2017 2 nd International Conference on Architectural Engineering and New Materials (ICAENM 2017) ISBN: 978-1-60595-436-3 The Study of Reservoir Immersion of a Hydropower Station Shi You Zhang, Ai Guo Li and Yan Guo Chen ABSTRACT According to the reservoir engineering geological conditions, several prediction methods of reservoir immersion were compared. The elevation of ground water was calculated by the finite element theory. The height of capillary water was determined by water content method, water characteristic curve method and field investigation method. On the basis, the immersion influence area was predicted, which can provide references in hydropower economic evaluation. 1 INTRODUCTION A power station is located in the middle reach of the Gan River. The main purpose of this project is electricity generation, incorporating with shipping, etc. The power station is second engineering grade with large (2) scale and the normal storage water level is 67.5m, dead water level 67.1m, minimum temporary running water level in flood period 66.0m, design flood level 67.93m, and check flood level 69.61m. The project consists of earth rockfill dams, ship lock, powerhouse, release sluice and bulwark, etc. Geologic Condition of Reservoir Region The reservoir backwater length of trunk stream is 35.8m when the normal storage water level is 67.5m. The reservoir bed is wide and flat, whose main 1 Shi You Zhang, Henan Electric Power Survey & Design Institute, Zhengzhou 450007, China Ai Guo Li,Changjiang Geotechnical Engineering Corporation, Wuhan 430010, China Yan Guo Chen, Yellow River Engineering Consulting Co., Ltd., Zhengzhou 450003, China 48
width is 400m to 700m range, and the valley is U-shaped. With developmental flood plain, the first stage terrace is widely spread on both sides of the reservoir, whose elevation is 64m to 73m range, and width ranges from 0.2km to 1.1km, while width in some areas is up to 1.7km. The strata of first stage terrace is Q4 alluvial deposit with a binary configuration. The upper formation are mainly low liquid limit clay (sand-containing or no sand), and thickness ranges from 3.0m to 9.1m. The downside is sand and gravel, which ranges from 3.0~9.5m. And major ingredient of underlying cretaceous red-bed soft rock is mainly pelitic siltstone, siltstone and sandy conglomerate, etc. The groundwater level height ranges from 61.0m~65.0m, and pore water is main type which exists in sand layer and gravel layer. Cause and Type of Immersion The type of reservoir is plain channel, and the elevation of first stage terrace ranges from 64.0m~71.0m. The water level of reservoir is higher than Gan River, and underground water is mainly supplied by rainfall. The underground water level is rising after reservoir filling, and the relation between recharge and discharge remains the same in the higher ground elevation area, which is the top-lifted flow type. In the area whose ground elevation is lower or slightly higher than normal storage water level, the relation between recharge and discharge changes, which is seepage type. The farmland, garden plot and residential areas are widely distribute around the strata of first stage terrace, which is divided by crop immersion area and construction immersion area. And the result has a big impact on economic evaluation of the hydropower station. To predict submerged range more properly, the judgment standard is determined by difference between various methods. The Immersion Prediction Because the area is plain river channel reservoir, the relation between ground water and surface water is close. The predicting outcomes have obvious differences by different method and standard. The value of rising groundwater is calculated firstly, then immersion possibility is estimated according to rising groundwater and immersion critical groundwater level. (1) Calculation of leachate mound The prediction method includes groundwater dynamics method and finite element method, etc. Comparing with Kamenski method and finite element method, the leachate mound of area 1, area 2 and area 3 are analyzed. 1) Kamenski method [2] Kamenski formula: K 2 2 2 2 h hx 0.5K 2 h hx K1M y yx 0. K 2 y yx (1) 1M 5 49
K1 : permeability coefficient of lower aquifer; K2 : permeability coefficient of upper aquifer; y:normal storage level; yx:thickness of aquifer on computation cross-section after banked-up water; M:lower aquifer; h: riverbed water level before filling; hx:thickness of aquifer on computation cross-section before filling. Normal water levle Initial calculation water level y K2 h hx yx Rising water level Initial water levle M K1 Impervious base Figure 1. Elevation of ground water predicted by Kamenski method. Considering that the stratum is binary-structured, with permeability coefficient of upper clay less than 1 10-4cm/s and permeability coefficient of lower sand gravel greater than 1 10-3cm/s, K2 is approximately equal to 0, and the formula is simplified: h h y (2) x y x It is assumed that relationship between recharge and discharge of underground water doesn't change, and hydraulic gradient is same. Take 2 section for example, on the basis of long-term measured data, hydraulic gradient is 3.7~36.0 in vertical direction, and predictions are shown that the groundwater level is 1.21m higher than normal level in area which is 200m to channel, 1.65m in area which is 400m to channel. 140 80 Normal water level 67.5m 70 BJ1 60 50 Gan River 18 60.97 2011.3.3 s K 2y 4 BJ2 Initial water levle 62.04 2011.3.31 Rising water levle 40 40 0 50 100 150 200 250 300 350 (m) al 4hl BJ3 62.59 2011.3.31 80 70 60 50 Figure 2. Profile prediction of ground water elevation in section 2 of reservoir. 2) FEM method It is assumed that the groundwater level of key simulation areas is uniform; groundwater flow system obeys Darcy's law and water equilibrium principle; groundwater movement is generalized as unsteady three-dimension flow. The 50
