PAK BENG HYDROPOWER PROJECT

Transcription

1 PAK BENG HYDROPOWER PROJECT Two-dimensional Sediment Numerical Simulation of Pak Beng HPP in Laos Mekong River September 2015

2 Two-dimensional Sediment Numerical Simulation of Pak Beng HPP in Laos Mekong River Research Report College of Water Resources and Hydropower Engineering, Wuhan University

3 CONTENTS 1 INTRODUCTION GENERAL SITUATION OF THE ENGINEERING PROJECT SEDIMENT PROBLEMS Water intake and sediment control of the power station Sediment deposition in the approach channel Downstream channel scouring RESEARCH TECHNICAL ROUTE AND CONTENTS BASIC DATA Runoff series Design flood at the dam site Sediment data Reservoir operation mode Discharge capacity for discharge structures MATHEMATICAL MODEL ESTABLISHING BASIC PRINCIPLE OF THE PLANE TWO-DIMENSIONAL MATHEMATICAL MODEL Basic equations in Cartesian coordinate system Basic equations in generalized curvilinear coordinate system Model discretization Boundary conditions PROBLEMS SOLVING RELATED TO THE MODEL Sediment carrying capacity Distribution pattern of sediment carrying capacity gradation Treatment pattern of bed load transport rate Treatment pattern of bed material load gradation Treatment of mobile boundary SELECTION OF CALCULATION CONDITIONS AND LAYOUT OF SCHEMES COMPUTATIONAL GRIDS OF THE MODEL Computational grids in the upstream of dam site Computational grids in the downstream of dam site CALCULATION CONDITIONS OF THE MODEL i

4 3.2.1 Water and sediment data for computation Outlet water level process LAYOUT OF CALCULATION SCHEMES CALCULATION RESULTS AND ANALYSES SEDIMENT SCOURING-SILTING PROCESS ANALYSES OF RIVER REACH IN THE UPSTREAM OF DAM FOR SCHEME ONE Flow regime and deposited topography analysis of river reach in the upstream of dam Sedimentation of powerhouse intake Variation analysis of sediment concentration through turbine and particle size distribution Analysis of sedimentation at the entrance of the upper approach channel Summary FLUSHING EFFECT ANALYSES OF FLOOD RELEASE AND SCOURING SLUICES WITHIN CHANNEL SCOURING SLUICES FOR SCHEME TWO Flushing effect analyses of flood release and scouring sluices Flushing effect analyses of channel scouring sluice SEDIMENT SCOURING-SILTING PROCESS ANALYSES OF RIVER REACH IN THE UPSTREAM OF DAM FOR SCHEME THREE The analysis of the flow regime in the downstream reaches of the dam site The analysis of the terrain of scouring-silting in the downstream reaches of the dam site The analysis of navigable currents condition of the lower approach channel Summary FLUSHING EFFECT ANALYSES OF THE CHANNEL SCOURING SLUICES FOR SCHEME FOUR CONCLUSIONS AND RECOMMENDATIONS CONCLUSIONS RECOMMENDATIONS ii

5 1 INTRODUCTION 1.1 GENERAL SITUATION OF THE ENGINEERING Pak Beng HPP is the first cascade power station in cascade development scheme of Mekong main stream which is recommended by Developmental research on runoff type hydroelectric projects of Mekong main stream. According to the plan, the development of cascade power stations of Mekong main stream mainly meets the demand of electric power in Mekong downstream valley basin, provides the navigation lock and other facilities in order to satisfy the current shipping requirements, as well as provides convenience for improving river shipping in the future. The project is grade I, large (2) scale and main hydraulic structures are the second class. The power station adopts runoff type development mode, and hub structures are composed of water retaining structures, outlet structures, power station structures, navigation structures and fish structures. The plane arrangement figure of the project is attached figure 1.1. The water retaining structures contain non-overflow dam section on the left bank, powerhouse dam section, flood release and scouring sluice dam section, ship lock dam section and non-overflow dam section on the right bank. The crest elevation of the dam is m, with the maximum height of 69 m and the dam length of m. Powerhouse dam section is located in the left main channel, while flood release and scouring sluice dam section is located on the right beach land. Moreover, ship lock dam section is arranged on the right bank, and non-overflow dam sections are arranged on both banks. In order to prevent bed load entering into intake of power station, an inclined debris barrier (with the crest elevation of 325.0m) is set in front of the powerhouse dam section, which divert the sand into flood release and scouring sluice, then the sand will be ejected to downstream through the sluice hole. The flood release and scouring sluice use the method of broad crest weir outflow, and the crest level of the weir is 317.0m.An emergency gate and a work gate are set in the scouring sluice, and they are both flat plate gates, with opening dimensions 15m 23m. A stilling basin is set behind the sluice, using the pattern of energy dissipation by hydraulic jump. Powerhouse is arranged in the main channel on the left side of river bed, and sixteen bulb tubular units are arranged in the house, with the intake elevation of m. In order to solve the problems of sediment deposition before the intake of power station and coarse sand passing through the units, eight sand ducts are arranged on the bottom of the intake (with intake elevation of 285.6m). An erection bay is arranged on each end of the main powerhouse, and an auxiliary powerhouse is arranged in the downstream side of the main powerhouse, with qualify platform on the top of which. Powerhouse consists of 7 dam sections, and the overall length is 410m. The ship lock, designed by standard of navigation of 500t ship, is single-line and single-stage, - 1 -

6 and arranged by the right bank side. The effective dimensions of the lock chamber are defined as 120m 12m 4m (length width threshold water depth), and upper lock head is arranged combined with the diversion dam. For the convenience of flushing sand in the approach channel, one channel scouring sluice is arranged on the right of the ship lock. The straight section of upstream approach channel is about 300m, while the total length of downstream approach channel is about 1142m, with bottom width of 45m, bending radius of 330m and bend angle of Opening dimensions of the channel scouring sluice are 15m 23m, and the channel scouring sluice uses the method of broad crest weir outflow, with the crest level of 313.0m. An emergency gate and a work gate are set in the scouring sluice, and they are both flat plate gates, with opening dimensions of the gates 15m 23m. A breast wall is arranged on top of the sluice hole. The channel scouring sluice is only put to use in the situation of suspending sailing. 1.2 PROJECT SEDIMENT PROBLEMS The project sediment problems of Pak Beng HPP mainly contain: protection of the reservoir and the impact on the boundary river between Laos and Thailand in the reservoir head, water intake and sediment control of the power station, sediment deposition in the approach channel, downstream channel scouring etc. The report has specially researched on the latter three problems Water intake and sediment control of the power station Pak Beng HPP is runoff type, and sediment deposition is little. Particularly after sediment was retained by the upstream cascade stations, incoming sediment greatly reduced, sediment particles size was fine, and most was ejected to downstream, so the problem of water intake and sediment control is not significant. In order to reduce coarse sand entering into the power station, a debris barrier is set in front of the water intake of powerhouse, which diverts the bed load into flood release and scouring sluice, then the sand will be ejected to downstream river course through the sluice hole. The flood release and scouring sluice has a large number of sluice holes and a big discharge capacity. Furthermore, sand ducts (with inlet elevation of 285.6m) are arranged between each two intakes (with inlet elevation of 288.9m), and the total number is eight. The sand crossing the debris barrier is ejected to downstream by opening the sand ducts Sediment deposition in the approach channel The river course where located the power station is navigation river course, and the ship lock is arranged on the right beach land. Approach channels are set in both upstream and downstream, and there is reflux in the entrance area of approach channels because the velocity in the approach channels is low, thus a large amount of silt deposit, and deposition is mainly in the entrances and exits of the approach channels. For this reason, One channel scouring sluice is arranged on the right side of the ship lock, using broad crest weir outflow, with the crest level of 313.0m.When the power station is finished, it s required to strengthen - 2 -

7 monitoring of sediment deposition in the approach channel, and flush the sediment in the entrance area of the approach channel by means of opening the channel scouring sluice when the discharge is small, and head difference is large Downstream channel scouring The sediment retaining effect of Pak Beng HPP is small, and most incoming sediment was ejected to downstream, so the construction of the power station won t basically change the conditions of incoming water and sediment. According to the similar project experience, when the power station is constructed, there isn t big influence on downstream channel scouring, but the problem of water lowering caused by partial riverbed undercutting in the downstream of dam still exits, withal, it has left room in the design of sluice bottom board of lower approach channel. The downstream Luang Prbang power station is connected with Pak Beng HPP, and after it s constructed, downstream channel scouring will be decreased because the downstream water level is raised. 1.3 RESEARCH TECHNICAL ROUTE AND CONTENTS The technical route of this sediment model is collecting relative data of the power station calculation region such as hydrology and sediment data, geology data, topography data and project planning report data and so on. On this basis, sediment simulation calculation of the region between 500m away from upper approach channel and Pak Beng water station in the downstream is conducted, adopting reasonable sediment numerical model to study the erosion-deposition variation in the river reach of dam area. According to the main project problems of water intake and sediment control of the power station, sediment deposition in the approach channel, downstream channel scouring, the specific research contents of the report are that: Combined with the reservoir dispatching operation mode, a study of sediment deposition before dam in a certain fixed number of years is made, in addition to the water flow regime under certain characteristic discharge. The sediment motion law before dam section is analyzed, emphatically quantitative analysis or qualitative assessment of sediment influx and deposition form in some important parts such as the front of power house intake, the debris barrier frontier, sand ducts and so on is carried out. Through analysis of numerical model calculation results and a combination of layout scheme of the hub, treatment suggestions and solving measures are put forward. The problem of sediment scouring and silting in entrance area of approach channel is studied. To be specific, sediment scouring and silting situation in upper and lower approach channel, especially the entrance area is studied. Besides, the flushing effect of channel scouring sluice and operation mode (including flushing flow rate and flushing time) is studied, and assessment suggestions to the arrangement of channel scouring sluice and effect are proffered, apart from optimized design scheme, operation mode and engineering measures suggestions to decreasing sediment deposition in approach channel

8 Combined with the reservoir dispatching operation mode, a study of sediment scouring and silting in the downstream river reach of power station in a certain fixed number of years is made, in addition to the water flow regime under certain characteristic discharge. The approach channel navigation flow conditions of the original design scheme are assessed, and suggestions to improving the navigation conditions are put forward. Except for special notation, Laos height datum is adopted in the report. 1.4 BASIC DATA Runoff series The monthly mean runoff series at Pak Beng HPP dam site are 1960~2004, and annual average discharge is 3160m 3 /s. The secular monthly mean runoff results at the dam site are seen from table1-1. Table 1-1 The secular monthly mean discharge at the dam site of Pak Beng HPP Item\Month year Mean discharge (m 3 /s) Distribution ratio (%) Design flood at the dam site Pak Beng dam site is located between Chiang Sean hydrology site and Luang Prbang hydrology site, thus design flood at dam site is obtained applying areal interpolation of frequency flood results between Chiang Sean hydrology site and Luang Prbang hydrology site. The design flood results of Pak Beng power station see table 1-2. Table 1-2 Design flood results at the dam site of Pak Beng HPP P (%) Peak discharge (m 3 /s) P (%) Peak discharge (m 3 /s) Sediment data (1) Sediment amount of dam site The annual average suspended load amount of dam site is t in natural situation, and there is no measured bed load data in each hydrology site of Lancang-Mekong river main stream. Taking into account of the consistency of design results of upstream Xiaowan, Nuozhadu and Jinghong, suspended load amount of Pak - 4 -

