STUDY ON THE LAYOUT OF FLOOD DISCHARGE 
FOR XIAOWAN PROJECT

Chen Jie, Sun Shuangke, Guo Jun and Zhou Sheng

China Institute of Water Resources and Hydropower Research

A1, Fuxing Road, Beijing, 100038, China

Tel: 86-10-68515511-1913, Fax: 86-10-68538685

Abstract: In the paper, interfaced with xiaowan Hydroelectric Project, model studies are mainly focused on the layout and type of flood-release facilities in the arch dam. Based on measurements of the mean dynamic water pressure and impact pressure, the fluctuating pressure on bottom plate of plunge pool, and the tail water fluctuation of power plant and so on, the layout and types of flood-release facilities are improved and optimized. By so doing, it is capable to improve the return flow in the plunge pool, which induces the low pressure area, avoid outlet nappes to be adjacent and overlapped, decrease the immersion angle, lower the impact pressure on the bottom plate of plunge pool, and as a whole to meet the design requirements.

Keywords: huge arch dam, large-discharge, flood dissipation, experimental study

1  INTRODUCTION

Since 1960’s, double curvature arch dam, one of the economic dam type, has been applied to many projects. But the discharge capacity released by the flood-discharging facilities in the dam is relatively seldom in the early projects. For example, the yinguli Project in the former Soviet Union, which is the highest double curvature arch dam with 272m height now, the maximum discharge capacity is 3150m³/s and merely 2200m³/s discharge is released by the flood-release facilities in the dam. ailkaiyong double curvature arch dam in the Honduras, the height of dam is 226m, the maximum discharge and the discharge released by the dam are 8590m³/s and 4590m³/s respectively. However, the several high arch dams in China such as Ertan Project under operation, xiaowan, Xiluodu, Goupitan Project under-design all exceed 230m and the discharge capacity exceeds 20000m³/s. In order to ensure the dam safety and save investment also, so it is important to study the flood-release facilities in the dam, increase the discharge capacity released by the dam and reduce the scouring of riverbed.

xiaowan Project, in Yunan Province in the southwest of China, is located on the middle-downstream of the Lancang River. The terrain at the dam site is of symmetrical V-shape. The type of the dam is parabolic double curvature arch dam. The project is mainly for power generation with installed capacity of 4200 MW. It consists of a 292m high double curvature arch dam which is the highest one in the world, a right bank underground power house, three types of discharge facilities which are 2 left bank spillway tunnels, 5 surface spillways and 6 middle-level outlets in the dam. A large plunge pool is located just downstream of the dam and a secondary dam creates a deep-water cushion for the energy dissipation. Total discharge capacity is 20684m³/s. Correspondingly, the flood discharging power is 40000 MW, which takes the leading ranking in the flood discharge power in the world at present.

The objectives of this study are to reduce one spillway tunnel by increasing the discharge capacity released by dam itself, to lower the dynamic water pressure acting on plunge pool and achieve non-lining or part lining for the bottom of the plunge pool, thus saving the investment. In order to achieve these objectives, in the paper, the immersion angle, unit discharge (or immersion discharge of per unit area) and effective volume of plunge pool (water depth) were regulated by improving the layout and types of the flood-release facilities and plunge pool. It aims at lowering the impact pressure and fluctuating pressure on bottom plate of plunge pool and consequently attaining the optimal effect under all operation conditions.

2  MODEL BRIEF DESCRIPTION

Based on Froude Similarity Law, the hydraulic model was built in the scale of 1: 120. Because the impact pressure is the main criterion to evaluate the energy dissipation of the arch dam, for the purpose of measuring the mean dynamic water pressure and the fluctuating pressure, 251 piezometric tubes were distributed on the bottom plate and bilateral slopes of plunge pool. Besides, more than one hundred pressure sensors being adjacent to piezometric tube, were installed in the nappes impact zone when surface spillways operate alone or surface spillways and middle-level outlets discharge together.

In preliminary design, 6 middle-level outlets with dimension of 65m (widthheight) are arranged symmetrically at three different elevations and angles of bucket. Main parameters are shown in Table 1. 5 surface spillways with 1115m in size, which are alternatively provided with six middle-level outlets, the crest level of which is El.1225m, the section curve upstream the crest is of an ellipse and is of a parabola downstream, followed by a slope, discharge water at different angle of bucket (longitudinal separation). At the same time, water flows, which is 4m far away downstream the crest are spreaded in horizontal direction (transverse spread). So the unit discharge is decreased. The surface spillways of NO.1 and NO.5 are symmetrically located, but NO.2 and NO.4 are unsymmetrical. Main parameters are shown in Table 2.

