Translated from Gidrotekhnicheskoe Stroitel 'stvo, No. 2, February 2011, pp. 9 -18. Basic information on the in-service tunnel spillway at the Irgana HPP, and also requirements for smoothing irregularities in its surfaces in contact with the flow are presented. A method is proposed for determination of requirements for the surface of the spillway with a tolerance for cavitation damages. Results of experimental investigations and analyses are presented. Keywords:Irgana hydroproject, surfaces of steeply inclined section of spillway in contact with flow, cavitation damages, flow parameters, rectangular projections, permissible height of irregularities.The in-service tunnel spillway at the Irgana hydroproject, which was built on the Avarskoe Koisu River in the Republic of Daghestan, consists of a water intake with a radial gate, steeply inclined section, and discharge tunnel. The spillway is intended to pass design flows of 2000 m 3 /sec in a 10,000-year flood, and 1320 m 3 /sec in a 1000-year flood. The spillway is designed for gravity flow over its entire length. The differential in water levels over its steeply inclined section is approximately 75 m (Fig. 1). The profile of the intake head of the spillway is configured in Kriger coordinates with a profiling head of 15.8 m. For a distance of 20.61 m along the horizontal, the spillway narrows down from 18 to 8 m in plan. In cross-section, the steeply inclined section of the in-service tunnel spillway (SISTS) is troughshaped with a dome configured as the arc of a circle with a radius of 4 m. The thickness of the reinforced-concrete lining of the steeply inclined section of the tunnel is 0.7 m, and the protective layer for the reinforcement in the lining does not exceed 0.1m. Its design is discussed in detail in [1].Requirements for the treatment of irregularities in the concrete surface of the SISTS, which is past over by a highvelocity flow, were formulated on the basis of hydraulic studies that had been performed early-on in the design stage, proceeding from prevention of cavitation. There were no particular difficulties encountered in fulfilling this condition either in preventing cavitation over the uniform natural roughness of the concrete surface, or in the case of shelves, but it was necessary to dress similar protrusions and local roughness (reinforcing rods and other metallic components, large aggregate). Fulfillment of this condition for projections on the most cavitation-prone sections -immediately downstream beyond the constriction in the planform of the SISTS -could be realized with their smoothing at an angle of less than 90°to the concrete surface where the height was 4, and even 2 mm. Smoothing of projections of such a height on the concrete surface could hardly be implemented over the area of the spillway surface in contact with the flow, which is approximately 2200 m 2 , including 1600 m 2 of lateral walls.Procedure for determination of requirements for spillway surface with tolerance for cavitation damages. A tolerance for irregularities compris...