mathematical model is built on the basis of conceptual model mentioned above and boundary condition: Hi Hi Hi Hi K K K Ss x x y y z z t Hi Lateral boundary condition: K v x, y, z, t n Gan River: Hi H1 x, y, z, t H Bottom boundary condition: 0 n Roof boundary condition: H x, y, z, t z Initial condition: Hi x, y, z,0 Hi0 x, y, z (3) H i : water head; K: permeability coefficient; H 1 : water head value of the first boundary condition; v: seepage velocity; H i0 : initial condition of unsteady flow; S S : storage; ε: source sink term; x, y, z, t: space coordinate and time variable; n: external normal direction of boundary. According to FEM method, long-term measured data of underground water level, rising water level of 2 section is simulated and the simulation results show: the underground water level presents a downward trend from simulation area to Gan River; underground water move in the direction of the inside, with hydraulic slope of 1 section about 0.05 ~0.1 and hydraulic slope of 3 section about 0.01 ~03, and the underground water level is about 67.5m. Figure 3. Prediction model of ground water elevation in section 1 of reservoir. (2) Critical depth of groundwater Critical depth of groundwater is calculated according to the following formula[1]: H cr H k H (4) where:hcr Critical depth of groundwater;hk The lifting height of capillary water above water level; H Value of safe superelevation. 51
Figure 4. Prediction model of ground water elevation in section 2 of reservoir. 1) The lifting height of capillary water 1Water content test method So in different depths of sampling points are analyzed by water content test and specific gravity test of samples, which are sampled in testing pit every 10~15cm above water level. Saturability characterize water filling degree of soil pore, and the lifting height of capillary water (Hk) is the depth corresponding to that Sr is greater than or equal to 80% above water level (In Figs. 5 and 6). By above mentioned method, the lifting height of capillary water of four sampling points are 0.231m 0.43m 0.523cm and 0.384cm. Figure 5. Variation of saturation with depth in section 1 of reservoir. Figure 6. Variation of saturation with depth in section 2 of reservoir. 52
2Water characteristic curve method Water characteristic curve is the relation curve between soil water content and soil suction. If the soil sorting feature is bad, the section whose water content reduces sharply as soil suction slightly increases. The suction value is the equal of the lifting height of capillary water (as shown in Fig 7 b). Negative pressure Negative pressure Fine grain sorting Bad grain sorting (a) Hk VWC Figure 7. Soil-water characteristic curve. (b) Hk VWC The relation curve between negative pressure and volumetric water content of the four sampling points are shown in Figure 8, 9, 10 and 11. According to the curve, the soil sorting feature is bad, and the lifting height of capillary water are 0.292m, 0.612m, 0.425m and 0.223m. Figure 8. Soil-water characteristic curve of the sampling point in section 1 of reservoir. 53
Figure 9. Soil-water characteristic curve of sampling point #1 in section 2 of reservoir. Figure 10. Soil-water characteristic curve of sampling point #2 in section 2 of reservoir. Figure 11. Soil-water characteristic curve of the sampling point in section 3 of reservoir. 3Field survey When surveying and mapping for geology is implemented, the height of the saturation of 42 still water is measured, and the lifting height of capillary water of clay ranges from 0.35m to 0.57m, with average value 0.47m. 4Integrated value According to water content test method, water characteristic curve method and field survey, the average lifting height of capillary water is 0.45m. The lifting height of capillary water is 0.5m when all methods are considered. 2) Value of safe superelevation The values of safe superelevation are different for different lithology and part, which depend on the foundation type in the residential area and are the sum of depth and thickness of root system in agricultural region. Because the thickness of root system of paddy and oilseed rape is 0.3~0.5m, the value of safe superelevation is 0.5m. The three or four-floor constructions are mainly brick-concrete structure and the foundation types include strip foundation, ring beam foundation and few pile foundation. The burial depth of strip foundation and ring beam foundation is mainly 0.3~1.0m, and the burial depth of pile foundation is 6m. 54
According to the lifting height of capillary water and value of safe superelevation, the critical value of ground water of agricultural region is 1.0m, of residential area 1.5m. (3) Prediction of immersion affected area According to critical underground water level, the immersion affected area is predicted. By finite element method, the elevation of immersion scope of agricultural region ranges from 67.5m to 68.5m, and the elevation of immersion scope of residential area ranges from 67.5m to 68.5m. But the elevation ranges from 67.5m to 70m by Kamenski method, and the first stage terrace in immersion area. The predicted immersion areas are shown in table I. TABLE I. PREDICTED IMMERSION AREAS BY KAMENSKI METHOD AND FEM METHOD. Immersion area( 10 4 m 2 ) Predicted Kamenski area FEM method method 1section 7.5/34.2 4.5/29.4 2section 7.3/33.8 6.5/27.5 3section 0/72.1 0/61.7 Note: Numerator is immersion scope of residential area, and denominator is immersion scope of agricultural region. CONCLUSIONS (1)The value of rising water level of first stage terrace is obvious after the value is predicted by two water storage methods. (2)The critical depth of groundwater is comprehensively determined by capillary rise of reservoir soil layer, which is got by the method of water content, characteristic curve of water analysis and field survey. (3)The predicting outcomes of two methods is shown: large-sized submerged sections is forming; the prediction immersion area is larger by Kamenski method than finite element method; the reservoir bank is usually first stage terrace, whose ground elevation is close to normal water level, and lower than normal water level on the local scale; ground water have weak confined aquifer, and the relationship between recharge and discharge is complex; the value of rising water level will be little higher because of the insufficient of Kamenski method. In a word, predicting the value of rising water by finite element method is a useful method of evaluation of hydropower economic benefit. REFERENCES 1. GIWP: Code for hydropower engineering geological investigation (China Planning Press, China 2006). 55
2. GIWP: Handbook of water conservancy and hydropower engineering geology (Water Resources and Electric Power Press, China 1985). 56