Volume 6 hydro power 6 18 – recent achievements in hydraulic research in china

Volume 6 hydro power 6 18 – recent achievements in hydraulic research in china Volume 6 hydro power 6 18 – recent achievements in hydraulic research in china Volume 6 hydro power 6 18 – recent achievements in hydraulic research in china Volume 6 hydro power 6 18 – recent achievements in hydraulic research in china

J Guo, China Institute of Water Resources and Hydropower Research (IWHR), Beijing, China

© 2012 Elsevier Ltd All rights reserved

Glossary

Aerator A special device used to tract the air into the

bottom floor of a spillway tunnel or chute spillway It

consists of air vent, offset of ramp

Flaring pier gate One type of energy dissipater The pier

on the downstream part is expanded and the width

between the piers is reduced It is applied on the surface

spillway to form a 3D flow

Jet flow collision Collision of jet flows from the surface

spillway and the middle outlet before impinging into the

plunge pool to increase the ratio of energy dissipation

Orifice spillway tunnel One type of energy dissipater

It consists of one or several orifices installed inside a

spillway tunnel

Plunge pool A water body formed by a secondary dam built just downstream of the dam for dissipation of energy Slit bucket One type of energy dissipater The width of the flip bucket is contracted symmetrically or asymmetrically

It can be applied in the outlet of the spillway tunnel, chute spillway, surface spillway, and middle outlet The flow through the slit bucket is contracted latitudinally and dispersed longitudinally

Spraying Rainfall is formed by splashed jet flow with a high intensity during the discharging

Vortex shaft spillway tunnel One type of energy dissipater A vortex chamber is connected to a vertical shaft and then a spillway tunnel The vortex chamber can form a rotating flow

6.18.1 Introduction

The hydraulic research has achieved noticeable improvements as the hydropower projects have been developing at a faster rate in China since the 1980s, mainly on the new energy dissipaters, aeration and cavitation mitigation, pressure fluctuation and flow-induced vibration, flow discharging spraying, and prototype observations [1] Table 1 gives the typical characteristics of Chinese hydropower projects, which are high dams in narrow valleys with large discharge flows

General Report of the 13th Congress of ICOLD [2] gives statistics of discharge facility applications worldwide with the physical parameters of L/H and P and their combinations (see Figure 1) The author has put the parameters of some selected projects from China and the United States into the same figure for comparison

Table 1 and Figure 1 show that (1) most dams are over 200 m high and some are nearly 300 m high; the highest dam under operation is Ertan Arch Dam with a maximum height of 240 m and the highest dam under construction is Jinping Arch Dam with a maximum height of 305 m; (2) the discharge flow is over 20 000 m3 s−1 and the largest one is 102 500 m3 s−1 in Three Gorges Project; this indicates that the unit width discharge flow is usually over 200 m3 (s-m)−1; (3) more than one type of discharge facilities are found in different types of dams, such as the surface spillways combined with middle outlet, chute spillway, or tunnel spillway; (4) some new types of energy dissipaters are involved, such as flaring pier gates with stilling pool or with roller compacted concrete (RCC) stepped spillway, flip buckets with plunge pool, orifice spillway tunnel, or vortex spillway tunnel; (5) high head and large gates are used

As the complicated hydraulics is the key issue in the design and operation, and the characteristics of energy dissipaters of dams in China are difficult to determine, efforts have been made during the designing stage based on the physical model experiments To verify the scientific research and designing solutions, several hydraulic field observations on large projects have been undertaken when they are in operation

Comprehensive Renewable Energy, Volume 6 doi:10.1016/B978-0-08-087872-0.00603-X 485

Outlets on dam

stilling basin

1–5 � 7

H, dam height (m), P, 0.0098AZ (MW); Q, discharge flow (m3s−1); Z, head difference between design reservoir water level and original river bed (m); ●○, Xiaowan Dam; ▲ Δ, Eartan; ■ □, Goupitan; Φ, Mossyrock Dam Original figure is taken from GR 50 of the 13th Congress of ICOLD [2]; the marked points are made by author for comparison