9 Beng dam site is calculated to be t, applying ratio of bed load to suspended load is 3%. Xiaowan, Nuozhadu are two biggest reservoirs among the eight planned cascade power stations in the middle and lower reaches of Lancang River, and have notable sediment retaining effect. Recently, Manwan and Dachaoshan power stations have been completed, while Jinghong and Xiaowan power stations were respectively reservoir impounded and power generated in the year of 2008 and 2009, besides, Nuozhadu is expected to generate power in According to the above power station development succession, Pak Beng HPP should be constructed and go to production after reservoirs such as Xiaowan and Nuozhadu. Taking into account of the sediment retaining influence of upstream cascade stations, Pak Beng dam site suspended load amount is t, and accounts for 26.6% of the natural situation. Bed load before Jinghong dam site is all intercepted and Pak Beng dam site bed load only comes from the region between Jinghong and Pak Beng dam site. In accordance with 3% of the region suspended load amount, it s estimated to be t, accounting for 24.2% of the natural situation. (2) sediment gradation According to data provided by the designer, the grading curve for sediment particles of suspended load sees figure 1-1. Fig. 1-1 The grading curve for sediment particles of suspended load There is no measured bed load data in each hydrology site of Lancang River and nearby hydrology sites of Pak Beng power station in Mekong River, thus bed load gradation is - 5 -

10 replaced by bed material gradation. The grading curve for sediment particles of bed load sees figure 小于某粒径沙重百分数 (%) D 50 =16.9mm D m =28.0mm 粒径 (mm) 0.1 Fig. 1-2 The grading curve for sediment particles of bed load Reservoir operation mode Considering producing power generation benefit of the power station and improving navigation conditions, the power station should run at the normal pool level of 340m in flood season, and lower water level to 335 m in withered season.in flood season, the incoming discharge is large, while the water head is small, flood operation dispatch is mainly considered from the perspective of shipping safety, decreasing reservoir deposition and reducing the submerging impact of backwater on Laos and Thailand land. When inflow of the reservoir is less than 13200m 3 /s which is the 3-year frequency flood, the inflow is less than navigation standard of ship lock, thus ship lock is in normal use. The reservoir water level should maintain at the normal pool level of 340m, and fall head is greater than the smallest fall head when units are in safe operation, thus power station is also in normal use. Flow releases through flood release and scouring sluice, sand ducts and units. When inflow of the reservoir is greater than m 3 /s, which is the flood of 3-year frequency, and less than m 3 /s, which is the flood of 5-year frequency, the discharge is greater than navigation standard of ship lock, thus ship lock stops to use. The reservoir water level should maintain at the normal pool level of 340 m, and fall head is greater than the smallest fall head when units are in safe operation, thus power station is in normal use. Flow releases through flood release and scouring sluice, channel scouring sluice, sand ducts and units

11 When inflow of the reservoir is greater than14900m 3 /s, which is the flood of 5-year frequency, fall head of the power station is less than the fall head when units are in safe operation, thus power station stops to use. Meanwhile, in order to reduce the submerging impact of backwater on Laos and Thailand land, it should open sluice gates for sediment release, and flow releases through flood release and scouring sluice, channel scouring sluice, and sand ducts Discharge capacity for discharge structures Discharge capacity for each outlet structure sees following table 1-3. Frequency flood (%) Table 1-3 Discharge capacity for each outlet structure of Pak Beng HPP Upstream level (m) Downstream level (m) Discharge from flood-dischar ging and sand-sluicing gate (m 3 /s) Discharge from channel scouring sluice (m 3 /s) Discharge from sand duct (m 3 /s) Discharge from units (m 3 /s) Discharge in total (m 3 /s)

12 2 MATHEMATICAL MODEL ESTABLISHING This research plans to adopt plane two-dimensional flow and sediment mathematical model to calculate, which was developed by the school of water resources and hydropower engineering of Wuhan University. The model can be used in both fixed and movable bed calculation, and has been applied in many projects, such as sand excavation plane two-dimensional flow and sediment mathematical model at Wuhan river reach, plane two-dimensional flow and sediment mathematical model of Wuhan Erqi bridge, flow and sediment mathematical model of waterway regulation engineering in Shashi Section of the Changjiang River, two-dimensional erosion and sedimentation calculation at Tunaozi reach in reservoir backwater end area of the Three Gorges Reservoir in the initial impoundment period, prediction for river bank-failure and an initial research on countermeasures in Jingjiang River channel after impoundment of Three Gorges Reservoir, rerouting and bend cut-off project of Mongolia ditch, flood control evaluation of Mawo harbor district in Anqing, waterway regulation and flood control evaluation in Maojiaokou reach of Hunan zishui, special topic of sediment research on Haji hydropower station in the dam area of Salween river, the calculation of flow and sediment mathematical model of Zhualai hydropower station in Yunnan Dongwang River, the calculation of flow and sediment mathematical model of Dongfengyan hydropower station in Min river and the calculation of flow and sediment mathematical model of Jianwei hydropower station in Min river and so on. 2.1 BASIC PRINCIPLE OF THE PLANE TWO-DIMENSIONAL MATHEMATICAL MODEL Basic equations in Cartesian coordinate system Considering the influence of lateral inflow, basic equations of two-dimensional flow and sediment simulation in Cartesian coordinate system are: Z M N Flow continuity equation: 0 t x y (2-1) Flow motion equation in x direction: M t um x vm y Z gh x 2 M D 2 x 2 M gn 2 y 2 M h u v 2 (2-2) Flow motion equation in y direction: N t un x vn y Z gh y 2 N D 2 x 2 N gn 2 y 2 N h u v 2 (2-3) Sediment continuity equation: - 8 -

13 HS MS NS t x y 2 2 HS HS S S* 2 2 x y (2-4) Non-equilibrium transport equation of bed load: g x bx g y by b b* g g (2-5) Riverbed deformation equation: Z t g x g b bxk byk K S K S* K (2-6) y Where H is the depth; u and v are velocity in x and y direction: M=uh,N=vh;Z is the water level; n is Manning's roughness coefficient; D is the turbulent viscosity; is the water density; S is the sediment concentration; S * is the sediment carrying capacity; is the settling velocity of sediment particles; ' is the sediment dry density; is the recovery saturation coefficient for suspend load; is the recovery saturation coefficient for bed load(dimensional); is the sediment diffusion coefficient; K S K S * K are respectively the settling velocity for sediment particles, sediment concentration, sediment carrying g capacity of every sediment groups; b g b * effective bed load transport rate; gbxk sediment group in x and y directions,and the expressions are: are unit width bed load transport rate and g byk are respectively bed load transport rate of every gbxk, gbyk gbk, gbk u v u v u v Basic equations in generalized curvilinear coordinate system In generalized curvilinear coordinate system, letbe J x y x y (2-7) y x, J y x, J x y x, J y (2-8) J In curvilinear coordinate, denote the components of unit width discharge is FM FN in and direction, then: FM y M x N J x M y N (2-9) FN y M x N J x M y N (2-10) - 9 -

14 In curvilinear coordinate, denote the components of velocity is U V in and direction: v u J v x u y U y x (2-11) v u J v x u y V y x (2-12) According to generalized curvilinear transformation relation, the above basic equations in Cartesian coordinate system could be transformed to: Flow continuity equation: 0 FN FM t Z J (2-13) Flow motion equation in direction: J h v u M gn M q M q DJ M q M q DJ Z Z ghj MV MU t M J x x (2-14) Flow motion equation in direction: J h v u N gn N q N q DJ N q N q DJ Z Z ghj NV NU t N J y y (2-15) Sediment continuity equation: * S S W J HS q HS q J HS q HS q J VHS UHS t HS J (2-16) Non-equilibrium transport equation of bed load: * g b b b b g g J g (2-17) Riverbed deformation equation: K K K byk y bxk x byk y bxk x b S S g g g g t Z * (2-18) The above equations are basic equations in generalized curvilinear coordinate system for plane two-dimensional flow and sediment mathematical model. Within: y x q ;

15 q 12 x x y y ; q x y M ; M N N denotes partial derivative, for example M M ; Other variables have the same meaning with the above Model discretization The above governing equations could be expressed in unified convection-diffusion equation form: J t H HU HV Jq H Jq H S U S P (2-19) Calculation physical quantities of the model are arranged with use of collocated grids (non-staggered grids), that is to say, calculation physical quantities are arranged at point P (Figure 2-1), which is the center of the control volume. With use of control volume method, the governing equations are integrated to the showed control volume along time and space, thus general discretization forms of the governing equations could be obtained. a P P a E E a W W a N N a S S a 0 P 0 P b (2-20) Among them: a a E N D A P ) C,0 ; a D A P ) C,0 e ( e e w ; w ( w w D A P ) C,0 ; a D A P ) C,0 n ( n n S ; s ( s s b S U. a P P ; a P ae aw an as ap SP. ; a 0 P 0 H p t Where = J,variables with superscript 0 denote the variables in time layer. P C / D C,and is grid Peclet number, e C w Cn and Cs coefficients in the surface of control volume, D e D w respectively denote convection Dn and Ds coefficients in the surface of control volume, and their expression are respectively: C HU ; C HU w ; C HU n ; HU D D e e s e 1 H J 1 H J s e w 1 H 1 H ; D w ; Dn J ; J n A P = 0,1 0.5 P ; w n C ; s s denote diffusion

16 N n W w v P H u e E s S Fig. 2-1 Schematic plot of the control volume Table 2-1 Variables, diffusion coefficients and source items in general equations of the model Equation SU continuity equation Flow motion equation in x direction Flow motion equation in y direction Sediment continuity equation Z Z ghj x x u D DJq M 12 DJq12M Z Z ghj x x v D DJq N S 12 DJq12N Jq 12 Jq JWS * HS 12 HS gn gn 2 2 H H S P H H u 4 3 u JW v v 2 2 J J In the solving process, momentum interpolation is adopted in the treatment of velocity in the interface of control volume in order to avoid water level fluctuation. To avoid the appearance of overflow value in the iteration process, under-relaxation technique proposed by Patankar and Spalding is applied, that is under-relaxation factor is introduced to discrete equations, in order to improve the degree of diagonal dominance of coefficients in discrete equations

