Wang Caihuan, Huang
Guobing and
Chen Yuanming
Yangtze River Scientific Research Institute, Wuhan,430010,China
Tel: 86-27-82829863, Fax: 86-27-82633828
Abstract：For the flood relief and energy dissipation characteristics of the Geheyan Project, the basic ways of energy dissipation and scour protection were studied, and the new type dissipater-combined dissipation of asymmetric flaring piers and a stilling basin was developed. This made thatch difficult problem of flood relief and energy dissipation of a high arch dam built on a narrow river channel had been solved. The successful experiences from this project can be used in conducting the energy dissipation and scour protection design of other high arch dams.
Keyword: high arch dams, flood relief, flaring gate pier, stilling basin, energy dissipation and scour protection
The Geheyan Project, located in a gorge type reach of the Qingjiang River, 9.0km away from Changyang county town of Hubei Province, is a large-scale water conservancy project. It consists mainly of powerhouses, concrete dam and ship lift. 4 turbine generators, each having an installed capacity of 300 MW, were installed in the Right Bank of the river. A two-stage ship lift was built in the left bank, allowing a 300t-grade ship team passes through. On the main riverbed, a concrete gravity arch dam was constructed. The dam is a first-order structure, on which 7 surface spillways and 4 deep outlets for flood relief as well as 2 bottom outlets for diversion and emptying were set up. The dam was designed according to a 1000-year frequency flood (the corresponding reservoir level was 202.8m E.L. and the flood discharge was 21900m3/s) and was checked using a 10000-year frequency flood (the level and the discharge were 204.7m E.L. and 23900m3/s respectively). The dissipater was designed according to a 100-year frequency flood (the level and the discharge were 202.0m E.L. and 13000m3/s respectively) and was checked using a 1000-year frequency flood.
The characteristics of flood discharge and energy dissipation for the Geheyan Project would include：(1) Large discharge, high water head and large power due to flood Only in surface spillways would the power reach 17450MW. (2) Dissipation zone would be located in a reach made of shale, which is weak in lithologic character and lower in anti-scour ability. The anti-scour velocity in the zone would be only 3.5m/s. The dam toe would have only a distance of 110～180m from the interface of limestone and shale downstream of it. Therefore, if adopting the scheme of jet for energy dissipation, all the scour pits would have been in the shale-made reach; if using the scheme of bottom current for dissipation, the stilling basin would have been partly located at the shale-made zone, and the scour zone downstream of it would have entirely been on shale. (3) Using the way of discharging flood through the arch dam body, all streams of discharged jets would be concentrated together along the radial direction of the dam axis. (4) The Geheyan Dam would be 151m in height. If using the ordinary stilling basin, the joint of the downstream dam surface and the bottom of the stilling basin would have been relatively complicated, and the flow-passing surface would have suffered from a high velocity flow of 45m/s, thus requiring it have a stronger cavitation resistance ability. (5) Because the shale-made reach downstream of the dam became wider and wider, and the shale-made bank slope was a deeply weathered layer, it would be not suitable for the downstream cofferdam to be built in a place for from the dam axis, therefore limiting the possibility of constructing a ordinary stilling basin. (6) After sealing the diversion tunnel, there would still be for three-period flood to pass through the dam gap. In order to prevent the dam toe from scour, it would be initial to establish a section of apron.
For the above mentioned characteristics, based on a great number of investigations on jet dissipation and bottom current dissipation, we presented the combined dissipater scheme, consisting of flaring gate piers, added jets and a tilling basin. The flaring gate pier was a new type dissipater firstly developed in China, and has been used in some water conservancy projects such as Panjiakou, Ankang, Wuqiangxi and Yantan. And high-energy dissipation rate and remarkable economical benefit have been obtained. However, the application of this new type dissipater to a high arch dam had not been carried out prior to the Geheyan Dam. The difficulty was that when an arch dam began to overflow there would be a problem of flow in-radial-direction concentration downstream of it. How to safely and economically solve the problem was a key of applying the faring pier technique to high arch dam.
Based on a great number of experimental studies on the arrangement of the Geheyan Project’s flood discharge and energy dissipation a comprehensive harnessing method, to disperse kinetic energy due to flood discharge for decreasing the scouring force from the discharged flow, and to reinforce the river bed for increasing its anti-scouring capability, were presented.