6.18.2 Energy Dissipation

6.18.2.1 Slit Bucket

As the valley is usually narrow in the west and there is a large discharge flow during the flood season, the normal energy dissipaters are not suitable The slit bucket is specially developed for such kind of situations and it can make the flow contracted at the end of the bucket and project it dispersing in the sky longitudinally The advantages are high efficiency of energy dissipation and less scours in the riverbed The systematic physical model studies are conducted to understand its hydraulic characteristics The model tests have found out that (1) the Froude number in front of the slit bucket should be larger than 3.5, (2) the angle of the bucket can be changed between –10° and +45°, and (3) the scour in the riverbed can be reduced by 1/3 to 2/3 compared to the normal bucket with an angle of 30° [3] This kind of dissipater was first applied in the sky-jump spillway of Dongjiang Project in the early 1990s with the unit width discharge flow reaching 600 m3 (s-m)−1 The prototype observations performed in 1992 (see Figure 2) show a good relationship between model and prototype on jet flow and scour patterns although the discharged flow does not reach the design value [4]

This new technique has been widely applied to more than 10 projects in China and also included in the ‘Design Specification for River-Bank Spillway’

The spillway tunnel in Xiaowan Project is located on the left bank Reducing the riverbed and bank erosion is one of the tasks during the design as the river valley is very narrow and the rock on the right bank further downstream of the energy dissipation zone

is not strong enough to resist erosion

Four types of flip buckets have been studied (see Figure 3) [5] As the injection angle of flow in type (a) is too small, it results in jet flow

to close the right and erodes the right bank in the original design A large backflow appears along the left bank with a maximum return

Figure 3 Four types of buckets in Xiaowan arch dams (a) Tongue shape bucket (b) Tilted bucket I (c) Tilted bucket II (d) Slit bucket (Hmax = 292 m,

Q = 3535 m3 s−1)

flow velocity of 14 m s−1 The maximum depth of scour pit is 15.6 m and close to the right bank Another two types of flip buckets, type (b) and (c), have been proposed and tested The scours in riverbed have not been improved ideally The scours are still close to the right bank

Type (d) slit bucket is finally adopted by the design through optimization The direction of jet flow is adjusted and dispersed along the river channel The riverbed and bank erosion has been reduced greatly The configurations of the final design are that (1) the width of edge is reduced from 14.0 to 4.45 m with the contraction ratio of 0.3178 The axis of slit bucket is asymmetrical with the tunnel axis The left-side wall is 3.2 m from the axis of the central line and the right-side wall is 1.25 m from the central line (2) Two steps of contraction are selected on the right-side wall in which the first contraction is 27.251 m long with a contraction angle of 2.86° and the second contraction is 25.0 m long and a further contraction angle of 7.08° is applied (3) One step of contraction on the left-side wall is 25.0 m long with an angle of 8.64°

The physical model tests with a scale of 1:45 show that flow surface is raised suddenly through the slit bucket, and flow is dispersed longitudinally in the range of 200 m downstream of river reach slightly close to the left bank without a return flow (Table 2) The maximum flow velocity along the right bank is less than 8 m s−1, which is reduced by 40% compared with the original design, and the maximum scour depth is 8 m in the case of low downstream river level In most cases, the flow velocities along both banks are less than 5 m s−1, which reduce the protection work greatly A slight scour is measured in the design and under check flood operation modes because the water depth downstream is much larger.Similar physical model tests have been performed on the Xiluodu 3# spillway tunnel and Jinping spillway tunnel Expected results have been obtained which reduced the scours downstream riverbed greatly Figure 4 gives the scours on riverbed by Jinping spillway tunnel under the designed reservoir water level [6] The maximum scoured depth on the proposed plan is 6.3 m

6.18.2.2 Flaring Pier Gate

This new type of energy dissipater, state of the art, is specially developed for Ankang Hydropower Project [7, 8] The stilling basin of the project is located on a curved river reach and the riverbed is with a low ability of anti-scourging The other reason is that the construction has been proceeding and the length of the stilling basin cannot be further lengthened A new concept of energy dissipation has been proposed for this project, that is, combining the flaring pier gates on surface spillway with stilling basin, to make the flow out of pier gates contracted laterally and dispersed longitudinally, which changes a two-dimensional (2D) flow into a 3D flow and increases the energy dissipation ratio (see Figure 5) The strong 3D turbulent flow can create aeration in the flow through lateral space High ratio of energy dissipation makes the length of the stilling basin to be shortened and the construction work reduced This new energy dissipater was applied to the Ankong Hydropower Project in the middle 1970s with the maximum unit width discharge flow of 254 m3 (s-m)−1 Finally, the length of the stilling basin is reduced by one-third In fact, the Panjiakou surface spillway

