An increase in the height of spiUways of high dams leads to an intense increase in the cavitation action of the flow. Experimental investigations by many authors have shown that cavitation erosion occurs at a stream velocity greater than 12-15 m/sec and its intensity increases proportionally as the 5-7 power of the velocity. With an increase in spillway height, for example, from 50 to 100 m, the rate of cavitation erosion increases by a factor of 6-8. and with an increase to 150 m by a factor of more than 40. If the concrete surface has irregularities and areas with an insufficiently gradual change of profile and also poorly streamlined structural elements intended for guiding the flow or for other purposes (stream deflectors, energy dissipation baffles, openings of discharge conduits, ventilation, drainage, and other pipes), this can lead to the occurrence of cavitation and to subsequent destruction of the spillway. Cavitation erosion in some cases can be so intense as to disrupt the normal operation of the structure. Our observations and the data of publications in the technical literature showed that within a reLatively short time cavitation cavities can reach a depth of 1.5-2 m and the volume of removed high-strength concrete, tens of cubic meters. Repair of cavitation damage requires considerable Labor and time.The greatest number of cavitation cavities occur below surface roughnesses. Special measures for controlling surface defects are usually taken when conslzucting spillways, which complicates and increases the cost of construction but does not rule out the appearance of some defects Later upon aging of the concrete under atmospheric and climatic effects. Expensive high-strength grades of concrete are usually used in regions with a rigorous climate and large temperature difference in order to increase the strength of the spillway surface.To control cavitation erosion below surface roughnesses and around spillway structural elements, it has proved to be effective and economic to supply air directly to the zoiae of expected cavitation or, if the number of possible cavitation zones is large, to saturate with air (aerate) the boundary layer of the flow depthwtse. The admixture of free air changes the physical properties of water: the water becomes a less elastic, compressible medium, absorbing well the cavitation impacts. Smoothing of the spillway or its anticavitation strengthening is not required in the case of discharging an aerated flow. With sufficient saturation of the flow with free air, individual irregularities of the spillway surface or an increase in its total roughness does not cause cavitation erosion. [2] showed that, on aerating water in the amount of 1.5-2.5~o, cavitation erosion of concrete specimens decreased considerably and stopped in the case of 7-8% air (Fig. i). Experiments conducted at the research department of the State PLanning, Surveying, and Scientific Research Institute (Gidroproekt) in cavitation tunnels with a cylindrical cavitation exciter on specimens of concrete and cement ...

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A spillway during its design service life should guarantee with one or another degree of reliability the passage of flood discharges without creating an emergency situation for other structures of a hydroelectric scheme.The probability of a random event, consisting in that not a single failure will occur during the entire set period of operation, is taken as the measure of reliability of the system [I]. Damages on the spillway itself can be such that they could be eliminated either during operation (passage of water) or in the period between floods.Thus it is necessary to determine the design service time (life) of the structure, since by definition it is senseless to speak about reliability without designating the service life. Not a single existing standard establishes the service life of a hydraulic structure.There are standards for depreciation deductions [2] in which the annual payments are determined by the period of wear or obsolesence of the structure.But there are not indications that after paying off the construction costs of the object it is necessary to reconstruct it or to construct a new one corresponding more to the requirements of developing scientific and technical progress.The second problem is the selection of the reliability measure.Here, as applied to surplusing works, the values of the probabilities of the design floods could be certain indirect estimates, if these probabilities were to have strict scientific substantiation and, what really matters, were based on an objectively developed scientific methodology.Finally, the problem of reliability as a whole is very complex.It is related, especially with respect to unique high-head dams, to the large volumes of the reservoirs, delivery of water, generation of electrical energy, to economic, national, and other factors.However, it is completely necessary to solve ito Only in this case can one consciously evaluate the design margins of safety; tO optimize the operating regime of hydroelectric schemes; to take into account the changes in the properties of the foundations, gain of strength by the concrete, etc.; as a result, to obtain the maximum economic effect.Actually, imagine that the service life is designated in conformity with standards.How will this affect the investigation and design of surplusing structures even if the standard values of design flood probabilities are not revised and the level of reliability of the spillways does not decrease?First of all we must determine what are the discharges of water and in the course of what time will they occur in the period of the guaranteed, no-failure operation of the spillway.This problem can be formulated in terms of probability theory as one of finding the mean residence time t i of a random function Q(t) (water discharge) above a specified level Qi during a fixed time T. According to [3] the solution of the problem for a stationary stochastic process is written in the form Fig. i. Cumulative flood hydrograph for different service lives of the structure.

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