Ao
Daguang, Yuan Xingzhi, Zeng Zhicheng and Li Bo
Scientific Research Institute of Pearl River Water Resources
Commission, Guangzhou 510611, China
Abstract: Given out in this paper is the successful experience in improvement of navigation channels in Pearl River, the largest river in South China. It is composed of 3 rivers and empties into the sea through 8 outlets、The reach to be improved is at the conjunction zone for the West and the North Rivers.
Many Kinds of shoals
grown there, and the flow condition is very complicated. Further more, it is one
of the main stems for flood disposal. The improvement is therefore very
difficult. The success is achieved after collecting a lot of hydrological and
topographical data to reveal the causes of shoal-growth. The engineering measure
is planned according to the features of the channel, resulting in good
effectiveness.
Keywords: experimental research, navigation channel, improvement
Pearl River is the largest River in South China, composed of the West, the North and the East Rivers, with the small rivers in the delta. It empties into South China Sea through 8 outlets. In the delta, rivers are dense, intersect with each other, exchanging water and sediment. The flow condition is very complicated.
The West River is the main stem in Pearl River System. The long term mean discharge at Makou station is 7551m3/s, and the mean sediment concentration is 0.31 kg/m3. The historical largest flood peak discharge is 54400 m3/s. While of Sanshui Station in the North River, it is 1233 m3/s, 0.21 kg/m3 and 17700 m3/s respectively.
Sixianjiao is an important way to link the West River and the North River, regulating the volume of water and sediment in the West River naturally (Fig. 1). In the case of flood occurring in the West River, the water in the West River flows into the North River. While in the case of flood occurring in the North River, the water in the North River flows into the West River. Since the tidal waves coming from Hongqimen and Modaomen arrive at Sixianjiao in different time, and the river bed is higher in the North River than in the West River, water flows from the North River to the West River in dry season. The annual net water volume flowing from the North River to the West River is roughly 9.4 billion tons. Because of the high sediment concentration in the West River than in the North River, the annual net sediment transport volume is roughly 1.92 million tons from the West River to the North River.
Ships from the West River goes into the North River through Sixianjiao Waterway and finally get to Guangzhou through a river reach of 76 km long, called Dongping Waterway. This navigation channel is an economic navigation line from the West River to Guangzhou. However, before improvement, many shoals exist in the channel and the water depth is less than 2 m. Only ships small than 300 ton can pass through.
Fig. 1 Plan of laoyazhou reach
The main problems about the reach to be improved are:
(1) Number of shoals are large and the reach is long
The total length of the river reach is 76 km, in which 46 km of the reach has many shoals in the channel. The shallow reach is as long as 9.8 km, comprising 21% of the length of the reach to be improved.
(2) The cause of growth is complicated for Laoyazhou Shoal
Laoyazhou Shoal is situated in the reach for the two rivers to link with each other. The flow and sediment transport condition is complicated. Every year, the riverbed has to be dredged for 3~4 times to maintain normal navigation. Argument always exists and no conclusion has been reached.
(3) Shallow and riskful west mouth of Sixianjiao Waterway
· Riskful The vortex flow threatens the safty of the sailing ships
· Shallow The rear of Qinsha Shoal is growing downstream, making the navigation channel shallower and narrower.
It is said that the noise from the west mouth can be heard even 10 km far away when flood occurs in the West River. The wracks sometimes drastically disappear.
(4) Close to North River Dyke, bearing heavy flood control pressure
The upstream reach is under the North River Dyke, which is an important guard of Guangzhou City and several important industrial cities to fight the flood from the West and the North Rivers. The improvement project has very important effect on flood control.
The reach to be improved is shallow, with wide shoals in it. In early 50’s, the minimum navigation depth was only 0.3~0.4 m. After dredged and locally improved, the depth has increased to 2.0 m since early 70’s. In order to develop the West and the North Rivers, the navigation has to be widened to 80 m wide and deepened to 2.5 m deep so that ships of 1000 tons can sail through. An systematical improvement plan has to be worked out. According to the field data, the sediment from the West and the North Rivers comes concentratively in flood season. The reach is filled up in flood season while scoured in medium and dry season. The key to reduce deposit and increase water depth is to raise the sediment carrying capacity in medium and dry season. Therefore, engineering measures should contain following items:
· Building groynes, longitudinal dams and lock dykes
· Bank protection
· Dredging
· Bomb ledge rocks
The purpose is to narrow the river channel at medium and low flow level, constrain the flow to scour the shoals so as to increase the navigation depth.
The plan is divided into 2 stages. In the first stage, river channel at low flow level is improved. If the result is still not satisfactory, the second stage will be carried out for the river channel at medium flow level.