According to the design, the Geheyan Dam would have 7 surface spillways for overflowing with a maximum flood discharge of 17060m3/s, the discharged power total 17450MW. The test conducted on the model found that: when using symmetric flaring gate piers, due to the surface outlet becoming gradually narrow in the section of the flaring gate pier the jet nappe out from it elongated longitudinally and dispersed fully in air, and formed three-dimensional submerged jump as it went into the stilling basin, thus being able to achieve a very good effect in energy dissipation. However, the fact was that the nappes out from 7 surface spillways concentrated in the middle part of the stilling basin along the radial direction of the dam axis. Therefore, when they fall into the basin, the kinetic energy per unit width would be completely unequal each other. And a large-scale backflow produced in each side of the basin, and then became refraction flow downstream of it.
According to hydraulic characteristics of flow out from the symmetrically contracted sluiceway in the stilling basin, a scheme to solve the flow in-radial-direction concentration problem using asymmetric flaring gate piers was presented. That is, using the sluiceway’s both boundaries to force the overflow to change its movement inertia along the radial direction so that all the jet nappes out from the surface spillways would be parallel each other, and would have the same direction as the river flow. Through careful investigation for the shrinkage dimension of both sides of each sluiceway, shrinkage values of the sluiceways were determined as show in table 1.
Table 1 The shrinkage parameters of the sluiceways
|
Number of surface spillways |
① |
② |
③ |
④ |
⑤ |
⑥ |
⑦ |
|
Shrinkage value at the left (m) |
5.9 |
5.3 |
4.7 |
4.5 |
4.3 |
3.7 |
3.1 |
|
Shrinkage value at the right (m) |
3.1 |
3.7 |
4.3 |
4.5 |
4.7 |
5.3 |
5.9 |
|
Width of sluiceway at the exit (m) |
3.0 |
3.0 |
3.0 |
3.0 |
3.0 |
3.0 |
3.0 |
|
Shrinkage rate of the sluiceway |
0.25 |
0.25 |
0.25 |
0.25 |
0.25 |
0.25 |
0.25 |
Experimental studies
showed that when the flood discharge through 7 surface spillways reached 7100m3/s
or even 17060m3/s, the 7 corresponding nappes would parallel fall
into the stilling basin. Apart from consuming a large amount of their kinetic
energy in air, the nappes also carried a lot of air reels with them into the
stilling basin, resulting in-its volume expanding sharply and energy being
dissipated fully, Besides, there was a good current pattern and the downstream
river channel was scoured slightly. The impact pressure (△P) due to the nappes at the bottom floor of the stilling basin only
was about 40KPa. △P/γ＝Pmax/γ-ht, in which Pmax is the maximum
hydrodynamic pressure at the bottom, and ht is the difference of
elevation between the downstream water level and the bottom. All this has showed
that the method of using asymmetric flaring piers to solve the discharge
in-radial-direction concentration and energy dissipation and scour protection is
very useful. The arrangement of the flaring piers is shown in Fig.1.
Fig. 1 Plan of the sluiceways of the Geheyan Arch Dam
The Geheyan Dam was planned to have 4 deep outlets for flood discharge, with a maximum discharge of 4000m3/s. In conducting the model tests, two types of tunnels, flat bottom type and downward bent type, were compared and studied, and several types of exits of the tunnels, such as laterally dispersed, laterally dispersed plus diversion piers, big flip bucket and laterally contracted were compared and studied. According to various indexes, such as the deep outlets’ discharge capability, flow pattern, pressure distribution and cavitation properties at their different parts, the discharge’s combination condition with that from the surface spillways, as well as improvement of downstream dissipating effect, the downward-bent-type tunnel had the advantage over the flat-bottom-type one, and at the exits of the deep outlets the scheme of using asymmetrically-and-laterally-contracted narrow gap dissipater had the advantage over the other ones. The lateral contraction parameters of the 4 exits are listed in Table 2.