17 2.1.4 Boundary conditions In plane two-dimensional flow and sediment mathematical model, boundary conditions usually contain treatment of inlet and outlet boundary, bank boundary and mobile boundary and so on. In this model: (1) Inlet boundary:according to the known inlet whole cross-section discharge, the transverse distribution of unit width inflow is given. Besides, sediment concentration of every grid at inlet cross-section is also given. (2) Outlet boundary:generally, water level at outlet section is given. (3) Bank boundary:bank boundary is non-slip boundary, and velocity is given as 0. (4) Mobile boundary:in this model, freezing method is used in the treatment of mobile boundary, which is according to the elevation of river bed at water level nodes, emergence of the grid unit can be judged. If not, roughness is the normal value, vice versa, the value of roughness is taken an infinite positive number. Meanwhile, in order to have no influence on the solving of inflow governing equations, a thin water layer is given at emergence nodes and the thickness is given 0.5cm generally. 2.2 PROBLEMS SOLVING RELATED TO THE MODEL Sediment carrying capacity Sediment carrying capacity is the cornerstone of sediment mathematical model, and the calculation accuracy has direct impact on sediment carrying capacity of suspend load and deformation of riverbed. For non-uniform sediment, total sediment carrying capacity and group sediment carrying capacity are contained. Zhangruijing equation is applied in the calculation of sediment carrying capacity, which is summarized by collecting test data of Yangtze River, Yellow River, and several reservoir and indoor flumes, and has universal applicability. However, the equation is sediment carrying capacity equation in the sense of river channel one-dimension, and the study of plane two-dimensional sediment carrying capacity is not mature nowadays, besides, there isn t uniform understanding and quite clear formulation for the basic concept and definition. In practical application, most researchers generally use one-dimensional sediment carrying capacity to calculate two-dimensional riverbed deformation, and for partial unreasonableness, the value of every parameter in sediment carrying capacity equation could be adjusted beforehand according to verification data, then scouring and silting deformation calculation of this river channel is carried out with these parameters, and this method could basically get rather precise calculation effect. m 3 U S k gh * (2-21) Where k, m are sediment carrying capacity coefficient and exponent; U is sectional mean

18 velocity, is sediment mean settling velocity, which is calculated using Zhangruijing equation Distribution pattern of sediment carrying capacity gradation Nowadays, the mechanism of flow group sediment carrying capacity or sediment carrying capacity gradation remains unclear. However, it can be seen clearly that sediment in water has two sources: one is coming from the upstream inflow, and the other is coming from diffusion of the river bed because of the turbulence and diffusion effect of inflow. flow sediment carrying capacity is taken as the sediment concentration of sediment transport in equilibrium and it s gradation should be related to these two sediment sources. Comprehensively considering the beforehand trial calculation values and upstream inflow sediment concentrations of every group, the proportions of every group are taken as sediment carrying capacity gradations, that is: ' k *k k ' (s*k sk ) k k d P kkd s s (2-22) Group sediment carrying capacity: S *, k Pk S* k kd (2-23) ' S Where k is the mean sediment concentration in the upstream section (in group k); S *k denotes the trial calculation value of group sediment carrying capacity assuming the inflow is clear water. k d denotes the number of the minimum grain size group of bed material load Treatment pattern of bed load transport rate Bed load transport rate adopts bed load transport rate experience curve of Yangtze River scientific research institute: V D V D gd m 1 h m d gb ~ d gd 1 m h U m 50, d 0.06 Where VD is bottom velocity, U is sectional mean velocity Treatment pattern of bed material load gradation Bed material load gradation adjustment adopts Weizhilin mode, which was once used in the model of prediction for sediment deposition in the initial impoundment period of the Three Gorges Reservoir. The composition of river bed is generalized as surface layer, middle layer and bottom layer, thus thickness and average gradation of every layer are respectively denoted

19 as h u, h m, hb and P uk, P mk, P bk. Surface layer is the exchange layer of sediment, while middle layer is the transition layer and bottom layer is the sediment scouring limit layer. It s regulated in every calculation period, interface of every layer are fixed, and the exchange of sediment is limited to the surface layer, while middle layer and bottom layer are temporarily non-influenced. Figure 2-2 is schematic figure of mixed layer thickness of bed material. In period end, move the surface and middle layers up or down according to erosion or deposition, and keep the thickness of these two layers unchanged, while change the bottom layer thickness according to sedimentation and scouring depth. The specific calculation process is: 0 assuming at initial time in certain period, the surface size grading is P uk, sedimentation and scouring depth in this period and sedimentation and scouring depth of the group k are respectively Z b and Z bk, thus in period end, gradation of the part above surface changes to: h P Z ' 0 ' u uk bk uk (2-24) hu Z b P Then redefine the position and composition of every layer, and their positions move according to the change of bed surface, because the thickness of surface and middle layers remain unchanged. h u 表层 h m 中层 h b 底层 冲刷时 h Z b b h 淤积时 b Z b Z b Z b Fig. 2-2 Mixed layer thickness of bed material Treatment of mobile boundary There are often central bars and point bars in river channels, the positions of waterlines (closing boundary) are always changeable, because the water levels change quite great in dry and flood seasons. Therefore it s required to solve the problem of automatic adjustment of waterlines in mathematical model. If there isn t good solution, it will cause concussion of the result and even distortion. In this model, freezing method is used in the treatment of mobile boundary, which is according to the elevation of river bed at water level nodes, emergence of

20 the grid unit can be judged. If not, roughness is the normal value, vice versa, the value of roughness is taken an infinite positive number. Meanwhile, in order to have no influence on the solving of inflow governing equations, a thin water layer is given at emergence nodes and the thickness is given 0.5cm generally. 3 SELECTION OF CALCULATION CONDITIONS AND LAYOUT OF SCHEMES 3.1 COMPUTATIONAL GRIDS OF THE MODEL Computational grids in the upstream of dam site The upstream calculation region of plane two-dimensional flow and sediment mathematical model of Pak Beng HPP in dam section is the 3 km river channel (about 2600 m upstream of the upper channel) before dam, and dam site is the entrance control section. Computational grids are generated based on elliptic differential equation, and total number is ( , where grid number is 106 in direction (main inflow direction) and 151 in direction (the transverse direction of main inflow). Meanwhile, to keep the consistence of grid size with the size of overflow outlet, sand flushing holes and debris barrier, the mesh refinement treatment is conducted in the nearby of above hub arrangements. Attached figure 3.1 is the topographic figure of the upstream calculation region of dam site. Attached figure 3.2a is the schematic fig.ure of the calculation grids in the upstream of dam site Computational grids in the downstream of dam site The downstream calculation region of plane two-dimensional flow and sediment mathematical model of Pak Beng HPP in dam section is the 2 km river reach (about 600 m downstream of the lower channel) below dam. Calculation grids are generated based on elliptic differential equation, and total number is 15251( ), where grid number is 101 in direction (main inflow direction) and 151 in direction (the transverse direction of main inflow). Meanwhile, to keep the consistence of grid size with the size of overflow outlet,sand flushing holes and debris barrier, the mesh refinement treatment is carried out in the nearby of above hub arrangements. Attached figure 3.2b is the schematic fig.ure of calculation grids in the downstream of dam site. 3.2 CALCULATION CONDITIONS OF THE MODEL Water and sediment data for computation (1) Water and sediment representative series The water and sediment series of five-year water and sediment process between 1984 and 1988 are used to simulate the entrance process in this period sedimentation calculation. These five-year data contain rich water and rich silts, middling water and middling silts, less water and less silts, and the annual average sediment discharge,

21 sediment concentration and sediment ratio in flood season of the representative series are close to the annual average value. The contrast between the average values in addition to distribution among years and long series annual average values sees table 3-1. According to development succession of the cascade power stations in Lancang River, Pak Beng HPP should be constructed and go to production after reservoirs such as Xiaowan and Nuozhadu, thus inflow of the reservoir for sedimentation calculation should use the representative discharge between 1984 and 1988 after the runoff regulation of reservoirs such as Xiaowan and Nuozhadu, and the inflow sediment is the representative sediment amount between 1984 and 1988 after sediment retaining by the upstream cascade stations. This series of water and sediment data are circular permutated to form 30-year series then taken as water and sediment data for computation in the upstream of Pak Beng power station. Attached figure 3.3a and 3.3b are respectively inlet flow and sediment concentration process figures in the upstream of dam. Water and sediment data for computation in the downstream of Pak Beng power station are obtained according to one-dimensional sediment mathematical model calculation results of Kunming Institute. Attached figure 3.4a and 3.4b are respectively outlet flow and sediment concentration process figures in the downstream of dam. Representative stage Table3-1 Contrast between the average values in addition to distribution among years and long series annual average values Discharge (m 3 /s) Percentage of flow discharge from June to October(%) Sediment mount (10 4 t) Percentage of sediment from June to October (%) sediment concentration (kg/m 3 ) 1984~ Annual average value (2) Sediment grading data a Sediment grading data in the upstream of dam According to the analysis of sediment data for calculation, suspend load is grouped to five, and bed load is the same. The suspend load has no measured data, so its gradation is replaced by bed material gradation. The separation particle size and gradation of suspend load and bed load sees table 3-2. Where, median diameter of suspend load d 50 =0.0077mm, average diameter d m =0.0224mm, maximum diameter is 2.45mm; median diameter of bed load d 50 = 16.9mm, average diameter d m =28.0mm, maximum diameter is160mm

22 Table 3-2 Separation particle size of sediment size distribution (mm) Sediment weight percent which is less than a certain particle size (%) Bed load Suspend load b Sediment grading data in the downstream of dam Sediment grading data in the downstream of dam are obtained according to one-dimensional sediment mathematical model calculation results of Kunming Institute Outlet water level process (1) Outlet water level process of calculating river channel in the upstream of dam According to operation pattern of Pak Beng HPP, considering producing power generation benefit of the power station and improving navigation conditions, the power station should run at the normal pool level of 340m in flood season, and lower water level to 335m in withered season. When inflow of the reservoir is smaller than m3/s, which is the flood of 3-year frequency, incoming discharge is relatively little, thus reservoir water level should keep at the normal pool level of 340 m; When inflow of the reservoir is greater than 13200m 3 /s, which is the flood of 3-year frequency, and smaller than14900 m 3 /s, which is the flood of 5-year frequency, reservoir water level should keep at the normal pool level of 340 m; When inflow of the reservoir is greater than14900 m 3 /s, which is the flood of 5-year frequency, it should open sluice gates for sediment release. (2) Outlet water level process of calculating river reach in the downstream of dam According to Developmental research on runoff type hydroelectric projects of Mekong river main stream, the average water level of downstream Luang Prbang power station is overlapped about 1 meter with Pak Beng HPP in dry season. Besides, the commissioning dates are close to each other, most operation period of Pak Beng HPP after completed will be influenced by Luang Prbang power station, thus outlet water level process is difficult to determine. Therefore, the impact of Luang Prbang power station is taken into account in the calculation of outlet water level, and one-dimentional water and sediment mathematical model is established in the river channel between somewhere 500m away from the lower approach channel in Pak Beng hubs and Pak Beng water level sites. According to dispatch and operation pattern of reservoir within data of release water and sediment process, the long series of scouring-silting variation of river channel is obtained, and water level process