Since the reach is situated in the conjunction zone of the West and the North River, the effect of water and sediment coming from the 2 rivers has to be considered. Therefore, the model should covers 10 km of the West River and 34 km of the North River. According the field data, the medium diameter of bed load is 0.4~0.5 mm and the main material of the shoals is bed load. The movable bed model is then design following the similarity criterion for bed load transport, that is,
·
Gravity similarity
·
Resistance similarity
·
Starting similarity
·
Silt discharge similarity
·
Sedimentation time similarity
The scale of the model is 1:400 horizontally and 1:80 vertically, with a distortion rate of 5. Wooden powder is adopted for model sand, satisfactory for starting.
Verification in fixed bed model shows that resistance and gravity similarity is quite good. Verification in movable bed model shows that the alluvial topography is rather similar to the prototype, whatever in pattern or in volume distribution. The verification result is satisfactory.
Based on verification, improvement plan alternatives are tested in fixed bed model. After the optimal one is selected, it is tested in movable bed model to see where the scoured-up material will go and what is the effect on other reaches. The selected alternative is then improved according to the modeling result.
Laoyazhou divides the main stem of the North River into 2 branches. The left branch links to the mouth of Dongping Waterway and the right branch links to the north mouth of Sixianjiao. Laoyazhou navigation channel, about 1 km long, links these two branches.
Laoyazhou Shoal is situated at the east outlet of the navigation channel, to the north of the channel. The length of the shoal varies in a range of 200~600 m. In order to maintain normal navigation, dredging has to be taken for 3~4 times every year. One time in 1987, 140 thousand m3 of silt was taken off. However, after a flood in the North River, the dredged area was filled up again, and navigation was heavily affected.
Since the reach links to the West River and the North River, the flood coming from different river make different contribution to the growth of the shoal. In modeling test, the flow pattern is observed of flood, medium and low flow from the two rivers. It is shown that in the case of flow coming from the North River, a large circular flow forms at the east mouth of the reach. The larger the discharge, the larger the range of circular flow is (Fig. 2). While in the case of flow coming from the West River, the flow passing through the navigation channel is smooth whatever in seasons of flood, medium or low flow, and the circular flow at the east mouth disappears. It then can be seen that the growth of Laoyazhou Shoal has its particular hydro-dynamic and sedimentation condition. The main cause of growth is that the circular flow forms and the sediment deposits at the east mouth when flow comes from the North River.
Analysis of the historical information indicates a close relation between the navigation condition and the evolution of the right branch. If deposition occurs in the right branch, the navigation channel will be scoured. If the right branch is scoured, the navigation depth will deteriorate. Therefore, in order to improve the navigation channel in Laoyazhou reach, the flow diversion to the right branch must be constricted and the deposition condition in the circular flow zone at the east mouth must be desdroyed. In addition, the river is generally 250 m wide in Sixianjiao, enough for flood disposal. The navigation channel in Laoyazhou reach is as wide as 350 m, obviously can be narrowed.
Fig. 2 The flow pattern in laoyazhou channel before improvement
In the fixed bed model, several improvement alternatives are tested and compared. In the optimal one, following engineering measures are planned:
· Building 2 short groynes at the right bank of the left branch
· Building a low lock-dam in the right branch
· Building 3 short groynes at the south bank of the east mouth
The
first 2 groynes can move the flow diversion point away from the bulgy bank,
reducing the flow-deflection effect, and destroy the condition of circular flow
formation. The test result indicates that after improvement, the flow at the
inlet of the channel is smooth and the circular flow vanishes. The dynamic axis
turns into a direction almost the same as the flow from the West River. Under
the effect of the lock-dam in the right branch, the discharge in the navigation
channel increases much in medium and low flow seasons. The increment of
discharge is as large as 2 times of the original in dry season and the velocity
increases remarkably near the shoal. A circular flow area forms near the groynes
at the south bank of the east mouth, where deposition will occur, favorable to
the maintenance of a stable navigation channel.
The optimal alternative is then tested in the movable bed model. The result shows that the river bed near the groyne head at the south bank of the east mouth is scoured and the navigation channel is gradually widened and extends westerly. The sediment lifted from the shoal is partly carried away by the flow and partly deposits in the reach 250 m away from the west mouth of the navigation channel, forming new shoals blocking ships passing. In the case of flow coming from the West River, part of the depositing sediment at the west inlet of the navigation channel is carried to the east inlet and deposits there. After a year, the shallow reach in the east inlet is 200 m long. The above situation shows that the key to the improvement of Laoyazhou Shoal is to make the discharge increase. However, the discharge should yet be controlled to avoid severe scouring or deposition. If not, the shoal will be scoured rapidly and extra-saturation of sediment will form at the west inlet of the navigation channel. In order to control the discharge, the crest elevation of the lock dam in the right branch should be lowered. In addition, according to the modeling, when a large discharge occurs in the North River, the navigation channel at the west inlet of the reach has a trend of moving southerly. While the navigation channel at the south part of the west inlet will be scoured first when a large discharge occurs in the West River. The channel is then unstable. Therefore, the navigation channel at the west inlet needs to be stabilized. For this purpose, the scheme tested is then improved as follows:
·
Lowering the crest
elevation of the lock-dam in the right branch to control the discharge
·
Building 4 more groynes
at the west inlet of the navigation channel to stabilize the channel
A finally proposed scheme is then formed.
