Laboratory investigations of lower-pool hydraulics during operation of the service spillway at the Bureya HPP

Abstract

The performance of the lower pool during operation of a spillway is discussed.The spillway dam of the Bureya hydroproject ( Fig. 1) consists of eight spans each 12 m wide, which are situated at the head of the spillway with bulkheads each 3 m thick, is designed for the passage of flows in conjunction with the HPP: Q 0.1% = 10,400 + 2100 = 12,500 m 3 /sec, and Q 0.01% = = 11,200 + 2100 = 13,300 m 3 /sec with a pool differential of 117 m. The pools are merged by a discharge of flow from the dam over a distance of 200 m in a diversion channel beyond a 140-m-wide spillway where the overall width of the river is 250 m, and the average depth 15 m in the case of maximumflow passage.During passage of construction flows through eight bottom openings, local washouts of the diversion channel reached 20 m at a distance of 80 m from the dam, washouts directly at its attachment amounted to 7 m, and up to 10 m near the attachment to the abutment, whereupon undermining of stabilizing slabs was also noted. Deposition of scouring products in the form of a ridge (bar) up to 6 m high with a crest elevation of approximately 136.5 m, partitioned the channel; this led to an increase in the level at the HPP as compared with the natural conditions of up to 3 m at a passing flow rate of 500 m 3 /sec.The decision to slope the area of impingement of the discharge flow of beyond the service spillway from the left bank, along which a dirt road passes, and from the split abutment between the diversion channels of the spillway and HPP, was made in connection with the negative experience acquired with scouring of the stabilization. For this purpose, diverting springboards common to two spans are being built beyond extreme spans 1 and 2, and 7 and 8 of the spillway dam. This will solve the indicated problem, but will result in a concentration of flows in the median section of the channel, and here, to an increase in the depth of scouring, especially during the passage of spring floods in the case of fully opened basic gates.The following should be obtained from laboratory investigations for cases where different flow rates are passed:-a prediction of development of local washouts of the channel, the formation of a bar, and the hydrostatic head on the HPP;-the velocity distribution of the flow, and the height of the waves in the section with a length of up to 900 m from the axis of the dam; -information on the effect of the jet dropping directly onto the abutment and left bank, an estimate of the amount of water impinging on the abutment and terminal section of the powerhouse, and a determination of the effectiveness of the wall protecting the left-bank abutment and platform from splash; and, -information on the influence exerted by the flow on the combined stabilization of the right bank, which consists of a concrete lining and rocks of no specified size.

Characteristic features of the cap on the service spillway at the Bureya hydroproject and some results of its laboratory and field investigations

“…The cap on the spillway dam at the Bureya hydroproject [1] -as a rule, passage of the basic design flow at the normal support level is ensured in conformity with Construction Rule and Regulation [2,Section 5.4.3]. Passage of a flow with a probability P » 5% of being exceeded is provided through the Bureya hydroproject at the normal support level; here, 9400 m 3 /sec is passed through the eight bays of the spillway.…”