A NEW METHOD OF ELIMINATING TORRENT SHOCK WAVES IN STEEP SLOPE BEND

 Qiu Xiuyun¹,  Hou Jie¹,  Sun Tao² and Zhou Zhu¹

(1.Xinjiang Agriculture University,Urumqi 830052 China;

2. Tianjin University Tianjin300072 China)

Abstract: Kaqun primary water power station takes a measure of connecting suspended dissipator with diversion baffle to eliminate shock waves in steep slope bend. This measure is taken to eliminate energy in the first part of the steep slope mainly, and be connected with dissipation and diversion in bend. A year’s actual operation indicates that this measure solves the problem of controlling the flow pattern of big-base-slope-torrent bend well, and the construction methods are easy, the investment of project is small, original structure remains unchanged basically, the project operates well in winter. So the measure can be spread and be applied in other similar projects.

Keywords: steep slope bend, torrent shock waves energy dissipation

1    INTRODUCTION

In actual project, usually limited by conditions of geologic, land form etc., the discharge steep slope of diversion type power station has to reverse an angle in plane, designed as curvilinear bend type. Because of water flow^’s characters of high unit-discharge, high velocity, concentrated energy etc., when passing though the steep slope reach, it can produce shock waves under the function of centrifugal force when passing though the bend, inevitably, flow pattern in channel growing bad. Not to control or control it incorrectly will cause heavy engineering accidents. Therefore it has actual significance to study how to eliminate torrent shock waves in steep slope bend. At present, methods are often used in projects as follows: channel bed freeboard method, channel bed transverse sector lifted method, curved diversion board method, composite curve method, inclined diversion aerification bucket method, limting radius of bend method and putting stilling pond in bend part method etc.. These methods have been practised successfully within certain ranges and on certain conditions. Connecting with Xinjiang kaqun primary power station’s discharge steep slope hydraulics model experiments and its application, this article will introduce a new solution of eliminating torrent shock waves in steep slope bend, that is suspended dissipator and diversion baffle method. This project had been put into service in Jane, 1996. More than one year^’s actual operation has testified that this solution is very successful in engineering practice.

2    WORKS GENERAL SITUATION

Kaqun primary water power station is a middle-small-sized power station on the west bank of the dilivery general canal of ye^’erqiang river. Its installed capacity is 3×7000kw,single-unit discharge 17.1m³/s,design discharge of the diversion canal 60m³/s,heightening discharge 70m³/s,maximum discharge 35 m³/s in winter.

The former discharge steep slope of the power station consists of three parts. The first part from 0+010.190 to 0+048.220,base width varies from 12m to 3m gradually. From 0+048.220 to 0+081.903,base width is 3m.From 0+010.190 to 0+031.712,base slope is 0.03,drop is 0.64m.From 0+031.712 to 0+081.903,this part ends, base slope i equals 1/3.779,drop is 13.09m.The end of this part joins a bend whose radius of axis is 18.181m,central angle 58,base width 3m,length of axis is 18.404m,transverse base slope is 1/3.779,and axis is horizontal. Following the bend is a part of horizontal slope, 10m long and its base width is 3m.The base slope of the secondary steep slope is 1/3,base width 3m.Its plan layout is shown in figure 1.This project was completed in Jane, 1995, and put into service on November of that year. During working, when discharge is 25 m³/s, starting from 0+088.832 of the bend , water flows against the wall of the concave bank basically, the trench’s bed plate of the convex bank has no water, and serious drift appears in the bend, when water enters into straight line part(the secondary steep slope),which chainage is 0+115.307,it begins forming intense refracting flow. As a whole, water flow of the secondary steep slope discharges basically in refracting-drum pattern, part of which impacts the curb wall, rushes out of the discharging chute, undercuts backfilled soil of two sides , forms mud-rock flow, then goes back to the dischaging chute at 0+185.307.The mud and rock washed off is taken into the stilling pool and the take-out channel of its end. Undercutting coming to 4 times when working and earthwork volumn deposited in the stilling pool amounting to about 1200m³ threaten safty and operation of the project. Therefore, decision was made to modify this project. Because of complicated hydraulic parameters and flow pattern of water of overflowing steep slope, hydraulic model experiments are needed to find out the measures of eliminating torrent shock waves in bend and solving the problem of energy dissipation. Considering this project is a modification one, after a hydraulic model experiment which geometric scale is 1:25,comparing several different scheme^’s outcomes of experiments, finally pose a new method that dissipates part of the energy by suspended dissipator at the first steep slope, and forces high-speed water in bend to veer by using the diversion baffle, that is a new measure which is connected with dissipation and diversion to eliminate torrent waves in bend, shown in figure 2.