After improvement,
the sediment carrying capacity of the flow at the east inlet is strong and the
navigation width is large. The transition zone in the middle of the reach
decreases and the width increases. After a hydrological year, according to
observation, Laoyazhou Navigation Channel is generally scoured, a navigation
line 2.5 m deep forming and the navigation width exceeds 80 m (Fig. 3). The
project has only a slight effect on the water stage in the near reaches. For
example, in flood season, the largest water stage rising can only be 3 cm.
Fig.
3 The flow pattern in laoyazhou reach after improvement
Sixianjiao is an important way between the West and the North Rivers. The west mouth links to the West River, and the north mouth links to Laoyazhou Reach. The south bank of the west mouth is a steep hill, extending into the river. The rear part of Qinsha shoal is 500 m upstream of the hill. Since the flow in the left branch of the West River goes towards the hill, 2 streams of vortex flow forms in flood period. One goes up to the surface from the bed near the hill, another goes upstream. The material carried by the vortex flow partly deposits on the rear of Qinsha Shoal. Under the effect of the vortex flow and the flood flow from the West River, the flow pattern is very complicated at the west mouth. The reach is then a risk section for navigation. After passing through Qinsha Shoal, the flow from the left branch of the West River diverges and part of the material carried by the flow deposits on the rear of Qinsha Shoal. In addition, the material deposits under the effect of the upstream-ward vortex flow. The rear of the shoal extends downstream. The smallest width for navigation at the west mouth was only 30 ~ 40 m in November, 1994. Navigation was severely affected. According to the model observation, when flood occurs in the West River, the flow is turbulent with high velocity near the west mouth, forming large or small vortexes. Between the downstream lower flow and the opposite vortexes, a transition zone exists, where the velocity is small and sediment deposits. The main problems to be solved in the improvement are “risk” and “shallow”. Theses two problems are related to the hydrodynamic condition of the flow in the left branch of the West River.
In order to seek the way for removing the vortex flow at the west mouth, controlling the downstream extension of Qinsha Shoal, and improving the navigation condition of the west mouth, many scheme alternatives are tested in the model. Finally, an optimal alternative is selected. That is, building a low lock-dam in the left branch at Qinsha Shoal to weaken the flow in the left branch. The hydrodynamics in the left branch can then be weakened and the flow overflowing the dam can scour the bed material to make the channel deeper. In addition, a new navigation channel is to be built by dredging.
After construction, the flow pattern at the west mouth is remarkably improved, and vortex flow vanishes. The problems of “risk” and “shallow” are solved. In addition, the sediment in the left branch, coming from upstream reaches, can be carried downstream through the new navigation channel. The sediment source of the shoals at the west mouth is then be controlled. In the cases of flood flow, medium flow and low flow, the dam-overflowing flow can scour the shoals. Two years later, the inlet navigation channel has been widened, the water depth on the shoals increases, and the shallow reach near the west mouth has been deepened due to scouring. The improvement for the navigation channel at the west mouth is proved to be sucessful.
The improvement of rivers in south China is often difficult because of the large number of shoals and complicated boundary and water/sediment transport condition. In this experimental research, remarkable improvement effect is achieved through a deep probing into the shoal-forming mechanics and making a particular plan for the improvement project. Laoyazhou navigation channel is improved by controlling the flow in the right branch, eliminating the circular flow, and destroying the deposition condition. The channel originally with serious deposition becomes a smooth, excellent navigation channel. Dredging is not necessary any longer. For the navigation channel at the west mouth of Sixianjiao, appropriate measures are taken to remove the vortex flow, relaxing the “risk” and “shallow” conditions. The 76 km long Dongping Waterway could meet the demand for ships of only 300 ton to sail through before improvement because of the large number of shoals and serious deposition. After improvement, the depth of the whole reach is larger than 2.5 m, can meet the demand for ships of 1000 ton. The navigation condition has remarkably changed, making the reach become a “Golden Waterway” in Guangdong Province. The successful improvement of Dongping Waterway largely promotes the local economical development and improves the investing environment. Achieved is a large economical and social benefit.
References
[1] Dou Guoren, “Research on the similarity laws for whole sedimentation model”, Science Announcement, 1978.
[2] Ao Daguang, Li Suchiong, “Experimental Research on the improvement of Laoyazhou Shoal”, Sediment Research, Vol. 1, 1996.
[3] Ao Daguang, “Scouring and Deposition Analysis for Modaomen Outlet in Pearl River Delta”, Symposium of 2nd International Conference on River Sedimentation, Public House of Water Conservancy and Electric Power, 1983.
[4] Ao Daguang, Wu Tiansheng, “Experimental Research on the Improvement Of The West Mouth of Sixianjiao”, Pearl River, Vol. 6, 1996.