23 obtained by one-dimentional water and sediment mathematical model is adopted in outlet water level process of plane two-dimensional water and sediment mathematical model. Attached figure 3.4c is water level process figure of calculation outlet. 3.3 LAYOUT OF CALCULATION SCHEMES For 30-year series of water and sediment process, inlet flow and sediment concentration within outlet water level both in upstream and downstream of dam are generalized as 1338 process level. According to research contents, five schemes are drafted out. Scheme one is calculating and analyzing flow regime in the upstream of power station, sedimentation(especially in important positions such as the front of power house intake, the debris barrier frontier, sand ducts and the upper approach particularly the entrance area and so on), sediment concentration through turbines within gradation situation in 30 years of reservoir operation. Scheme two is analyzing the flushing effect of united application of flood release and scouring sluices within channel scouring sluices when flood release as the dam meets with a big flood on the basis of 5-year deposited topography obtained by scheme one. Scheme three is calculating and analyzing flow regime in the downstream of power station, sedimentation (especially in important positions such as the lower approach particularly the entrance area and so on) in the 5 years of reservoir operation. Scheme four is analyzing the flushing effect of channel scouring sluices under different working conditions on the basis of 5-year deposited topography obtained by scheme three

24 4 CALCULATION RESULTS AND ANALYSES 4.1 SEDIMENT SCOURING-SILTING PROCESS ANALYSES OF RIVER REACH IN THE UPSTREAM OF DAM FOR SCHEME ONE In scheme one, 30-year series of scouring-silting process in the reservoir area before dam were calculated based on original forms. According to results, deposited topography of river reach in the upstream of dam in the fifth year, tenth year, fifteenth year, twentieth year, twenty-fifth year, and thirtieth year after power station runs were analyzed, within flow regime under characteristic discharge, sedimentation regime in important positions such as the front of power house intake, the debris barrier frontier, sand ducts and the upper approach particularly the entrance area and so on, in addition to sediment concentration through turbines and gradation situation Flow regime and deposited topography analysis of river reach in the upstream of dam To analyze deposited topography of river reach in the upstream of dam and the effect of debris barrier, analyses of deposited topography of river reach in the upstream of dam and funnel-shaped before flood releasing and scouring sluices in the fifth year, tenth year, fifteenth year, twentieth year, twenty-fifth year, and thirtieth year were carried out. Attached figure 4.1 is schematic figure of funnel-shaped axes before dam, among which, line A is profile line in order to analyze the regime of funnel-shaped before flood releasing and scouring sluices, line B is profile line in order to analyze the regime of funnel-shaped before intake of number 5 unit, line C is profile line in order to analyze the regime of funnel-shaped before intake of number 14 unit. In this calculation, when the inflow of the reservoir is less than 6424 m 3 /s, which is full capacity operation discharge of the units, the opening of units is in accordance with the order from right to left, namely number 1 unit is opened prior to the others. To analyze the problem of flow regime after deposition year by year, three typical flow rate stages are chosen in this report: (1).Discharge is 1290 m 3 /s, water level before dam is 335 m, and this discharge is close to the lowest navigable discharge. At this flow rate stage, flow is released through the units of number1, 2, 3and 4. (2). Discharge is 3077 m 3 /s, water level before dam is normal pool level of 340 m, and this discharge is close to the annual average discharge of the dam site. At this flow rate stage, flow is released through the units from number 1 to 8. (3). Discharge is m 3 /s, water level before dam is normal pool level of 340 m, and this discharge is close to discharge corresponding with the flood of 3-year frequency. At this flow rate stage, flow is released through units, sand ducts, flood release and scouring sluices and channel scouring sluice. (1) The 0 year analysis

25 Attached figure 4.2a is the local initial topography elevation fig.ure of river reach in the upstream of dam, and Attached figures 4.2b, 4.2c and 4.2d are respectively local flow field figures at flow rate stages of 1290 m 3 /s, 3077 m 3 /s and m 3 /s. It can be seen from the figures that, flow regime before power house is rather good at the 3 typical flow rate stages. At the two flow rate stages of 1290 m 3 /s and 3077 m 3 /s, flood release and scouring sluices and channel scouring sluice aren t opened, thus backflow exists and sediment is apt to deposit. (2) The 5th year analysis Attached figures 4.3a, 4.3b and 4.3c are respectively local flow field figures at flow rate stages of 1290m 3 /s, 3077m 3 /s and 13272m 3 /s. Attached figures 4.3d and 4.3e are respectively local topography elevation fig. of river reach in the upstream of dam and sediment deposition thickness distribution fig. in the end of 5th year. It can be seen that sediment mainly deposits in the original main stream and the upstream side of debris barrier, and more in the upstream side of debris barrier, where deposition thickness is about 16 m, and distribution exhibits zonal distribution. The elevation in the upstream side of debris barrier is 315m, which is lower than the crest elevation of 325.0m, it shows that 5 years after the reservoir operates, debris barrier has played an effective role in intercepting bed load, however, there is still suspended load deposited in front of power house, and part will be taken to units. The initial topography elevation of unit intakes beforehand is 282.2m. 5 years after the reservoir runs, sediment deposition thickness before intake of number 5 unit will be less than 1m, and deposited elevation is 282.7m. Sediment deposition thickness before intake of number 14 unit will be about3.5m, and deposited elevation is 285.7m. Sediment before flood release and scouring sluices will deposit to the elevation of 318.2m (when discharge is large, flood release and scouring sluices are opened, and sediment in front of which will be flushed downstream, thus there is nearly no deposition before sluices. This number is the sedimentation situation in the end of 5th year, the same below.) Sediment in the entrance area of upper approach will deposit to the elevation of 330.0m The following figures 4-1a, 4-1b and 4-1c are respectively longitudinal profile figures of funnels before flood release and scouring sluices and unit intakes in the end of 5th year. The calculation shows that: in the 5th year, sediment before flood release and scouring sluices will deposit to the elevation of 318.2m, the gradient of the funnel before which is rather small and gradient of longitudinal profile is about 1/40, ranging about 200m. The longitudinal profile gradient of the funnel before intake of number 5 unit is about 1/2.8, ranging about 60m. The longitudinal profile gradient of the funnel before intake of number 14 unit is about1/3.0, ranging about 60m

26 高程 (m) 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 5 年后计算地形 5 Years Operation Elevation (m) 距进水口距离 Distance (m) (m) Fig. 4-1a The longitudinal profile figure of the funnel before flood release and scouring sluices in the end of 5th year 原始地形 Original 5 年后计算地形 5 Years Operation 距进水口距离 (m) Figure 4-1b The longitudinal profile figure of the funnel before number 5 unit in the end of 5th year

27 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River 310 Elevation (m) 原始地形 Original 5 年后计算地形 5 Years Operation 距进水口距离 Distance (m) (m) Fig. 4-1a (3) The 10th year analysis The longitudinal profile figure of the funnel before number 14 unit in the end of 5th year Attached figures 4.4a, 4.4b and 4.4c are respectively local flow field figures at flow rate stages of 1290 m 3 /s, 3077 m 3 /s and 13272m 3 /s. Attached figures 4.4d and 4.4e are respectively local topography elevation fig. of river reach in the upstream of dam and sediment deposition thickness distribution fig. in the end of 10th year. Seeing from figures, sediment of river reach in the upstream of dam continues to deposit. The elevation in the upstream side of debris barrier will be 319 m, which is lower than the crest elevation of m, it shows that 10 years after the reservoir runs, debris barrier still can intercept the bed load, however, there is still part of suspended load entering into units. Sediment before intake of number 5 unit will deposit to the elevation of m, while sediment before intake of number 14 unit will deposit to the elevation of m, which exceeds bottom board elevation of m of power station intake. The main reason is that, units close to non-overflow dam section on the left bank aren t opened frequently, flow regime before which is rather bad, thus backflow exists, and sediment is apt to deposit. The elevation of sediment deposition before flood release and scouring sluices changes little, for the reason that flood release and scouring sluices are opened on and off, thus sediment before which are taken to downstream by the flow continuously. In the entrance area of upper approach, sediment continues to deposit, and will deposit to the elevation of m,which is close to the reservoir operation level of 335 m in dry season (from December to May).The deposited sediment will affect navigation in dry season. The following figures 4-2a, 4-2b and 4-2c are respectively longitudinal profile figures of funnels before flood release and scouring sluices and unit intakes in the end of 10th year

28 高程 (m) 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River The calculation shows that: in the 10th year of operation, sediment before flood release and scouring sluices will deposit to the elevation of 318.4m, the gradient of the funnel before which is about 1/30. The longitudinal profile gradient of the funnel before intake of number 5 unit is about1/2.7. The longitudinal profile gradient of the funnel before intake of number 14 unit is about 1/ Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 Distance (m)(m) Fig. 4-2a The longitudinal profile figure of the funnel before flood release and scouring sluices in the end of 10th year Elevation (m) 原始地形 Original 10 年后计算地形 Years Operation 距进水口距离 (m) Distance (m) Figure 4-2b The longitudinal profile figure of the funnel before number 5 unit in the end of 10th year

29 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 Distance (m) (m) Fig. 4-2c The longitudinal profile figure of the funnel before number 14 unit in the end of 10th year (4) The 15th year analysis Attached figures 4.5a, 4.5b and 4.5c are respectively local flow field figures at flow rate stages of 1290 m 3 /s, 3077 m 3 /s and 13272m 3 /s. Attached figures 4.5d and 4.5e are respectively local topography elevation figures of river reach in the upstream of dam and sediment deposition thickness distribution fig. in the end of15th year. The elevation in the upstream side of debris barrier is 321 m, which is lower than the crest elevation of m. It shows that at this time debris barrier can still intercept the bed load to enter into the power station. Sediment before intake of number 5 unit will deposit to the elevation of m, while sediment before intake of number 14 unit will deposit to the elevation of m. The elevation of sediment deposition before flood release and scouring sluices changes little. In the entrance area of upper approach, sediment continues to deposit, and will deposit to the elevation of 337.0m, which exceeds the reservoir operation level in dry season. The deposited sediment will suspend navigation in dry season. The following figures 4-3a, 4-3b and 4-3c are respectively longitudinal profile figures of funnels before flood release and scouring sluices and unit intakes in the end of 15th year. The calculation shows that: the gradient of the funnel before flood release and scouring sluices is about 1/28. The longitudinal profile gradient of the funnel before intake of number 5 unit is about 1/2.7. The longitudinal profile gradient of the funnel before intake of number 14 unit is about 1/

30 高程 (m) 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 Distance (m) (m) Fig. 4-3a The longitudinal profile figure of the funnel before flood release and scouring sluices in the end of 15th year Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 Distance (m) (m) Figure 4-3b The longitudinal profile figure of the funnel before number 5 unit in the end of 15th year

