With the development of industry and agriculture and the growth of population, some areas in China, especially in North China, are increasingly feeling the shortage of water resources. For non-dangerous reservoirs, the problem of drought can be solved by storing floods as valuable water resources as possible in flood season. However, if the reservoir stores as much water as possible, some people worry that the earth-rock dam will overflow. Indeed, this kind of worry is understandable. However, through a scientific calculation, that is, the risk analysis and calculation of overflow dam, the quantitative data of overflow risk of earth-rock dam under the combined action of flood and wind and waves can be obtained, which can eliminate this worry and provide more than 99.999% safety and reliability for scientific and reasonable water storage of reservoir. In this way, we think that decision-making departments at any level will be more at ease.
II. Brief Introduction to Risk Analysis Theory of Overflow Dam
Overflow means that the water level in front of the dam exceeds the dam crest, and the water overflows the dam crest and flows downwards. Risk refers to the possibility of reservoir dam overflow. The risk of dam overflow refers to the probability that the water level in front of the dam exceeds the dam crest during the analysis period. The main risk factors causing dam overflow come from the uncertainty of inflow flood, wind and waves, storage capacity and discharge capacity. As for the inflow flood, everyone admits that it is random, so I won't go into details. Regarding the discharge capacity, although in the traditional reservoir design, the discharge capacity of discharge structures including spillway and spillway is treated as a certain quantity, strictly speaking, the discharge capacity is uncertain. Its uncertainty comes from the uncertainty brought by simplifying the real three-dimensional water flow into one-dimensional water flow model, the uncertainty of roughness value, the scale effect of model test and the allowable errors of various geometric dimensions in construction. The author thinks that all these random factors affecting the discharge capacity can be treated by treating the discharge coefficient of the discharge building as a random variable in a certain range. In the traditional reservoir calculation, the storage capacity or reservoir area is considered to be deterministic. But in fact, they are uncertain. There is random error in the contour map of reservoir area measured manually; When using contour map to calculate storage capacity, there are simplification errors in trapezoidal method or Simpson method. The reservoir area suffers from floods every year, which inevitably leads to erosion and siltation. However, due to the limited manpower and material resources, it is impossible to accurately measure the underwater topography of the reservoir area every year, so scouring and silting will also cause the uncertainty of the reservoir capacity. The wind, when to blow, from what direction, how fast and how many levels, is still random. For earth dams, the backwater height E of water surface caused by wind and the climbing height Rp of wind waves along the slope dam surface are naturally random. It should be pointed out that under the condition of general reservoir water level, the backwater of water surface and the climbing of wind waves caused by general wind will not cause dam overflow. Only when the flood comes and the water level of the reservoir rises to a certain value can the action of wind and waves cooperate with the flood to add fuel to the fire and lead to the risk of dam failure. Therefore, the premise of statistical wind series should be to count the wind force in each flood, but due to the lack of this information at present, for safety reasons, the maximum wind series in flood season is generally used. For the risk of dam overflow, only the wind blowing to the dam body plays a role in the dam overflow accident, so for the risk of dam overflow, the effective wind should be the largest wind system blowing to the dam body in flood season.
Strictly speaking, there is also uncertainty in the elevation of dam crest. It comes from measurement error and dam crest settlement, but for the completed project, its discreteness is very small, which can be regarded as a constant and does not affect the calculation accuracy.
In this way, the reservoir flood regulation process is a stochastic process, and its flood regulation calculus equation is a stochastic differential equation. We require not only the mean value of each random variable, but also the value of its variance. The dam-break risk of earth-rock dam must be obtained by hourly numerical integration under the combined action of flood sequence with check flood or design flood as the upper limit and effective wind sequence blowing to the dam body in flood season. In the calculation, when the control elevation of dam crest is Zc 1 and the control elevation of breakwater crest is Zc2, the corresponding flood level will get the corresponding flood risk values respectively. Subtract the calculated risk value of overflow dam with 1, and get the safety and reliability of overflow dam under the combined action of flood series and effective wind series in flood season under the given flood level. The concrete dam overflow risk model and its calculation method are limited in space, so I won't go into details. Please refer to the relevant literature. At present, there is no national or industrial standard for the safety and reliability of overflow dams. After analyzing the statistical data of overflow dam and dam break at home and abroad, we put forward that the acceptable risk of overflow dam is 10-6. This is equivalent to the magnitude of irresistible earthquake risk, or it is acceptable that the safety reliability of overflow dam is above 99.999%. The research and practice on the risk of overflow dam also laid a scientific foundation for formulating the safety and reliability specification of overflow dam in the future. According to the limited water level of the reservoir in flood season stipulated in the design, the above risk analysis method and value standard can be used to evaluate the overflow safety of earth dams. If its safety reliability is lower than 99.999%, measures must be taken as soon as possible to ensure safety; If the reliability is much higher than 99.999%, it shows that there is still water storage capacity. For large reservoirs and medium-sized reservoirs with solid dam bodies and good managers and flood forecasting systems, it is considered to increase the flood limit water level under the premise of ensuring the safety and reliability of overflow dams as high as 99.999%, so as to stop floods as much as possible, so as to reduce the downstream flood control pressure, reserve valuable water resources, and achieve both beneficial and harmful effects.
Third, the engineering example
The following table lists the successful applications of the above methods in douhe reservoir and Qinghe Reservoir. In the risk analysis of dam overflow, there are two kinds of critical elevation Zc for safety control: the first one is to take the crest elevation Zc 1 of earth dam, which only resists the backwater height of flood and wind waves; The second is to take the elevation Zc2 of the breakwater wall top to resist the flood, the height of wind and the climbing of wind and waves. Of course, the calculated risk of dam overflow is different with different critical elevations.
Fourth, dispel doubts
Some people may ask: in this way, isn't the super-high security gone? On this issue, we make the following two explanations: First, in the traditional reservoir operation calculation, except that the flood is a random event with a certain frequency, the reservoir capacity, reservoir area, flood season customs and discharge capacity of discharge structures are all treated as deterministic quantities. Once the flood frequency is given, the flood hydrograph becomes deterministic quantities. In this case, people adopt safety superelevation, that is, adding a height to the highest water level of reservoir calculus as safety superelevation to ensure safety, which essentially includes all uncertainties that have not been considered when determining. On the other hand, when we regard flood, wind tunnel height and climb, storage capacity and storage area, and discharge capacity as random quantities, and the flood dispatching process as a random process, these uncertainties have been taken into account, so it is not necessary to adopt safe superelevation.
Secondly, to take a step back, even if the dam can be regarded as a high public hazard according to the classification method of potential public hazards commonly used abroad, the above analysis method is also applicable, but the critical elevation Zc can be selected below the dam top or the breakwater top, so the height difference between this elevation and the dam top or breakwater top is equivalent to the redundant safety superelevation. However, whether the risk of dam overflow should be maintained at the order of 10-6 at this time needs further study. In my opinion, it seems unnecessary to maintain such a high standard.
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