Construction of high earth coffer dams in two stages on mountain rivers with flood overflow at an intermediate elevation

Conclusions

• 1. The erection of coffer dams in two stages with overtopping at an intermediate elevation removes the existing limitations in the time of erection and permits increasing the total height of the coffer dams, thus decreasing the sizes of diversion structures, and often shortening the construction time of hydrocenters.

• 2. In the case of the Toktogul’sk HES the decision to pass the flood through the tunnel and over the crest of a partially erected earth coffer dam made it possible to dam up the Naryn River at the beginning of 1966, and thereby to begin preparatory work in the pit of the dam one year ahead of time.

• 3. The earthen 30-m high Stage I coffer dam, through which a discharge (600 m³/sec) exceeding the design value by 30% was passed, withstood the flood without any damage. This shows the dependability of the adopted design of the spillway coffer dam and the good quality of the work.

• 4. The successful passage of the flood at the Toktogul’sk through the partially erected earth coffer dam makes it possible to recommend this method for other projects. The maximum effect can be expected in the construction of earth or rock-filled dams, where the coffer dam, as a rule, becomes part of the dam.

• 5. The 1966 flood at the Toktogul’sk HES passed with discharges exceeding those of the design and with heads lower than the design; this requires a special analysis of the discharge capacity of tunnels and pipes of large cross sections.