31 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 Distance (m) (m) Fig. 4-3c The longitudinal profile figure of the funnel before number 14 unit in the end of 15th year (5) The 20th year analysis Attached figures 4.6a, 4.6b and 4.6c are respectively local flow field figures at flow rate stages of 1290 m 3 /s, 3077 m 3 /s and 13272m 3 /s. Attached figures 4.6d and 4.6e are respectively local topography elevation figures of river reach in the upstream of dam and sediment deposition thickness distribution fig. in the end of 20th year. The elevation in the upstream side of debris barrier is 323 m, which is lower than the crest elevation of m. Sediment before intake of number 5 unit will deposit to the elevation of m, while sediment before intake of number 14 unit will deposit to the elevation of 291.5m. The elevation of sediment deposition before flood release and scouring sluices changes little. In the entrance area of upper approach, sediment will deposit to the elevation of 339.0m, and water depth won t satisfy the navigation condition. The following figures 4-4a, 4-4b and 4-4c are respectively longitudinal profile figures of funnels before flood release and scouring sluices and unit intakes in the end of 20th year. The calculation shows that: the gradient of the funnel before flood release and scouring sluices is about 1/27. The longitudinal profile gradient of the funnel before intake of number 5 unit is about 1/2.7. The longitudinal profile gradient of the funnel before intake of number 14 unit is about 1/

32 高程 (m) 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 (m) Distance (m) Fig. 4-4a The longitudinal profile figure of the funnel before flood release and scouring sluices in the end of 20th year Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 (m) Distance (m) Figure 4-4b The longitudinal profile figure of the funnel before number 5 unit in the end of 20th year

33 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 年后计算地形 Years Operation 距进水口距离 Distance (m) (m) Fig. 4-4c The longitudinal profile figure of the funnel before number 14 unit in the end of 20th year (6) The 25th year analysis Attached figures 4.7a, 4.7b and 4.7c are respectively local flow field figures at flow rate stages of 1290 m 3 /s, 3077 m 3 /s and 13272m 3 /s. Attached figures 4.7d and 4.7e are respectively local topography elevation fig. of river reach in the upstream of dam and sediment deposition thickness distribution fig. in the end of 25th year. At this time, thickness of erosion and siltation changes little, and there is still sedimentation in some regions. The elevation in the upstream side of debris barrier is 325 m, which is close to the crest elevation, and it shows that at this time debris barrier is going to be of no effect in intercepting the bed load to enter into the power station. Sediment before intake of number 5 unit will deposit to the elevation of m, while sediment before intake of number 14 unit will deposit to the elevation of m. The elevation of sediment deposition before flood release and scouring sluices changes little. In the entrance area of upper approach, sedimentation thickness changes little, and water depth won t satisfy the navigation condition. The following figures 4-5a, 4-5b and 4-5c are respectively longitudinal profile figures of funnels before flood release and scouring sluices and unit intakes in the end of 25th year. The calculation shows that: the gradient of the funnel before flood release and scouring sluices is about 1/27.3, ranging about 200 m. The longitudinal profile gradient of the funnel before intake of number 5 unit is about 1/2.8. The longitudinal profile gradient of the funnel before intake of number 14 unit is about 1/

34 高程 (m) 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 2525Years 年后计算地形 Operation 距进水口距离 Distance (m)(m) Fig. 4-5a The longitudinal profile figure of the funnel before flood release and scouring sluices in the end of 25th year Elevation (m) 原始地形 Original 25 25Years 年后计算地形 Operation 距进水口距离 Distance (m)(m) Fig. 4-5b The longitudinal profile figure of the funnel before number 5 unit in the end of 25th year

35 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation 310 (m) 原始地形 Original 2525Years 年后计算地形 Operation 距进水口距离 Distance (m) (m) Fig. 4-5c The longitudinal profile figure of the funnel before number 14 unit in the end of 25th year (7) The 30th year analysis Attached figures 4.8a, 4.8b and 4.8c are respectively local flow field figures at flow rate stages of 1290 m 3 /s, 3077 m 3 /s and 13272m 3 /s. Attached figures 4.8d and 4.8e are respectively local topography elevation fig. of river reach in the upstream of dam and sediment deposition thickness distribution fig. in the end of 30th year. At this time, debris barrier is of no effect, part of bed load enters into the front end of power house, but with the effect of sand ducts, sedimentation before intakes of power house isn t heavy, and little bed load enters into units. Sediment before intake of number 5 unit will deposit to the elevation of m, which is close to the bottom elevation of sand ducts. Sediment before intake of number 14 unit will deposit to the elevation of 293.5m. The elevation of sediment deposition before flood release and scouring sluices changes little. In the entrance area of upper approach, sediment will deposit to the elevation of m, which is close to the reservoir operation level of 340 m in flood season (from June to November). The following figures 4-6a, 4-6b and 4-6c are respectively longitudinal profile figures of funnels before flood release and scouring sluices and unit intakes in the end of 30th year. The calculation shows that: the gradient of the funnel before flood release and scouring sluices is about 1/27.3, ranging about 200 m. The longitudinal profile gradient of the funnel before intake of number 5 unit is about 1/2.84, ranging about 60 m. The longitudinal profile gradient of the funnel before intake of number 14 unit is about 1/3.8, ranging about 60 m

36 高程 (m) 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation (m) 原始地形 Original 3030Years 年后计算地形 Operation 距进水口距离 Distance (m) (m) Fig. 4-6a The longitudinal profile figure of the funnel before flood release and scouring sluices in the end of 30th year Elevation (m) 原始地形 Original 3030Years 年后计算地形 Operation 距进水口距离 Distance (m) (m) Figure 4-6b The longitudinal profile figure of the funnel before number 5 unit in the end of 30th year

37 高程 (m) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Elevation 310 (m) 原始地形 Original 3030Years 年后计算地形 Operation 距进水口距离 Distance (m) (m) Fig. 4-6c The longitudinal profile figure of the funnel before number 14 unit in the end of 30th year It can be seen from beforehand results that: when the reservoir runs 15 years, topography in the upstream side of debris barrier is lower than the crest elevation, thus debris barrier can intercept the coarse particle to enter to water turbines; When the reservoir runs 20 years, as the topography of original main river channel deposits further, deposited elevation is close to the elevation of upper debris barrier, which shows the upper debris barrier is going to be of no effect in intercepting the coarse particle. The elevation of lower segment, which is close to dam, is still higher than deposited elevation of main river channel. The sediment retaining weir could play the role of intercepting the coarse particle, but the effect is weakened. When the reservoir runs 25years, debris barrier is out of action. The existing observational data of erosive funnel topography of hydroelectric projects think that, the longitudinal gradient of erosive funnel is close to repose angle of sediment under water. The longitudinal gradient of erosive funnel is chosen to compare with repose angle of sediment under water in the report. The model calculation results show that, longitudinal profile gradient of funnel-shaped before intake of power house is about 1/2.84, and the angle is about 19.4 o. Besides, d 50 of sediment in the reservoir region is about mm, and the relevant repose angle is about 18 o. The gradient of erosive funnel is equivalent to repose angle of sediment under water Sedimentation of powerhouse intake The powerstation is placed on the left side of the main channel, and there are sixteen bulb turbine units inside the powerstation. To make sure the powerstation inlet clear, eight sand ducts are arranged under the dam, and each sand duct is arranged between two bulb turbine units, mainly for excreting the sediment deposition before the powerstation inlet. The elevations of powerstation inlet and sand duct inlet floor are respectively 288.9m and 285.6m. Attached figure 4.9 and figure 4.10 are respectively profiles of the powerhouse and sand duct. Attached figures 4.11a to 4.11f are respectively the distribution of local sediment deposition

38 thickness before the powerstation at the end of the 5 th, 10 th, 15 th, 20 th, 25 th and 30 th year. The patterns of sediment deposition before the number 5 unit can be seen in figures 4-1b to figures 4-6b of this chapter. The elevation of initial terrain before unit bulb turbine inlet is m, and increases to about 282.7m after five years of the reservoir operation. As the calculating results of the model, the debris barrier can effectively intercept coarse sediment from entering the turbine, and sediment deposition before the powerhouse is less. After ten years of the reservoir operation, the elevation of initial terrain before unit bulb turbine inlet increases to about m, and suspended sediment further deposit before the powerhouse. After thirty years of the reservoir operation, since the sediment retaining weir is out of action, the elevation of initial terrain before unit bulb turbine inlet increases to about m, which is equal to the elevation of sand duct inlet floor. The patterns of sediment deposition before the number 14 unit which is near the non-overflow dam in the left bank can be seen from figure 4-1c to figure 4-6c in Section of this chapter. After five years of the reservoir operation, the sedimentation elevation in front of the water inlet will be about 285.7m. After ten years of the reservoir operation, the sedimentation elevation in front of the water inlet will be about 290.0m, which is more than powerhouse intake bottom elevation 288.9m. After fifteen years of the reservoir operation, the sedimentation elevation in front of the water inlet will be about 290.9m. After twenty years of the reservoir operation, the sedimentation elevation in front of the water inlet will be about 291.5m. After twenty-five years of the reservoir operation, the sedimentation elevation in front of the water inlet will be about 292.9m. After thirty years of the reservoir operation, the sedimentation elevation in front of the water inlet will be about 293.5m. The sedimentation in front of the number 14 unit is more than that of the number 5 unit. This is because the opening order for the units is from right to left in the model calculation, which leads the number 1 unit is opened firstly. The units near the non-overflow dam in the left bank are not opened often. As a result, the flow regime in front of this place is bad and the backflow is formed so that sedimentation happened easily (The local flow field in front of number 14 unit can be seen in attached figure 4.12). To solve the sedimentation problem of the water inlet of the number 14 unit near the non-overflow dam in the left bank, the operation time for these units should be increased to avoid the total sedimentation in the water inlet Variation analysis of sediment concentration through turbine and particle size distribution Unit 1 which is close to flood release and scouring sluices is chosen to analyze. The variation of annual average sediment concentration through turbine of unit 1 is shown in table 4-1. It indicates that annual mean sediment concentration through turbine of this power station increases as the reservoir running time increases

39 Table4-1 Annual average sediment concentration through turbine of unit 1 Operating year Sediment concentration through turbine (kg/m 3 ) At the end of five years At the end of ten years At the end of fifteen years At the end of twenty years At the end of twenty five years At the end of thirty years Traditionally, sediment with particle size larger than 0.05mm has more influence on the water turbine when through it. To analyze the inflow sediment situation, variation of particle size distribution which covers particle size larger than 0.062mm is elected according to the sediment grading data in this report. Table 4-2 gives variation of proportion of sediment particle size larger than 0.062mm of unit 1. It shows that the proportion increases as reservoir running time increases. This points out that the function of debris barrier has been weakened. Table 4-2 Variation of proportion of sediment particle size larger than mm of unit 1 Operating year Proportion of sediment particle size larger than 0.062mm (%) At the end of five years At the end of ten years At the end of fifteen years At the end of thirty years Analysis of sedimentation at the entrance of the upper approach channel At the end of the 5th year, the entrance of the upper approach channel will deposit to m. At the end of the 10th year, the sediment continues to deposit to m, which is close to the operation water level of 335 m in dry period (from December to May), that is to say, the sediment at the entrance will affect the navigation in the dry period. At the end of the 15th year, sediment deposition will reach the elevation of m, which is higher than the reservoir operation water level in the dry period, and as a result, the deposition at the entrance of the approach channel may cause suspension of shipping in the dry period. At the end of the 20th year, the deposition at the entrance of the upper approach channel will reach the elevation of 339.0m, and the water depth does not meet the navigation condition in the flood period. At the end of the 25th year, the deposition at the entrance of the upper approach channel changes little; At the end of the 30th year, the entrance of the upper approach channel will deposit to about m, which is close to the reservoir operation water level in flood period (from June to November). After the operation of the hydropower station, there will be accumulative sediment deposition at the entrance of the upper approach channel, thus dredging engineering is needed for navigation requirements Summary (1) The sedimentation on the upstream of the dam is shown in Table 4-3. (2) Variation of sediment concentration through turbine. After five years of the reservoir operation, the annual average sediment concentration through the turbine of the unit

