were investigated by means of models at the construction site hydraulic laboratory, and, as a result, they were applied for the construction work. This is only an insignificant part of the problems that had to be solved.At the hydraulic development construction site, close cooperation was developed between the Vietnam and Soviet specialists, and the operation problems were successfully solved. At the construction site weekly-daily planning was introduced, and its fulfillment is controlled at the executive level by the construction staff. The most complex technical questions as well as the strategic construction plans are examined at joint meetings of the construction staff and of the State Staff with participation of the ministers and the Government of the SRV, high State officers, and official representatives of the USSR and the SRV. The site of the Hoabinh hydraulic development is located in a narrow canyon on a meander of the Da River, whose width during the low-water period is 350 m, the 10% discharge probability being 14,690 mS/sec. The river hydrograph in the site zone (Fig. 1) is characterized by frequent large floods and a long low-water period (October--May). According to the exploration data, the bed mass of gravel--pebble deposits is covered from the top by fine and sity sands in layers up to 15 m thick, which had to be removed at the dam base. Discharge of the construction flows only through diversion tunnels, which is traditional for construction of rock-earth dams, did not ensure discharge of flood flows even with a cofferdam more than 50 m high. Moreover, such a cofferdam could not be included as part of the fill of the future dam, since in its base fine and silty sands would have been left which had a low bearing capacity and were prone to rarefaction under seismic action. Under this scheme, placement of the dam fills could have been started only after putting the diversion tunnels into operation, which would have delayed by 3---4 years the start of construction of the dam fills and would not have permitted effectively using the soils from useful excavations. Under the specific construction conditions, the optimal solution was a combined scheme of discharge of the construction flows that was closely coordinated with the dam construction methods and times, and with the periods of construction of the grout curtain in the dam base.The components of discharge of the construction flows were: The river channel, narrowed from the right bank side; the diversion canal, excavated in the right-bank rock side; two pressure diversion tunnels 116.5 m 2 in area each; twelve deep sluices of the permanent spillway, 6 x 10 m in size, with a chute and a deflector.Five characteristic stages of discharge of the construction flows can be established. 1980--1982. The river flows were discharged along the natural channel, narrowed from the right bank by the first-stage dam fills [1]. During this stage, a diversion canal with the bottom at El. 21 m, 61 m wide, and about 700