Table 2 Lateral contraction parameters of the exits of the deep outlets
|
Number of deep outlet |
1 |
2 |
3 |
4 |
|
Contraction value at the left (m) |
1.6 |
1.5 |
1.0 |
0.9 |
|
Contraction value at the right (m) |
0.9 |
1.0 |
1.5 |
1.6 |
|
Gap width (m) |
2.0 |
2.0 |
2.0 |
2.0 |
|
Lateral contraction rate β |
0.44 |
0.44 |
0.44 |
0.44 |
Experimental studies showed that at the exits of the deep outlets using asymmetric narrow gap dissipaters may result in the jet nappes fully dispersing longitudinally and not radically concentrating laterally, and all the nappes goring towards the same direction as the river flow. Because the water passing cofferdam (i.e. the secondary dam) downstream of the Geheyan Project would be 24.0m higher than the basin’s bottom, when the 4 deep outlets discharged the pressure at the bottom would distribute in the same way as hydrostatic pressure, the maximum impact pressure in the zone impacted by the jets being only about 10 kPa. This showed that using the asymmetrically contracted narrow gap dissipater had a remarkable effect on protecting the deep outlet’s discharge from radial concentration and dispersing its kinetic energy before it going into the stilling basin.
The Geheyan Dam would have 2 bottom outlets for flood discharge, which were mainly planned to be used for diversion during later construction and emptying the reservoir. Because the 2 diversion bottom outlets would be built in No.14 and No.15 dam sections located at the central river, with a nappe-into-basin width about 1/4 of the stilling basin width, the main stream would concentrate. This would result in large backflow producing in both left and right zones of the basin and refraction flow appearing. On the model, several schemes had been studied, including laterally expanded exit and bucket combination, Laterally expanded exit and diversion pier combination, direction-turned plane and inclined bucket combination as well as laterally contracted exit, etc. But all these schemes were abandoned finally because of the nappes not fully dispersing laterally and the flow conditions in the basin being undesired. In order to make the nappe going into the basin fully disperse longitudinally and laterally, and to avoid the main stream from concentrating and the occurrence of refraction flow, based on summing up the above mentioned schemes a new scheme-warped angle-adding bucket lip was tested. The experimental results showed that the nappe out from this bucket dispersed in all directions, and the streams into the basin were uniform and had different directions; the maximum total width of the two nappes when going into the basin was 76m, 2/3 of the basin width, while the maximum length was 30m; in the basin the current was flat without poor flow patterns such as backflow; the pressure at the basin bottom behaved like hydrostatic pressure without obvious peak values.
Since 1996 when the Geheyan Project was completed fully, a great number of observations and investigations have been conducted on safety and energy dissipating effect of its flood relief structures. The observed results show that using the asymmetric flaring piers on the project’s surface spillways for overflowing and using the asymmetric sill-dropping and narrow gap dissipater on the deep outlets for flood relief have not yet been harmful to the flood relief structures themselves, and no cavitation phenomenon has been found on the flow passing surfaces. Dissipation conditions of the nappes out from the surface and deep outlets and flow patterns in the stilling basin generally conform to the results achieved on the model, but the atomization phenomena of the prototype are more serious than that of the model without harmfulness to the switch station and the highway going into powerhouses. Going through the trials of the 1998 catastrophic flood, the flood relief structures were safe and sound, and the downstream river channel was scoured slightly. After the 1998 flood, the stilling basin was pumped out and inspected. It was found that a little damage appeared in its bottom, which can result from the remaining sediment in the basin during construction.
Through careful researches the Geheyan Project has successfully used the asymmetric flaring pier and narrow gap dissipater technique, which together with the stilling basin formed a combined dissipater, thus solving the difficult problems from the project, such as shorter front edge of the overflow, large flood relief power, radial concentration of the discharge and poor anti-scour ability of the downstream river channel, and achieving good energy dissipating effect and economical benefits. These successful experiences will play an active role in developing the energy dissipation and scour protection technique of high arch dam.
References
[1] Chen Chanting, Large Flood Discharge Structures in High Dam. Beijing: Water Conservancy and Electric Power Publishing House, 1988.
[2] Liu Peiqing, Flow Characteristics in Plunge Pool And Its Comprehensive Assessment Indexes. Journal of Yangtze River Scientific Research Institute, 1997(1).
[3] Yang Qing, Flood Relief And Energy Dissipation Design of The Geheyan Project, Yangtze River, 1992(2).
[4] Xu Bingheng, Proceedings of Release Works and High Velocity Flow. :Chengdu Scientific And Technical University Press, 1994,P120.