40 (near the flood-discharging and sand-sluicing gate) is about 0.323kg/m 3. After ten years of the reservoir operation, the annual average sediment concentration through the turbine of power station was about kg/m 3. After fifteen years of the reservoir operation, the annual average sediment concentration through the turbine of power station was about kg/m 3. After twenty years of the reservoir operation, the annual average sediment concentration through the turbine of power station was about 0.334kg/m 3. After twenty-five years of the reservoir operation, the annual average sediment concentration through the turbine of power station was about kg/m3. After thirty years of the reservoir operation, the annual average sediment concentration through the turbine of power station was about kg/m 3. It shows that with the running time of the reservoir increasing, the annual average sediment concentration over the machine of power station increases. (3) Variation of particle size through turbine. After five years of the reservoir operation, the sediment through the unit 1 (near the flood-discharging and sand-sluicing gate) whose particle size is greater than 0.062mm accounts for 1.32%. After ten years of the reservoir operation, the sediment through the turbine whose particle size is greater than mm accounts for 1.45%. After fifteen years of the reservoir operation, the sediment through the turbine whose particle size is greater than mm accounts for 1.58%. After thirty years of the reservoir operation, the sediment through the turbine whose particle size is greater than mm accounts for 2.95%. It shows that with the running time of the reservoir growing, the proportion of sediment whose particle size through the turbine is greater than mm increases

41 Table 4-3 The sedimentation on the upstream of the dam Operating years At the end of five years At the end of ten years At the end of fifteen years At the end of twenty years At the end of twenty five years At the end of thirty years Sedimentation on the upstream of the sediment retaining weir The sedimentation elevation is about 315m. The sedimentation elevation is about 319m. The sedimentation elevation is about 321 m. The sedimentation elevation is about 323m. The sedimentation elevation is about 325 m, and the sediment retaining weir is going to lose its effectiveness. The sediment retaining weir lost its effectiveness. The sedimentation in front of the water intake The sedimentation elevation in front of No.5 generating unit is 282.7m and is m in front of No.14 generating unit. The sedimentation elevation in front of No.5 generating unit is 283.3m and is 290m in front of No.14 generating unit. The sedimentation elevation in front of No.5 generating unit is m and is m in front of No. 14 generating unit. The sedimentation elevation in front of No.5 generating unit is m and is m in front of No.14 generating unit. The sedimentation elevation in front of No.5 generating units is m and is m in front of No.14 generating unit. The sedimentation elevation in front of No.5 generating units is m and is m in front of No.14 generating unit. The sedimentation in front of the flood-discharging and sand-sluicing gate The sedimentation elevation in front of the gate is about m, and the slope of the scour funnel in the front is about 1/40. The sedimentation elevation in front of the gate is about 318.4m, and the slope of the scour funnel in the front is about 1/30. The sedimentation elevation in front of the gate varies slight, and the slope of the scour funnel in the front is about 1/28. The slope of the scour funnel in the front is about 1/27. The slope of the scour funnel in the front is about 1/27.3. The slope of the scour funnel in the front is about 1/27.3. The sedimentation in the entrance of upstream navigation approach channel The sedimentation elevation is 330 m. The sedimentation elevation is 334 m, which affects the navigation in dry season. The sedimentation elevation is m, which blocks the navigation in dry season. The sedimentation elevation is m, which doesn't match the navigation conditions in flood season. The sedimentation thickness varies slightly. The sedimentation elevation is m

42 4.2 FLUSHING EFFECT ANALYSES OF FLOOD RELEASE AND SCOURING SLUICES WITHIN CHANNEL SCOURING SLUICES FOR SCHEME TWO In scheme two, the scouring-silting situation of united application of flood release and scouring sluice within channel scouring sluices when flood release as the dam meets with a big flood is analyzed on the basis of 5-year deposited topography obtained by scheme one Flushing effect analyses of flood release and scouring sluices Attached figure 4.13 is sediment deposition thickness distribution figure in dam section before flood release and scouring sluices are opened. Attached figure 4.14 is sediment deposition thickness distribution fig. in dam section after flood release and scouring sluices are opened. It can be seen from the diagram, when flood release and scouring sluices are opened, most deposited sediment will be flushed by flood and there is hardly any deposition in a certain range of sluices, as bottom elevation of flood release and scouring sluice is the same level with original river bed. It shows that: the capacity of flood release and scouring sluices is great, and it will open sluice gates for sediment release, thus scouring effect is obvious, and deposition before dam is little. Furthermore, the scouring-silting situation in debris barrier after flood release and scouring sluices are opened is analyzed in the report. Attached figure 4.15 is the distribution fig. of flow velocity in the debris barrier nearby after flood release and scouring sluices are opened. It can be seen from the diagram, when flood release and scouring sluices are opened, most deposited sediment before sluices will be flushed by flood. However, for the deposited sediment in the debris barrier nearby, only the part close to dam section will be taken away, and flushing effect of upper debris barrier isn t obvious, for flow velocity is small. It s suggested that the angle of debris barrier should be adjusted or artificial desilting be carried out, in order to solve the problem that flushing effect of upper debris barrier isn t obvious when flood release and scouring sluices release sediment Flushing effect analyses of channel scouring sluice The report gives sediment deposition thickness distribution figures in the entrance area of upper approach channel before and after channel scouring sluices are opened (attached figures 4.13 and 4.14). By contrast, when the channel scouring sluices are opened, the deposited thickness in the entrance area of upper approach changes little, and little deposited sediment in the entrance area of upper approach will be flushed by downstream. It shows that: flushing effect of the channel scouring sluices isn t good at the large discharge. The report analyzes the discharge capacity of channel scouring sluice in 0 year. In 0 year, when the inflow of reservoir is 14904m 3 /s,power station stops to generate electricity, and the reservoir will open sluice gates for sediment release. By calculation, releasing discharge through channel scouring sluice is 1380m 3 /s. Attached figure 4.16 gives the fig. of flow

43 断面平均流速 (m/s) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River velocity distribution in the upper approach channel, it can be seen from the fig., flow velocity is rather large, which shows the discharge capacity of channel scouring sluices is great. The report analyzes the discharge capacity of channel scouring sluice in the end of 5th year based on 5-year deposited topography. In the end of 5th year, when the inflow of reservoir is m 3 /s,the reservoir will open sluice gates for sediment release. By calculation, releasing discharge through channel scouring sluices is 450m 3 /s, which shows the deposited sediment in the entrance area of upper approach affects inflow capacity, and releasing discharge through channel scouring sluice could not reach the designed discharge. Figure 4-7 gives the diagram of section average velocity distribution along the way in the upper approach channel, in the 0 year and 5th year. It could be seen that, in the 0 year, section average flow velocity is about 2.5 m/s in the approach channel; in the 5th year, section average flow velocity is only about 1.3 m/s in the approach channel, which shows in the end of 5th year, when flood releases in the situation of big flood, as the releasing discharge through channel scouring sluices could not reach the designed discharge, section average flow velocity in channel scouring sluices is rather small, and flushing effect isn t good. In order to improve the sediment siltation problem in the entrance area of upper approach channel, it s suggested to optimize the length of diversion dike and the types of the breakwater head to reduce the dredging engineering quantities. Velocity (m/s) 距闸门距离 Distance (m) (m) 第 0 Year 年第 5Year 5 年末 Fig. 4-7 Distribution of the section average flow velocity in the upper approach channel (unit: m/s) 4.3 SEDIMENT SCOURING-SILTING PROCESS ANALYSES OF RIVER REACH IN THE UPSTREAM OF DAM FOR SCHEME THREE Based on the original terrain, the sediment scouring-depositing process in the downstream of the dam site in 5 years in scheme three is worked out. According to the results, the terrain of the sediment scouring and depositing processes in the downstream reaches of the dam site in

44 the fifth year of the power plant will be analyzed and further studied. So will the flow regime under some characteristic discharge and the depositing regime of the sediment in lower approach channels, especially in some vital places like entrance areas, as well as the navigable flow conditions in approach channels after 5 years The analysis of the flow regime in the downstream reaches of the dam site In order to analyze the flow regime in the downstream reaches of the dam site of the initial year when the power plant works, three typical flow rate stages are selected in this report: (1) m 3 /s, which is the corresponding flow rate when floods occur every other year, and under which the ship lock is used normally and the flow is released through flood release and scouring sluices, sand ducts and units. (2) m 3 /s, which is the corresponding flow rate when floods occur every three years, and under which the ship lock is not used and the flow is released through flood release and scouring sluices, channel scouring sluices, sand ducts and units. (3) m 3 /s, which is the corresponding flow rate when floods occur every five years, and under which the power plant is not used while the reservoir adopts the operation mode of free sediment discharging, and the flow is released through flood release and scouring sluices, channel scouring sluices and sand ducts. See table 4-4 to get the specific scheduling mode. Attached figure 4.17a is the elevation fig. of the original terrain in the downstream reaches of the dam site. Attached figure 4.17b, attached figure 4.17c and attached figure 4.17d are the flow field charts in the downstream reaches of the dam site under the three typical flow rate stages respectively. Number Table 4-4 Discharge in total Reservoir regulation mode under the typical flow rates Discharge (m 3 /s) Units 1-4 Units 5-8 Units 9-12 Units Sand ducts Channel Flood-discharging scouring and sand-sluicing sluices gates Export water level (m) As can be seen from the figures, under the flow rate stage of m 3 /s and the ship lock is used, while the flow is released through flood release and scouring sluices, sand ducts and units, the flow regime in the downstream reaches of the main channel and the flood release and scouring sluices is relatively good, but there is reflux in the point bar of the channel, which easily lead to sediment deposition. The area next to the dam inside the approach channel is stagnant area. There is reflux in the entrance area, where the flow rate of the flow is about 1.2 m/s at most and the flow rate of the flow outside the entrance area is limited at 0.4 m/s at most. So under such flow rate stage, the navigable flow condition cannot be satisfied

