RESEARCH ON THE NAVIGATIONAL CONDITION IN THE DIVERSION CHANNEL OF TGP BY MODEL SHIP TESTS AND PROTOTYPE VERIFICATION

Yan Wei, Xu Qinqin, and Chen Yongkui

Yangtze River Scientific Research Institute, Wuhan 430010, China

Telephone and Fax Number: 86-27-82633828

Abstract: The diversion channel is being used as a navigational pass during the second construction phase of the Three Gorges Project (TGP). By means of model ship and hydraulics tests, the essential problems on the sailing line optimization, the methods of enhancing navigable discharges, protection measures of anti-scouring during flood period and scouring effects on navigation, etc., have been appropriately solved. The important main  data for design and construction were provided based on the experimental results which have been well verified by prototype barges-tow tests and the navigational practices during1997 -1998. The test results obtained from physical model and prototype indicated that the maximum navigable discharge in the diversion channel is restricted by up-bound passage. If excessive navigable discharge is required, quickening the vessel speed with special methods is an effective way. The navigational practices reveal that the model ship test results can correctly reflect the prototype situation, including the effect of the flowing water on vessels and the maneuver characteristics of the vessel. Therefore, model ship test is a effective method in the researches of sailing line optimization, navigable discharge enhancement, scouring effect on navigational condition, etc.. The model test results revealed that some regions in the diversion channel would be scoured due to the high flowing water velocity and the navigational condition would be affected. Therefore, some protection measures were conducted in prototype. The prototype investigation results indicate that the protection measures are effective. The navigational condition in the diversion channel after the flood reason in 1998 does not varied with the only exception of a special section in the channel downstream of the dam where the channel was scoured a bit, and the navigation difficulty in this section rises a little. The anti-scouring protection of this section should be strengthened.

Keywords:  the Three Gorges Project (TGP), navigable diversion channel, model ship test, sailing line optimization, prototype verification

1    INTRODUCTION

The second construction phase of TGP adopts the navigational plan of the diversion channel as well as a temporary navigation lock. It is very simple and reliable for vessels to pass the diversion channel when the discharge through the channel is low. Moreover, the transportation capacity of the channel is very large. Every year, about 4/5 of the whole transportation loads will be undertaken by the diversion channel in the construction of TGP. Therefore, it is very important and essential to study and solve the navigational problems in the diversion channel to ensure regular navigation in the second construction phase of TGP.

The profile of the diversion channel adopts the composite cross section, that is, the bottom elevation of the same channel cross section is not equal. The minimum bottom width of the channel is 350m. To meet the demands of design, besides researching on arrangement types of the diversion channel, Yangtze River Scientific Research Institute has done a lot on studying hydraulic and navigational characteristics of the diversion channel for many years before and during the construction phase. After having ascertained the arrangement plan of the diversion channel and the type of the profile's embankment head, theoretical analysis reveals that navigational condition optimization and navigable discharge enhancement of the diversion channel rely on reducing the resistant forces on the vessels passing the diversion channel. The resistant forces stem from the water flow velocities, flow patterns and water level slope in the diversion channel, and the ship maneuver characteristics, etc.. Therefore, a remote control model ship was used in the research procedure because model ship tests can represent the influence factors mentioned above. This paper presents the research results by the way of model ship tests in sailing line optimization, navigable discharge enhancement, and reduction of the influence on the navigational condition due to flood scouring in the channel. The research results have been well verified by prototype barges-tow tests and the navigational practices during 1997 –1998, which represents that the design, research and construction levels of the diversion channel with navigational requirement in the second construction phase of TGP possess the advanced level of science, and the research results have important practical values.

2    MODEL SHIP TESTS

2.1    Determination of the sailing lines and their optimization in the diversion channel

The distribution of the water flow velocity in the diversion channel obtained from the hydraulic model test in the condition of 20000m3/s discharge( navigable discharge for barges-tow3×1000t+1942kW) is shown in figure1.

By doing many model ship tests on general layout hydraulic model with the same scale as the model ship, two primary sailing lines are discovered, one is along the left bank, and the other the right bank. The rudder area of the remote control ship model was adjusted according to the requirement of the size scale correction before tests. It is evident from the navigation parameters obtained by model tests that any one of the two lines can fulfill the f navigational design demands. However, there exist rough navigation sections in both lines, particularly the sections of 1250-1600m downstream of the dam axis, and 225-505m upstream of the dam axis for the right bank line. The test data indicate that the sailing velocities relative to the bank in the rough sections are lower due to the higher resistance. Therefore, the sailing lines need to be optimized. After the optimization tests of sailing lines, a third line is selected which avoids the rough navigational section 1250-1600m downstream of the dam axis the right bank line (see figure1).

The first sailing line (line 1, left bank) is the best in the three sailng lines because it can avoid the main rough navigation section. Furthermore, the sailing time passing the diversion channel is the shortest. The second (line 2, right bank) has a broad range of vision, and it will not pass cross the main stream in the channel and disturb the down-bound sailing routes. However, this line demand the vessels to overcome the rough section 1250-1600m downstream of the dam axis and the rouge section of 225-500m upstream of the dam axis. Moreover, the sailing time passing the channel is longer than that of the first line. The third line is better than the second for the vessels can avoid the most difficult navigational sections. Therefore, the third line can be taken as spare up-bound sailing line for large discharge condition in the channel. The sailing line for down-bound navigation is along the middle leaning right side of the channel upstream of the dam axis, but along the middle in the 260-1250m section downstream of the dam axis to avoid disturbing the up-bound sailing lines (lines 2 and 3) and being pushed to the bank by the swirling water in the curve reach of the diversion channel.