                 Table 1  Main parameters of middle-level outlets

                Table 2  Main parameters of surface spillways

3  OPTIMIZATION STUDY OF ENERGY DISSIPATION

3.1  Study on the layout and type of flood-release facilities

Study on middle-level outlets

By means of model experiment, the size of middle-level outlets were selected to be 6m7m instead of 6m5m in the preliminary design and a 1/4 ellipse curve was added to the entrance upper boundary for upstream emergency gate. At the same time, for the purpose of improving the flow patterns and pressure distribution, the top curve between the intake to gate groove were descended. Besides, the height of emergency gate was also decreased to 11.475m from 13m. Thus save the investment.

In order to measure the pressure distribution in middle-level outlets, more than 40 piezometric tubes were installed along the middle of the top and bottom plate in NO.4 middle-level outlet with the highest working water head. The experimental results show that negative pressure is not appeared and pressure distribution is normal and reasonable under all pool levels. It verifies that the optimization of type configuration is correct.

Study on surface spillways

Optimization on the layout and type of surface spillways was performed also. In the early stage, the experimental results indicated that the flow patterns and impact pressure of plunge pool were the most sensitive to the layout of surface spillways, especially the shape of bucket. It has three deficiencies for the layout of surface spillways. The first is that the NO.2 and NO.4 surface spillways are unsymmetrical so as to induce the backflow in the plunge pool and the low-pressure zone. The layout also is convenient for the operation. The second is that the angles of depression of the surface spillway NO.2 is too large thereby leading to the impact pressure of the plunge pool increases obviously. The third is that outlet angles of spread of the surface spillway NO.2 and NO.4 are too large, to thus leading to two nappes overlapped by two adjacent flows thereby limiting nappes to spread near the bottom plate and increasing the impact pressure as well.

In order to overcome above deficiencies, firstly the surface spillways of NO.2 and NO.4 are arranged symmetrically, their angles of depression are 20and outlet angles of spread are 7.68, the outlet horizontal section is in the shape of half-ellipse. With these improvements, flood discharge will be operation easier and also will avoid the backflow and low-pressure area. However, at the 1240m level, while surface spillways discharge flood alone, the maximum mean impact pressure appeared near the middle of the plunge pool section that the chainage is 0+130m, namely the site that two nappes are adjacent and overlapped, are near 159.8kPa. But as the water level increases, its value decreases on the contrary. It is the cause that ,for the surface spillway NO.2 and NO.4, the outlet angle of spread (7.68)  is too large and the outlet dispersion section adopted a half-ellipse curve. Thus two nappes begin to overlap at the low pool levels. It reduces the jet-trajectory distances and limits nappes to spread, thus resulting in the increase of impact pressure reaching to the highest at the 1240m level. When water level increases, the maximum mean impact pressure gets lower because the overlap and collision of two nappes are intensified and nappes are more dispersed.

For the problem exit after the first stage modification, the surface spillways of NO.2 and NO.4 are further improved. A 1/4 ellipse replaces the shape of half-ellipse of outlet. But the results show that the problem of the adjacent overlap remains unsolved if only the shape of outlet is modified. The dynamic water pressure and impact pressure are lowered little while surface spillways discharge flood alone. In order to avoid nappes to adjacent overlap and not to shrink the leading edge width of outlet, the outlet angle of spread of the both are reduced to 4.53from 7.68. The shape of outlet retains a 1/4 ellipse. And the left sidewall of NO.2 and the right side wall of NO.4 are extended to upstream edge dispersion section. With these modifications, nappes are not overlapping and the impact pressure declines obviously.

3.2  Flow patterns and nappe characteristics

By above optimization, nappes of middle-level outlet fall into plunge pool in three layers. Five surface spillways are symmetrically arranged at three differential elevations. Nappes also split into three layers. Each nappe does not attach and overlap to the other. Under all reservoir water levels, nappes characteristics parameters, either middle-level outlets and surface spillways discharge flood alone or together, are shown in Table 3.

(1) For the surface spillway, the minimum jet-trajectory distance is 85m from the toe of dam. Because their outlets adopt dispersion bucket, nappes disperse sufficiently and unit discharge is small which changes in the range of 33 to 42m³/s.m. It is helpful to lower the impact pressure of the bottom plate in plunge pool.

(2) For middle-level outlets, the immersion site of nappes is far from the toe of dam. Unit discharge is between 33 to 42m³/s.m.

(3) When surface spillways and middle-level outlets discharge flood together, the minimum jet-trajectory distance of nappes is 100m from the toe of dam. Although unit discharge increases, with the collision and aeration, the total immersion range and main range of nappes also increase obviously, but immersion discharge of per unit area is not so large thereby mitigating the impact in the bottom plate. In the immersion zone, main nappes fall to the vicinity of the bottom plate, and the water flow is strong turbulent. But the fluctuation gradually attenuate at the chainage of 0+350m, where the plunge pool turns at to the left 10for the purpose of decreasing the excavation. The flow after secondary dam is relatively calm. The tail water fluctuation of power plant is small and meets the demand of design.