is the first one that adopted this kind of energy dissipater in the world Further inventions have been made by combining with bottom outlets in Wuqiangxi and Baise Hydropower Projects, or RCC stepped spillway in Shuidong and Dachaoshan Hydropower Projects Dachaoshan Hydropower Project is an RCC gravity dam with a maximum height of 111 m and a unit width discharge flow of 193.6 m3 (s-m)−1 The energy dissipater is a flaring pier gate with stepped spillway, and the roll bucket is adopted in the downstream Special measure has been taken in the design that the first step is two times higher than normal ones, so that it will make the flow project over several steps and a large cavity is formed under the jet flow; thus, more air enters the bottom of the flow The hydraulic field observation was carried out under normal water level in 2002 when the reservoir was filled for the first time The observed results

[9] show that (1) the pressure variations on steps have been changed a lot, and the pulsation pressure is as high as 10 kPa (see Table 3); and (2) the air concentration on steps is over 30%, which is much higher than the chute spillway The analysis indicates that the first high step plays an important role in cavitation mitigation on steps (see Figure 6) Slit bucket can also be applied to surface spillway to reduce the scouring downstream Guangzhao RCC Dam is a good example The dam height in Guangzhao is 195.5 m with a maximum discharge flow of 9857 m3 s−1 Three surface spillways and two bottom outlets are adopted in the design

Traditional flip bucket is used in the beginning of the design As a 30° bucket angle is taken in the middle one and 22° in the side one, the elevation difference between the middle one and the side one is 2.75 m The buckets on both sides are slightly contracted from the width of 16 to 13 m

Table 2 Scour depth and location by slit bucket in Xiaowan Project

Location of maximum depth of scoured pit Maximum scoured pit

Reservoir water level Downstream water level

1624 1620

Figure 5 3D energy dissipation by flaring pier gate (a) Plan view, (b) side view, (c) section A–A, (d) section B–B

Table 3 Pressure and air concentration on steps in Dachaoshan Hydropower Project

Pave/σ Pmax/ Pmin

on the wall

Main jet flow zone

The physical model tests show that (1) the overburden in the energy dissipation zone is almost scoured to the downstream with

a maximum scouring depth of 26 m, and solid rock on the riverbed is scoured by 5 m; (2) toes of side banks are also scoured; and (3) the scoured materials are accumulated around the tailrace with a maximum height of 18–20 m, which will severely affect the operation of power plant

The proposed slit bucket [10] is designed with (1) bucket angle of –10° applied for all three with a contraction ratio of 0.3; and (2) unsymmetrical contraction on side buckets and symmetrical contraction on the middle one with the width of edge of 4.8 m

Figure 7 gives the comparison of scouring pattern by two types of flip buckets under the check flood operation mode It indicates less scouring by slit bucket

6.18.2.3 Jet Flows Collision with Plunge Pool in High-Arch Dams

As some high-arch dams are constructed in narrow valleys, the collision of energy dissipation by jet flows of surface spillways and middle outlets and a large plunge pool downstream is often chosen The very successful project is the Ertan high-arch dam The design criteria on the slab of plunge pool are that the maximum impinging pressure must be less than 15 � 9.81 kPa Commendable efforts on the arrangements of the spillways, middle outlets, and plunge pool have been made and measured by the physical models during the design stage, such as the impinging angle between surface spillway and middle outlet, the shape of flip bucket of surface spillway, the length of plunge pool and the elevation of the floor considering the excavation, and the height of secondary dam The final solution on the arrangement of discharge facilities in Ertan Dam are seven surface spillways and six middle outlets, and the length of plunge pool is 330 m with a 32 m high secondary dam (see Figure 8) Different flip buckets are adopted in every opening of the surface spillway The maximum discharge flow through the surface spillways and middle outlets is 16 300 m3 s−1

Figure 7 Comparison of scour pattern by two types of flip buckets under the check flood operation mode (a) Scour in the original design, (b) scour in the proposed slit bucket

Figure 8 Design of discharge structures and plunge pool in Ertan Project: (a) general design of energy discharge structures and (b) comparison between the prototype measurements and model tests in Ertan plunge pool

(68.2% of total discharge) and the critical situation is the independent operation of surface spillway with the maximum impingent pressure of 14.0 � 9.81 kPa under the check flood reservoir water level