2.2    Research on enhancement of navigable discharge

The navigable discharges of design standard are Q=20000m3/s for Yangtze barges-tow

(3×1000t+1942kw) and Q=10000m3/s for local barges-tow respectively. It has been proved that the ascertained diversion channel scale has already fulfilled the design demands of navigable discharges based on the test results, the calculation and analysis of the resistance to the vessels passing the channel. The navigational conditions have been improved after the optimization of the sailing lines and local configuration adjustment of the diversion channel. For further increase of the navigable discharge, it is needed to subtract the load barges or accrete the power of the tow, etc.. The test results of barge-subtracting measures are listed in table 1 and table 2.

2.3    Test verification of the navigation condition after flood scoring and accretion in the diversion channel during the flood season in 1998

The navigation condition (flow pattern and velocities) based on the underwater landscape measured on the prototype in September 1998 is basically accordant with that of design plan of the diversion channel, and the navigation condition along the lift bank sailing line (line 1) is a bit better than that in design condition, the same for the right bank sailing lines (lines 2 and 3) with the exception of the 1500-1750m section downstream of the dam axis where the flow velocities and water level slope increase due to flood scoring(see table 3).

The ship model test results reveal the same tendency. The ship speeds relative to the bank decrease in the 1500-1750m section when passing along the sailing line 2 or 3.

3    PROTOTYPE VERIFICATION

3.1    The flow patterns and velocity distributions in different discharges

The discharges through the diversion channel in 1998 varied from 3500-63000 m³/s, with the navigational discharges from 3500-33200 m³/s. The flow patterns, velocity distributions and water level slopes were measured on the prototype. The prototype results are identical to those of physical model, and the comparisons are listed in table 6.

The velocity distributions and water level slopes obtained on prototype are closer to thos from hydraulic model test. For example, the maximum velocity and maximum slope along the sailing line of right bank obtained from model tests is 3.99m³/s and 1.41‰ respectively in discharge of 20000 m³/s , and 3.61m/s and 1.55‰ respectively on prototype in discharge 0f 19000m³/s.

3.2    The trial trip in tide season

A trial trip on the prototype was conducted during 20-24th, September 1998 to confirm the sailing lines determined by model tests, combined with the hydraulic investigation in the diversion channel. It is also carried out to try to determine maximum navigable discharge by prototype trial trip for the purpose of enhancing the transportation capacity of the channel. The discharge in prototype observation varied form 20000 m3/s to 50000m3/s, and different types of vessels were used in trial trip.

 The prototype investigation results coincide essentially with those obtained in model tests, which means that the design and construction of the diversion channel fulfill the design standard.

4    CONCLUSIONS

(1) So many hydraulic problems have been met in taking the diversion channel as navigational pass during the second construction phase of TGP. All the problems have been properly solved by careful design and long period hydraulic model tests, especially by ship model tests. Verification on the prototype with trial trip and by navigational practice reveal that the design and construction of the diversion channel fulfill the design standard, and model ship test is an effective measure in the research of navigational condition and can give a good forecast.

(2) The test results obtained from physical model and prototype indicated that the maximum navigable discharge in the diversion channel is restricted by up-bound passage. If excessive navigable discharge is required, quickening the vessel speed is an effective way or use some special equipment such as barge shifter.

(3) The vessels on the prototype pass the diversion channel basically along the sailing lines determined by model ship tests. The navigation practices indicate that the model ship test results can correctly reflect the prototype situation, including the effect of the flowing water on the vessel and the maneuver characteristics of the vessels. Therefore, model ship test is a effective method in the researches of sailing line optimization, navigable discharge enhancement, scouring effects on navigational condition, etc..

(4) The model test results revealed that some regions in the diversion channel would be scoured due to the high flowing water velocity and the navigational condition would be affected. Therefore, some protection measures were conducted in prototype. The prototype investigation results indicate that the protection measures are effective. The navigational condition in the diversion channel after the flood reason in 1998 does not varied with the only exception of  1500-1750m section downstream of the dam axis where the channel was scoured a bit, and the navigational difficulty in this section rises a little. The anti-scouring protection of this section should be strengthened.

References

[1]    Zhu Guangcui, Liao Zhenlian. Study on Navigation Hydraulic Characters in Diversion Channel during the Construction Phase of TGP. Journal of Yangtze River Scientific Research Institute. 1989(3).

[2]    Liu Lizhong, Liu Laiyi. Study on Navigation Hydraulic Characters Test in Diversion Channel during the Construction Phase of TGP.. Journal of Yangtze River Scientific Research Institute. 1995(4).

[3]    Chen Yongkui. Analysis on the Navigation Resistance against Real Barges-tow during the Second Construction Phrase of TGP. Journal of Yangtze River Scientific Research Institute. 1995(4).

[4]    Chen Yuanqing, etc. Hydraulic Observation on Prototype of the Diversion Channel of TGP. Beijing: Ocean Press, 1998.

 Table 1    The barges-tow velocities relative to bank along left bank sailing line in different discharges
                 (up-bound )     m/s

V1 -- velocity at the position 100m downstream of the downstream head of the longitudinal cofferdam

V2 -- velocity at the position 100m upstream of the downstream head of the longitudinal cofferdam

Table 2    The barges-tow velocities relative to bank along right bank sailing line in different
                discharges (up-bound )     m/s

Table 3    The water level slopes and  maximum flow velocities along the sailing lines in different
               discharges

L—distance to the dam axis; +, upstream; -, downstream

D—distance to the right bank

 Table 4  Navigational parameters along right bank sailing line in different discharges (up-bound)

  Table 5  Navigational parameters along left sailing line in different discharges (up-bound)

Table 6   Comparison of the model and the prototype results

Chainage unit: km before +; m after +.

Fig.1    Velocity distrbutions and upbound sailing lines in the condition of discharge=20000m³/s