      Table 3  Nappes characteristics parameters and tail water fluctuation

3.3  Impact pressure acting on plunge pool

Because the impact pressure is the important criterion to evaluate the energy dissipation layout plan of arch dam, the maximum mean impact pressure of plunge pool with reinforced concrete lining can not exceeds 309.8kPa as specified abroad. But the value was controlled under 159.8kPa when evaluated the ERTAN project in China. Such criterion is applied to all other arch dam with high-head and large-discharge in China.

Under all operation condition, the maximum mean dynamic water pressure P_{m_max} and impact pressure P_{m_max}, the fluctuating pressure mean square root σand the maximum impact pressure P_max（P_max=P_{m_max}+3σ, the occurrence probability of which is 0.27%）measured with pressure sensors are shown in Table 4. The data in the Table 4 indicate that the magnitude and tendency of the maximum mean impact pressure measured with pressure sensors and piezometric tubes are agreeable and confirm the reliability of the data each other. Whether surface spillways and middle-level outlets alone or together, the value of P_{m_max}, σ and P_max all are far less than the design control value (P_{m_max} <159.8kPa，σ<7.59.8kPa，P_max <329.8kPa). It indicates that the optimization plan is promising.

        Table 4  The pressure characteristics values （unit: 9.8kPa）

Fig. 1   Longitudinal distrbution of Pm_max Pm_min
(H_upstream = 1243.00m)

According to Table 1~2 and Figure1~3, under all operation condition, the main impact range in the plunge pool is not large. The range is 0+120m～0+165m, 0+280m～0+330m, 0+180m～0+230m respectively when surface spillways and middle-level outlets discharging alone or together. Because the impact pressure is lower when middle-level outlets discharge flood alone, if lining is adopted to the plunge pool, the main protection ranges will be concentrated from 0+120m to 0+230m. The thickness of plate as well as the diameter and number of anchor bars can be reduced downstream. Upstream chainage 0+120m, the water surface surge is small and the dynamic water pressure fluctuates slight as well, thus the area need not be protected. The area between chainage 0+110m（or 0+100m）to 0+120m can be served as transition zone. In order to avoid nappes impacting directly to the bottom plate, the plunge pool should be always full of water with small opening before flood discharge.

Fig. 2  Longitudinal distribution of Pm_max,s for surface spillways discharging alone

Fig. 3  Longitudinal distribution of Pm_max, s  for surface spillways and
 middle-level outlets together

In addition, according to power frequency spectrum of fluctuating pressure and the probability density distribution acquired with sensors, the fluctuation in the impact zone is low frequency and the probability density distribution approach to normal distribution.

4  CONCLUSIONS

The study of energy dissipation scheme in arch dam is actually by optimizing the layout and types of flood-release facilities and plunge pool, regulating the immersion angle, unit discharge (or immersion discharge of per unit area) and the effective volume of plunge pool (water depth), to make the vertical velocity component of flow as small as possible when the nappe touches to the bottom and lower the impact pressure and fluctuating pressure of plunge pool bottom plate, thus attaining the optimal effect under all water levels. In the study, after the layout and type of surface spillway and middle-level outlets are optimized, the experimental results show that nappes disperse sufficiently in the longitudinal and transverse direction, immersion unit discharge or immersion discharge of per unit area is small. Under all operation condition, nappes, either surface spillways or middle-level outlets, fall into plunge pool in three layers. Nappes collision is quite good as expected. The maximum mean impact pressure, the fluctuating pressure mean square root and the maximum impact pressure all are far less than the design control value. So an optimization scheme of energy dissipation for arch dam is acquired.

References

[1]  The study on increasing flood discharge in arch dam and the cancellation of one spillway tunnel (the study of discharge distribution in arch dam) for Xiaowan Project, Hydraulics Department, China Institute of Water Resources and Hydropower Research (iwhr), 1998.4.

[2]  The experimental study on flood discharge for Xiaowan Project, Hydraulics Department, iwhr, 1995.5.

[3]  The study on increasing flood discharge in arch dam and the cancellation of one spillway tunnel (interim report) for Xiaowan Project, Research Department, Kunming Hydraulic Investigation and Design Institute，1998.3.

[4]  The study of vibration and a new type plunge pool for high-head and large-discharge arch dam, Water Resource and Harbor Engineer Department, Tianjin University, 1996.3.

[5]  The optimization study on flood discharge for Xiaowan Project, Hydraulics Department, iwhr, 1999.8.