Fig.1    Layout plan of former discharge steep slope 

Fig.2    Layout plan of restructed project

3    SUSPENDED DISSIPATOR AND DIVERSION BAFFLE

3.1    The grid^’s layout pattern and its composition

The suspended dissipator is put at the bottom of the first steep slope, clear height is 18cm apart from the bed plate of the trench, the axis-direct distance is 1m along the steep slope. dissipating grid strips welded on the longitudinal bracing beam of the trench^’s bottom use No.18 common rolled I-steel, whose length is the same to that of trench^’s bottom. Bracing beam uses No.18 common rolled channel steel, staying against the side wall, fixed by expansion bolts whose diameter is 20mm on the curb wall and bed plate, its layout is shown in figure 3.The difference between suspended dissipator and “the method of artificial roughness in trench’s bottom” is that there is a clearance between the suspended dissipator and the trench^’s bottom.

Fig.3    Layout of suspended dissipator

Diversion baffle is put in the concave bank of bend, 2m clearly hight distance from the bottom plate of the bend, composed of sector-shaped plate which is 1.5m wide(half of the width of the bend),13.5m long along the axis and the plate has the same curvature as the bend’s. Diversion baffle is made of 8mm common roled steel plate. To guid water to flow from the concave bank to the convex bank, under the baffle are installed transverse grid strips which is as wide as the board, interval is 0.3m,using No.20 common rolled I-steel. Diversion baffle is fixed on 6 pieces of No.20 common rolled I-steel beams, shown in figure 4.

In order to control the splash of water at the end of the first steep slope (the entrance of the bend),an inclined tongue-shaped clip plate is fixed here. The clear height of the plate ^‘s entrance and exit are 4.6m and 3.0m respectively, its length is 7m,shown in figure 5.Its materials use No.18 and No.32 common rolled I-steel as main beams, face slab uses 20mm-thick common rolled steel plate welded on beams.

Fig.4    Layout of diversion baffle

3.2    Experiments’ result and analysis

3.2.1  Function of suspended dissipator

The use of suspended dissipator takes an effect of artificial roughness, making the energy inside of water flow offset reciprocally and lowing the mean velocity in the section and the bottom, so the aim of dissipating some of the energy has been achieved. When torrent enters into the bend, its centrifugal force is decreased when it passing by the bend is decreased because of energy^’s decrease, thus, refracting of water flow in bend is weakened. Experiments indicates that when discharge Q equals 70m³/s, maximum mean velocity at the end of the steep slope is 10.29m/s(without suspended dissipator ,maximum mean velocity in this place is 14.82m³/s).Because velocity has decreased, water depth at the end of the steep slope increases from 1.81m(before putting suspended dissipator )to 2.93m.The damping ratio of water flow passing by the steep slope part is 52.29%.But the function of the suspended dissipator  isn’t entire identical with “the method of artificial roughness in the trench’s bottom ”.Because there is a space between the suspended dissipator  and the trench’s bottom, when water flow passes by the steep trench, grid strips divide it into upper and lower two layers whose velocities don’t equal, so the two streams of water run into each other and produce friction by the gravity, and cause vortexes at the downtream of the grid strips, consequently, the loss of energy increases. The other advantage of the suspended dissipator is that when discharge is low water passes completely under the grid strips. Thus, reducing the splash of water and preventing the phenomenon of suspended ice on the two side walls of steep trench in winter, at the same time, the strips can make ice cakes broken. These characters are especially important during winter’s working in cold areas.