45 The area between the stagnant area and the entrance recirculation area is the reverse recirculation area. Attached figure 4.18 shows the flow field chart near the entrance of the approach channel. Under the flow rate stage of m 3 /s and the ship lock is not used, the flow is released through flood release and scouring sluices, channel scouring sluice, sand ducts and units. The flow regime in the main channel and in the downstream reaches of the power house is relatively good. There is reflux only near the point bar of the channel. Under the flow rate stage of 14904m 3 /s, the flow is released through flood release and scouring sluices, channel scouring sluice and sand ducts. Because the flow rate of the currents released through flood release and scouring sluices is huge and the flow velocity is large, the flow pattern near the approach channel is in disorder The analysis of the terrain of scouring-silting in the downstream reaches of the dam site Attached figure 4.19a and attached figure 4.19b show the elevation of the terrain and the distribution of the depth of sediment scouring-silting in the downstream reaches of the dam site in the end of the fifth year respectively. As can be shown in the figures, after the dam is set up, the silt in the upper reaches is intercepted and the sediment concentration of the released discharge decreased, which lead to erosion in the lower reaches of the dam site, mainly in the original main channel, of a maximum scouring depth about 3.3m, deposition existing in some point bar. The calculation results of the model show that under the mode of reservoir dispatching drafted by the feasible report, the effects of sand scouring are not obvious when the flow rate is big and the channel scouring sluices are opened. After the power plant is in operation, there will be accumulated deposition in the approach channel, and the maximum depth of silt deposition near the entrance of the approach channel after five years is about 5.0m. Attached figure 4.20 shows the maximum depth distribution of silt deposition near the approach channel. Seen from the fig.ure, the main deposition of the silt is in the water mouth, and the depth of deposition in the approach channel decreases as the distance to the entrance increases The analysis of navigable currents condition of the lower approach channel The report analyzes the currents condition of the lower approach channel in the minimum navigable discharge after five years. Typical flow rate stage is selected as 1290 m3/s. The minimum navigable flow rate of 1260 m 3 /s is most correspondingly close to this flow rate stage, when the currents are released through four units. Table 4-5 shows the minimum water depth, width and bending radius of the channel when the navigable stage is needed. Table 4-6 shows the maximum flow rate limit in the entrance area of the approach channel. The navigation structure in Pak Beng HPP is a ship lock of single line and single stage, whose level is Ⅳ, with a maximum weight of navigable ship 500 tons

46 Referring to the table, the Ⅳ level water depth of the channel is no less than 1.6m~1.9m, and the width of the straight line segment is no less than 30m~45m. Table 4-5 Channel dimensions of natural and channelized river Table 4-6 Maximum velocity limits at water surface in the entrance area of the approach channel Attached figure 4.21a and attached figure 4.21b respectively show the isopleths of water depth and flow field chart near the entrance of the approaching channel under such flow rate stage. The calculation results show that under the minimum navigable discharge, the width of the channel when the water depth is 2 m deep is about 45 m, and backflow flow rate near the entrance is no more than 0.4 m/s. Although they can satisfy the navigation condition, they tend to obstruct navigation Summary This chapter studies and analyzes the deposited terrain of river reaches in the downstream of

47 dam site in the fifth year, the flow regime under some characteristic discharges and the deposited regime of the sediment in lower approach channel, especially in some vital places like entrance areas, as well as the navigable flow conditions in approach channels after 5 years. (1) The flow regime in the downstream reaches of the dam site. By analyzing the flow regime in the downstream reaches of the dam site of the first year under three typical flow rate stages when the power plant works, it is known that when floods occur less that every two years, the navigable flow condition cannot be satisfied because the reflux flow rate in the mouth of the approach channel is more than that of the maximum limit 0.4m/s. (2) The variation of scouring and silting in the downstream reaches of the dam site. The operation of the power plant has lead to erosion in the downstream reaches of the dam site, the maximum scouring depth of the original channel being about 3.3m with deposition existing in some point bar. There is accumulated deposition in the approach channel, and the maximum depth of silt deposition near the entrance of the approach channel after five years is about 5.0m. The depth of deposition in the approach channel decreases as the distance to the water entrance increases. (3) The condition of navigable currents of the lower approach channel. Under the minimum navigable discharge when the power plant works for five years, the width of the channel when the water depth is 2m deep is about 45m, and backflow rate near the entrance is no more than 0.4m/s. Although they can satisfy the navigation condition, they tend to obstruct navigation. 4.4 FLUSHING EFFECT ANALYSES OF THE CHANNEL SCOURING SLUICES FOR SCHEME FOUR Scheme four is analyzing the flushing effect of channel scouring sluice under different working conditions on the basis of 5-year deposited topography in the downstream of dam obtained by scheme three. Among them, the total releasing discharge of working condition 4.1 is 13272m 3 /s, and under this discharge level, the flow releases through flood release and scouring sluices (13 holes), the channel scouring sluices1 hole), sand ducts and units. The flushing time is one day. The total releasing discharge of working condition 4.2 is 7860m 3 /s, under which full capacity operation of units is considered and sediment is flushed by opening the channel scouring sluices at the same time. Under this discharge level, the flow releases through the channel scouring sluice (1 hole) and the units. The flushing time is one day. The total releasing discharge of working condition 4.3 is 1380m 3 /s. Under this discharge level, all flow releases through the channel scouring sluice (1 hole). The flushing time is one day. The total releasing discharge of working condition 4.4 is 3077m 3 /s, which is close to annual

48 average discharge. Under this discharge level, the flow releases through the units (4 sets), the channel scouring sluice (1 hole). The flushing time is one day. Table 4-7 gives the distribution of releasing discharge under each working condition. Discharge in total (m 3 /s) Table 4-7 Distribution of releasing discharge Export water level (m) flushing time (day) Discharge from units (m 3 /s) Discharge from sand ducts (m 3 /s) Discharge from flood-discharging and sand-sluicing gates (m 3 /s) Discharge from channel scouring sluices (m 3 /s) working condition working condition working condition working condition Attached figure4.22 shows the scouring-silting changes of sediment near the entrance area of lower approach channel in the case of working condition 4.1. It can be seen from the figure that: under large discharge, as the downstream water level is high, the sediment depositing at the entrance isn t washed away, instead, there is small amount of sediment depositing in the entrance of the approach channel, with the maximum thickness of 0.2m. At this moment, the effect of channel scouring sluice is not good. Attached figure 4.23 shows the scouring-silting changes of sediment near the entrance area of lower approach channel in the case of working condition 4.2. It can be seen from the fig.ure that: under this discharge, part of the sediment depositing at the entrance is washed away by the flow, and the maximum scouring thickness is about 0.1m. The effect of channel scouring sluice is not obvious. Attached figure 4.24 shows the scouring-silting changes of sediment near the entrance area of lower approach channel in the case of working condition 4.3. It can be seen from the fig.ure that: under this discharge, the maximum scouring thickness is about 5m, and the sediment depositing at the entrance is washed away by the flow in one day. The effect of channel scouring sluice is obvious, however, the generation benefit of power station will be affected considering all flow releases through the channel scouring sluice. Attached figure 4.25 shows the scouring-silting changes of sediment near the entrance area of lower approach channel in the case of working condition 4.4. It can be seen from the fig.ure that: under this discharge, the maximum scouring thickness is about 5m, and the sediment depositing at the entrance is washed away by the flow in one day basically. The effect of channel scouring sluice is obvious. From the beforehand calculation results, it can be seen that: under large discharge, as the downstream water level is high, the effect of channel scouring sluice is not good. For example,

49 when the discharge is about 7860 m 3 /s, full capacity operation of units is adopted and sediment is flushed by opening the channel scouring sluices at the same time, the maximum scouring thickness of 1 day is about 0.1m. The flushing effect is not ideal. On the contrary, in the case of small discharge, the flushing effect is obvious as water level in the downstream river channel is low. For instance, when the total releasing discharge is about 1380 m 3 /s and all flow releases through the channel scouring sluice (1 hole), the sediment depositing at the entrance will be washed away in one day; when the total releasing discharge is about 3077 m 3 /s, the water flow releases through the channel scouring sluice (1 hole) and the units, and sediment depositing at the entrance will be washed away in one day basically. The effect of channel scouring sluice is obvious. Therefore, in order to improve the flushing effect of channel scouring sluice and to solve the sedimentation problem in the entrance area of lower approach, it s advised to choose flushing time at a small discharge. At this time, the water level in the downstream river channel is low, and main flow releases through the lower approach channel. The discharge area in the approach channel is small, and the flow velocity is large, as a result, the flushing effect is obvious. This operation mode may have some influence on the power generation effect, to reduce the loss of the electric energy, the sand scouring in the lower approach could be conducted at mid-night, when load is at a low ebb. It is necessary to point out that, the above calculation results are obtained without considering that the sediment depositing in the entrance area of the upper approach channel will have impact on the discharge capacity of the channel scouring sluice. With the analysis of the flushing effect of the channel scouring sluice in the upstream of dam, the sediment depositing at the entrance will affect the inflow capacity, causing the releasing discharge through channel scouring sluice could not reach the designed discharge. Thus, the flushing effect of the channel scouring sluice in the downstream of dam will be affected. It should be taken seriously, dredge and desilt timely to improve the deposition problem in the entrance area of upper approach channel

50 5 CONCLUSIONS AND RECOMMENDATIONS 5.1 CONCLUSIONS This report used a plane two-dimensional flow and sediment mathematical model for the sediment calculation in the upstream and downstream of Pak Beng HPP after the reservoir is established. According to the two -dimensional mathematical model calculation of the dam, the results of scouring-silting process in the upstream and downstream of the dam are obtained, and main conclusions are as follows: (1) The silting situation of the debris barrier. At the end of the 5th year, the elevation in the upstream side of debris barrier is 315m, which is lower than the crest elevation of 325.0m. It shows 5 years after the reservoir runs, debris barrier plays an effective role in intercepting bed load. At the end of the 15th years, the elevation in the upstream side of debris barrier is 321m, and at this time the debris barrier can still play the role of intercepting the bed load to enter into the hydropower station; At the end of the 25th years, the elevation in the upstream side of debris barrier is 325m, which is close to the crest elevation, it shows that at this time, debris barrier is going to be of no effect in intercepting the bed load to enter into the power station. After the power station operates 30 years, debris barrier is of no effect. When flood release and scouring sluices are opened, for the depositing sediment in the debris barrier nearby, only the part close to dam section will be taken away and flushing effect of upper debris barrier isn t obvious, for flow velocity is small. (2) The silting situation before power station. The silting situation before intakes of units close to flood release sluices is better than the situation of units close to non-overflow dam section on the left bank. For example, after the power station operates 30 years, sediment before intake of number 5 unit will deposit to the elevation of 285.5m, which is basically the same level with the bottom elevation of sand ducts, the longitudinal profile gradient of funnel-shaped before which is about 1/2.84, ranging about 60 m. For number 14 unit, after the power station operates 10 years, sediment before intake of unit will deposit to the elevation of 290 m, exceeding bottom board elevation of m of power station intake. After 30 years, sediment before intake of unit will deposit to the elevation of m, the longitudinal profile gradient of funnel-shaped before which is about1/3.8, ranging about 60 m. The silting situation of units close to non-overflow dam section on the left bank is serious and the problem is prominent, which needs to take seriously. (3) The sediment concentration through the units and particle size. With the time of the reservoir operation increasing, the annual average sediment concentration through the units increase from kg/m 3 at the end of the fifth year to kg/m 3 at the end of the 30th year, the sediment through the units whose particle

