diversion dike becomes a vertex and reflux region where the flow is turbulent. When a dike is added, the flow area is compressed, and the water flowing away from the diversion dike is increased significantly.Figure 8(b) shows the flow field at the exit after installing the diversion dike at the exit of the northern branch. The water level of the downstream junction is dropped by about 0.010 m-0.018 m, thus the outlet surface slope of the northern branch is increased. Thus,the backwater problem caused by the stronger current flows in the southern branch is effectively solved, and the velocity of the outflow of the northern branch is increased. With the shoal cutting and a diversion dike at the entrance of the northern branch, the discharge capacity is further reinforced. The compressed flow area at the exit of the southern branch naturally raises the water level. The velocities at the left in the downstream channel are reduced due to the right-deviation of the northern outflow and the position of the maximum velocity in the section shifted to the right.This may have a negative impact on the downstream river regime.

Fig. 8 Flow field with diversion dike

Fig. 9 Deposition and erosion in three months

Figure 9 shows the erosion and the deposition three months after adopting the protective measures with a discharge of 27 310 m3/s. The effects of the protective measures are diminished with the increase of the diversion ratio. Compared to the current situation, the diversion ratio is reduced by about 0.28%but is still about 4.29% higher than when no protective measures are implemented. From a macroscopic view, for most parts of the northern branch, one sees the deposition as a physical process. The erosion and the deposition coexist in the southern branch.Because of the water-retardation effect of the diversion dike, a lower upstream velocity causes the deposition in the upper area of the diversion dike. The deep groove near the dike is eroded to a certain extent,meanwhile, one sees the deposition at the downstream beach due to the sheltering effect of the diversion dike from the remaining flow. At the exit of the southern branch, the stronger current flowing in the southern branch is compressed, the main streamline is shifted,and the increased velocity causes a serious erosion at the exit of the southern branch.

3.3 Diversion dike and submerged dam

In order to improve the hydrodynamic performance, the submerged dike is added at the entrance of the southern branch. The combination of the protective measures increases the diversion ratio up to 3.47%, which is lower as compared with that caused by the diversion dike due to the large water blocking area of the submerged dam shown in Fig. 10. Because of the increase of the local resistance caused by the submerged dam, the hydrodynamic condition is weakened. When the flow above the dam face is high,the velocity of the flow at the dam face and the river on both sides of the dam is increased significantly,with the natural water level rising upstream and the local flow pattern being influenced. The transverse slope between the northern and the southern branches,to facilitate the inflow of the northern branch, is reduced. However, the simulations show an increase of 50% in the velocity and a chaotic flow field with negative impacts on the downstream river regime.

Fig. 10 Flow field at the entrance with protective measures

Figure 11 shows the erosion and the deposition three months after adopting the protective measures with a discharge of 27 310 m3/s. The simulations show that the channel scouring and silting by self-adjusting can reduce the diversion ratio by approximately 0.32%. Still, the diversion ratio is increased by approximately 3.17% as compared with the case of no engineering measures. The siltation is found in the area with the shoal cutting and the dredging, as well as in the upstream area in front of the submerged dam. The deposition occurs along the sand ridges on the left side in the southern branch,where a layer of about 0.4 m-0.6 m is added to the bed surface. Due to the impact of the submerged dam and the diversion dike, the erosion occurs in the area upstream and to the right of the dam. The topographic changes in the lower reach of the submerged dam are more complex. The deposition exists mainly between the submerged dam and the second Yangtze Bridge,and more substantially in the left bottomland. Since the sediment-carrying capacity of the flow over the submerged dam is rapidly reduced, an area 300 m-600 m downstream is significantly eroded, with an erosion depth of almost 0.5 m. Meanwhile, the deposition also occurs in the junction area of increased riverbed levels.The effect of the protective measures is also diminished with respect to the added diversion ratio.

Fig. 11 Deposition and erosion in three months

4. Conclusions

This paper reviews the evolution of the characteristics of a goose-head pattern braided channel in the lower Yangtze River, and analyzes the main reasons that cause a series of hydrodynamics changes in a goose-head pattern braided channel. The diversion ratio, the variation of the water and the sediment and the change of the bed form are simulated,the enhanced effect of the hydrodynamics performance is discussed, as well as a preliminary prediction of the fluvial processes, the key protection zones and the protective measures. The following conclusions are drawn:

(1) The water-impoundment of the TGR reduces the sediment loads, decreases the large discharge frequency, and limits the tributary development in view of the goose-head pattern braided channel so the decreasing of the flow discharges and the sedimentation becomes an important factor for the deterioration of the tributary. The simulation results indicate that the channel scouring and silting and self-adjusting is mainly reflected in two aspects. First, the diversion ratio of the tributary shows a slightly decreasing trend,the hydrodynamic action becomes weaker and the degree of the siltation gradually increases. Second, the growth and the decline of the tributary with stable shoreline and bottomland are mainly controlled by the inflow energy and the deflection degree of the tributary at the diverging point.

(2) Reasonable methods can help to relieve the impact of the TGR for the goose-head pattern braided channels, and then improve the hydrodynamic performance. The use of dikes changes the local boundary bed conditions. when the length and the angle of the diversion dike change, the north branch's diversion ratio changes greatly, and only when the angle is increased to 115°, the diversion dike can work and no matter how long the dike is, the northern branch diversion ratio keeps at a fixed value at the angle of 115° The diversion dike at the entrance can effectively intercept a portion of the flow into the main flow, and provide an advantage for the development of the tributary. The diversion dike at the exit can effectively solve the backwater problem due to the stronger current flows in the main branch. Meanwhile, the use of a submerged dam will further enhance the effect.

(3) Reasonable engineering measures also result in the erosion at the main channel near the division dike and in front of the submerged dam. This is expected due to the change of the flow pattern in these areas. As a result, larger areas of the deposition occur before the erosion areas. The deposition before the dike is likely to be caused by the decrease and the deflection of the water velocity.