3.2.2  The function of diversion baffle

When water flowing by the bend, there is a rather high hydraulic head between the concave and convex bank. The diversion baffle put in the concave bank of the bend firstly takes on an effect of oppression on water flow of this bank. The effect makes water pressure under the plate increase quickly forcing water to flow towards the convex bank. What’s more, put under the plate, when water flows by the bend, 0.3m-interval transverse grid strips lead to a interference wave to eliminate disturbance waveform caused by the bend, make water flow under the plate separate a lateral velocity component guiding part of the water to flow from the concave bank to the convex bank, lower the concave bank’s water level and prevent the producing of shock waves in the bend. At the same time, the putting of transverse guide strips under the diversion baffle takes a roughening action, enlarging the loss of energy of water in bend. As a result of above-metioned synthetic effects, the flow pattern of water in secondary steep slope is steady. With high discharge, diversion baffle is submerged in water, some of water passes on the board, the other under it. At this time, the flow pattern of water flow in bend is steady basically. Experiments indicate that for heighening discharge 70m³/s,the maximum water depth of the concave bank of the bend is 4.05m,the maximum hydraulic head of two banks is 1.22m(without suspended dissipator  and diversion baffle, the maximum hydraulic head is 5.47m).When discharge is 35m³/s,there is very thin water on the board, transverse water-surface gredient head is 0.67m.With low discharge, water flows under the board completely, in this situation the suspended dissipator in the steep slope has eliminated most of energy, so when water flows by the bend, it has been in steady state. And the phenomenon of refracting in secondary steep slope disappears basically.

3.2.3  The function of tongue-shaped clip plate in the entrance of bend

The experiment observation indicates that when discharge is high, blowing spray splashed by discharging water at the end of steep trench(the entrance of the bend)is disadvantageous to the steep trench and the structure of the side wall, and it is easy to shape suspended ice in winter, so tongue-shaped clip plate is put at the entrance of the bend(shown in Fig.5).Experiments indicate that the plate can restrain the splash of spray, and can dissipate energy, guide and steady water flow.

The result of experiments indicates that using multiple measures of suspended dissipator and diversion baffle etc. in steep slope bend has rather good effect to eliminate torrent shock waves of bend. Attached list1. gives the experiment result of Kaqun primary power station’s the discharge steep slope at different discharges, including before and after putting suspended dissipator and diversion baffle.

Fig.5    Layout of tongue-shaped

Attached List 1

4    ACTUAL OPERATION OF THE RESTRUCTED PROJECT

Restructed Kaqun primary power station was put into service in Jane,1996.Actual operation indicates that when discharge is 5m³/s,10m³/s,35m³/s,60m³/s respectively, problems of refracting, contribution of flow pattern is nonuniform and water’s impacting the wall of discharging trench etc. are all solved well, water flow passes by the bend and the steep slope smoothly, the situation of operation tallies with result of model experiments bascially, which solves actual problems of operation well. The project woked well in that winter. In this area, mean annual minimum temperature of winter is minus 18℃,it is minus 16℃ in that year, water temperature is 0.1℃,maximum ice cake is 1(length)×2(width)×0.2(thickness)meter or so. When ice cakes enter into the first steep slope, they hit on I-steel grid strips. The grid is just like a smashing-ice machine under the function of water, smashing ice cakes and dischanging them into the stilling pond of the downstream. So phenomenon of blocking ice up never takes place.

5    SUMMARIZATION

Measure of eliminating torrent shock waves in steep slope bend mentioned in this article is a new one to solve the problem of water flow in big-base-slope-torrent bend. It uses the method that dissipates energy in the first steep slope mainly and combines dissipating with guiding in the bend. Both experiments and actual operation indicate that this method is well used in big-base-slope-torrent bend, the priblem of cotrolling flow pattern in big-base-slope-torrent bend is solved well. As far as this project is concerned, it saves project cost largely, retrieves a direct economic loss more than ¥1,000,000.At the same time, the investment of project is small, the construction methods are easy, original structure remains unchanged basically. Restructed project, tested by practice, can pass by all discharges on design conditions, each technical parameter and hydraulic important factor comes up to the demands of original design. Especially in winter, its working is rather good. This project won “the award of excellent engineering design of Xinjiang Uigur autonomous region”.

References

[1]  hydraulics teaching and research group of QingHua university,Hydraulics, People’s educational publishing house,1981.

[2]  Li chongzhi etc., Hydraulic drop and steep slope, Hydraulic and electric power publishing house,1988.

[3]  [the soviet union] C.M.Cluckuu],Hydraulic calculation of high-head hydraulic structure, Hydraulic and electric power publishing house,1984.

[4]  Li jianzhong, Ning lizhong, High-velocity hydraulics, Northwest industry university publishing house ,December of 1994.

[5]  Xun xingzhi, Briefing of controlling of shock waves in flood way open channel bend bend, ShanXi hydroelectric power,No.3,1986.

[6]  Liu chongzhen, Methods and applications of improving shock wave in torrent bend, LiaoNing hydrotechnics,No.1,1981.