51 size greater than mm increase from 1.32% at the end of the fifth year to 2.95% at the end of the 30th year. (4) The silting situation in the entrance area of upper approach. After the reservoir runs, there will be accumulative deposition in the entrance area of upper approach. At the end of the 10th year, the depositing elevation will reach 334 m, and the navigation will be intercepted when it s not at the flood season (the water level before dam is 335 m). At the end of the 20th year, water depth in the entrance area of approach doesn t satisfy the navigation condition for the whole year, and the dredging projects measures should be taken. (5) The silting situation in the entrance area of lower approach. After the reservoir runs, there will be accumulative deposition in the entrance area of lower approach. After 5 years, the maximum depositing thickness near the entrance area of lower approach is about 5.0 m. Under the lowest navigation discharge, the channel width in the approach channel and the velocity of backflow near the entrance area can still satisfy the navigation condition, but there is navigation obstruction trend. (6) The scouring-silting situation before the flood release and scouring sluices. The flood release and scouring sluices has a great releasing capacity, and it will open sluice gates for sediment release, thus flushing effect is obvious, and the deposition before the dam is little. After 30 years, the gradient of funnel-shaped before flood release and scouring sluices is about 1/27.3, ranging about 200 m. There is need to say, because the 30-year series formed by circular permutation of water-and-sand process are adopted in the computation, and the sediment deposition elevation of certain year in the beforehand conclusions is obtained based on this series. After the reservoir is completed and put into operation, the actual water-and-sand process may have great difference with the adopted series, and the scouring-silting process will differ with the computed results either, thus sediment deposition problem more serious than the computed results may appear. 5.2 RECOMMENDATIONS (1) In order to solve the deposition problem near the non-overflow dam on the left bank, It s suggested that runtime should be increased to avoid the inlet of the generating units be deposited by sediment. Besides, in order to reduce the backflow problem at the downstream of the power house, the units should be opened at interval, to let the flow release uniformly. (2) To improve the sediment deposition problem in the entrance area of upper approach and reduce the dredging quantities, it s suggested that length of diversion dike and the types of the breakwater head should be optimized

52 (3) To improve the flushing effect of channel scouring sluice and to solve the sedimentation problem in the entrance area of lower approach, it s suggested that flushing time be chosen when the discharge is small. This operation mode may have some influence on the power generation effect, so the sand flushing in the lower approach could be conducted at mid-night, when load is at low ebb, in order to reduce the loss of the electric energy. (4) In order to solve the problem that flushing effect of the upper side of debris barrier isn t obvious when flood release sluices are opened, It s suggested that the angle of debris barrier should be adjusted or artificial desilting be carried out regularly. (5) Because the sediment deposition elevation of certain year in the beforehand conclusions is obtained based on the series of water-and-sand process. Actually, there may be some sedimentation problems more serious than computed results. The results obtained by mathematic model can only give reference for the power station operation, at the same time, those who run the power station should reinforce the underwater topography monitoring of the reach near the project, to ensure the safety operation of the power station

53 Attached Fig. 1.1 Plane arrangement figure of the hydropower station

54 Attached Fig. 3.1 Plane figure of topography in the upstream calculation region (unit: m)

55 Attached Fig. 3.2a Figure of grids in the upstream calculation region(the calculation region is divided into grids, while only grids are given in the figure) Attached Fig. 3.2b Figure of grids in the downstream calculation region(the calculation region is divided into grids, while only grids are given in the figure)

56 流量 (m 3 /s) 含沙量 (kg/m 3 ) 流量 (m 3 /s) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Discharge (m 3 /s) 时间 Time ((Year) 年 ) Attached Fig. 3.3a Figure of inlet flow rate process in the upstream of dam Sediment Concentration (kg/m 3 ) Attached Fig.3.3b 时间 ( 年 ) Time (Year) Figure of inlet sediment concentration process in the upstream of dam Discharge (m 3 /s) 时间 Time (Year) ( 年 ) Attached Fig. 3.4a Figure of inlet flow rate process in the downstream of dam

57 水位 (m) 含沙量 (kg/m 3 ) Two-dimensional Sediment Numerical Simulation of Pak Beng Hydropower Station in Laos Mekong River Sediment Concentration (kg/m 3 ) Time 时间 (Year) ( 年 ) Attached Fig. 3.4b Figure of inlet sediment concentration process in the downstream of dam Water Level (m) 时间 Time (Year) ( 年 ) Attached Fig. 3.4c Figure of outlet water level process in the downstream of dam

58 c A Attached Fig. 4.1 Schematic figure of funnel axes Attached Fig. 4.2a Elevation figure of local topography near the dam in the 0 year (unit: m)

59 边滩 边滩 m/s Attached Fig. 4.2b Local flow field figure of 1290 m 3 /s in the 0 year 边滩 边滩 m/s Attached Fig. 4.2c Local flow field figure of 3077 m 3 /s in the 0 year

60 边滩 边滩 m/s Attached Fig. 4.2d Local flow field figure of m 3 /s in the 0 year 边滩 边滩 m/s Attached Fig. 4.3a Local flow field figure of 1290 m 3 /s in the end of 5th year

61 边滩 边滩 m/s Attached Fig. 4.3b Local flow field figure of 3077 m 3 /s in the end of 5th year 边滩 边滩 m/s Attached Fig. 4.3c Local flow field figure of m 3 /s in the end of 5th year

62 Attached Fig.4.3d Elevation figure of local topography near the dam in the end of 5th year (unit: m) Attached Fig.4.3e Distribution figure of local sedimentation thickness near the dam in the end of 5th year (unit: m)

63 边滩 边滩 m/s Attached Fig. 4.4a Local flow field figure of 1290 m 3 /s in the end of 10th year 边滩 边滩 m/s Attached Fig. 4.4b Local flow field figure of 3077 m 3 /s in the end of 10th year

64 边滩 边滩 m/s Attached Fig. 4.4c Local flow field figure of m 3 /s in the end of 10th year Attached Fig.4.4d Elevation figure of local topography near the dam in the end of 10th year (unit: m)

65 Attached Fig.4.4e Distribution figure of local sedimentation thickness near the dam in the end of 10th year (unit: m) 边滩 边滩 m/s Attached Fig. 4.5a Local flow field figure of 1290 m 3 /s in the end of 15th year

66 边滩 边滩 m/s Attached Fig. 4.5b Local flow field figure of 3077 m 3 /s in the end of 15th year 边滩 边滩 m/s Attached Fig. 4.5c Local flow field figure of m 3 /s in the end of 15th year

67 Attached Fig.4.5d Elevation figure of local topography near the dam in the end of 15th year (unit: m) Attached Fig.4.5e Distribution figure of local sedimentation thickness near the dam in the end of 15th year (unit: m)

68 边滩 边滩 m/s Attached Fig. 4.6a Local flow field figure of 1290 m 3 /s in the end of 20th year 边滩 边滩 m/s Attached Fig. 4.6b Local flow field figure of 3077 m 3 /s in the end of 20th year

69 边滩 边滩 m/s Attached Fig. 4.6c Local flow field figure of m 3 /s in the end of 20th year Attached Fig.4.6d Elevation figure of local topography near the dam in the end of 20th year (unit: m)

70 Attached Fig. 4.6e Distribution figure of local sedimentation thickness near the dam in the end of 20th year (unit: m) 边滩 边滩 m/s Attached Fig. 4.7a Local flow field figure of 1290 m 3 /s in the end of 25th year

71 边滩 边滩 m/s Attached Fig. 4.7b Local flow field figure of 3077 m 3 /s in the end of 25th year 边滩 边滩 m/s Attached Fig. 4.7c Local flow field figure of m 3 /s in the end of 25th year

72 Attached Fig. 4.7d Elevation figure of local topography near the dam in the end of 25th year (unit: m) Attached Fig.4.7e Distribution figure of local sedimentation thickness near the dam in the end of 25th year (unit: m)

73 边滩 边滩 m/s Attached Fig. 4.8a Local flow field figure of 1290 m 3 /s in the end of 30th year 边滩 边滩 m/s Attached Fig. 4.8b Local flow field figure of 3077 m 3 /s in the end of 30th year

74 边滩 边滩 m/s Attached Fig. 4.8c Local flow field figure of m 3 /s in the end of 30th year Attached Fig.4.8d Elevation figure of local topography near the dam in the end of 30th year (unit: m)

75 Attached Fig.4.8e Distribution figure of local sedimentation thickness near the dam in the end of 30th year (unit: m)

76 Attached Fig. 4.9 Profile figure of powerhouse

77 Attached Fig Profile figure of sand duct

78 Attached Fig. 4.11a Distribution figure of local sedimentation thickness in front of the powerhouse in the end of 5th year (unit: m) Attached Fig. 4.11b Distribution figure of local sedimentation thickness in front of the powerhouse in the end of 10th year (unit: m)

79 Attached Fig. 4.11c Distribution figure of local sedimentation thickness in front of the powerhouse in the end of 15th year (unit: m) Attached Fig. 4.11d Distribution figure of local sedimentation thickness in front of the powerhouse in the end of 20th year (unit: m)

80 Attached Fig. 4.11e Distribution figure of local sedimentation thickness in front of the powerhouse in the end of 25th year (unit: m) Attached Fig. 4.11f Distribution figure of local sedimentation thickness in front of the powerhouse in the end of 30th year (unit: m)

81 powerhouse debris barrier Attached Fig Local flow field figure in front of the powerhouse water inlet in the left non-overflow dam section flood release and scouring sluice debris barrier upstream approach channel Attached Fig Distribution figure of sedimentation thickness before flood releasing and sand scouring sluices in addition to channel scouring sluice are opened (unit: m)

82 flood release and scouring sluice debris barrier upstream approach channel Attached Fig Distribution figure of sedimentation thickness after flood releasing and sand scouring sluices in addition to channel scouring sluice are opened (unit: m) debris barrier Attached Fig Velocity distribution of flow near the debris barrier after flood releasing and sand scouring sluices are opened (unit: m/s)

83 channel scouring sluice Attached Fig Velocity distribution of flow in the upper approach channel (unit: m/s) Attached Fig. 4.17a Elevation figure of initial topography of calculation region in the downstream dam (unit: m)

84 m/s Attached Fig. 4.17b Flow field figure of m 3 /s (unit: m/s) m/s Attached Fig c Flow field figure of m 3 /s (unit: m/s)

85 m/s Attached Fig. 4.17d Flow field figure of m 3 /s (unit: m/s) 1 downstream approach channel Attached Fig Flow field figure in the vicinity of approach channel outlet

86 Attached Fig. 4.19a Topography elevation figure of calculation region in the downstream dam in the end of 5th year (unit: m) Attached Fig. 4.19b Sediment scouring-silting thickness distribution figure of calculation region in the downstream dam in the end of 5th year (unit: m)

87 downstream approach channel Attached Fig Maximum thickness distribution of sediment deposition in the vicinity of approach channel after 5 years (unit: m) Attached Fig. 4.21a Water depth contour in the vicinity of approach channel outlet (unit: m)