The Ertan Arch Dam was completed in 1999 and hydraulic field observation was carried out in the same year The field observation results are in good agreement with the model’s results [11], shown in Figure 8 The field observations are carried out under the design reservoir water level with a discharge flow of 8000 m3 s−1 (four surface spillways and four middle outlets) The design concept of energy dissipater in Ertan Dam is accepted by other high-arch dams, such as Jinping (305 m), Xiaowan (292 m), Xiluodu (278 m), Baihetan (277 m), Goupitan (232 m), and Laxiwa (250 m), which all have large plunge pools with a length of about 400 m and secondary dams with a height of about 40 m

As the pressures on the vertical wall of the differential buckets in Ertan are quite low, even negative, the differential flip buckets between surface spillways are recommended and studied on Xiaowan, Goupitan, Xiluodu, and Baihetan arch dams The angles of buckets change from –35° to 10°, which makes the jet flows separated along the plunging pool and the impinging pressures reduced greatly For example, the maximum discharge flow through seven surface spillways and eight middle outlets

in Xiluodu Project has increased from 30 000 to 33 800 m3 s−1 with the bucket angles of surface spillways from –30° to 10° and the maximum impinging pressure being controlled under 13.0 � 9.81 kPa The angles in Xiaowan Arch Dam are from –20° to 10° and in the Baihetan from –35° to 20°; the maximum discharge flow can be increased by about 10% The bucket shape is also an important factor to spread the flow to lateral directions and reduce the impinging pressure Figure 9 gives the flow

Figure 9 Flow pattern by surface spillways of Baihetan Arch Dam

Bed rock surface

Grout curtain Drainage curtain

Drainage holes Plunge pool

Deep well pump house 1155.0

1014.0

1032.0

980.0 965.0

3035404550

Prototype Model test

(b)(a)

Temporary bottom outlet

Max impingement pressure (9.81∗kP)

6.18.2.4 Orifice Spillway Tunnel

The principle of energy dissipation of orifice spillway tunnel is sudden contraction and then sudden expansion through the orifices It was first applied in the Mica Dam in the 1980s but the discharge capacity was less than 1000 m3 s−1 The first large-scale orifice spillway tunnel was adopted in Xiaolangdi Project by reconstruction of diversion tunnel in the 1990s

Xiaolangdi Project has a rockfill dam with a maximum height of 154 m and a total discharge capacity of 17 063 m3 s−1 All discharge structures are located on the left bank, including one chute spillway, three spillway tunnels, three orifice tunnels, and three silt flushing tunnels The powerhouse is also located on the left bank The main consideration on the orifice spillway tunnel is cavitation The objectives of studies include optimization of the number, interval, orifice plate shape, adoption of abrasion-resistant concrete, and inclined ratio on the top of the chamber to increase the pressure of the tunnel The final design of the orifice tunnel is that three orifice plates are installed in the horizontal pressurized tunnel with an interval of 3D (D is the diameter of the tunnel,

D = 14.5 m) The contract ratios of these are 0.690, 0.724, and 0.724, respectively, which result in a strong rotation, shear and turbulent flow, dramatic energy dissipation, and reduction in velocity to about 10 m s−1 (see Figure 10) More details of the research had been considered during the design, including the different scales of conventional model tests, depressurized model tests, and intermediate prototype observation in the Baozhusi silt tunnel

The orifice spillway tunnel was first operated in April 2000 and hydraulic field observations have been carried out with the working heads of 70 and 100 m on 1# tunnel and 100 m on 2# tunnel [13, 14] The parameters observed are pressure and flow noise in the pressurized tunnel; pressure, cavitation noise, air entrainment, and air concentration in the open flow tunnel; and strength and stress on the radial gate

The model test results and field observations show that they are in consistency with the energy dissipation ratio and pressure distribution (see Figure 11) A slight cavitation noise is still observed at the gate opening ratio from 0.96 to 0.99 (see Figure 12) Sound increment of spectrum level at 11.6 to 27.0 dB in a high-frequency band is observed But no cavitation damage is found during inspection after several rounds of operation

The scale effect on cavitation has been a cause for concern during the design Several physical model experiments, under the normal atmosphere condition and depressurized condition, are carried out with the model scale of 1:40 to 1:30 [15] An intermediate test on the silt flushing tunnel in Pikou Project was performed for further analysis of scale effect Table 4 shows that

Figure 10 Pressure and hydrophone sensors arrangement in Xiaolangdi 2# orifice tunnel

1# orifice plate 2# orifice plate 3# orifice plate

(m)

Figure 11 Pressure coefficients of 2# orifice tunnel in Xiaolangdi Project