Problems associated with the rehabilitation of the bottom outlets of the Poiana Uzului Dam in Romania

After some 30-40 years of uninterrupted operation, important hydraulic works are in serious need for rehab or retrofit interventions. Works of this type can be made difficult or even prevented by various unforeseen situations, unknown to the specialists because they were not previously accompanied by events that affected neither the operation of the hydraulic systems nor the safety of the constructions and installations. The paper presents the experience of a group of specialists and the way in which the aforementioned unforeseen problems were solved during the rehabilitation works of the bottom outlets of a large dam in Romania. Poiana Uzului dam is the highest buttress dam in Romania (80 m max. height, commissioned in 1972), creating a multipurpose storage reservoir of circa 88 hm³ used for water supply for population and industry, hydroelectricity and flood protection. According to the national dam safety norms, the dam is ranked into the 1st class of importance, which corresponds to a design discharge value of Q0.1% (maximum discharge value with an annual exceedance probability of 0.1%). According to the Romanian design standard STAS 4273-83, for the 1st class of importance, the dam discharge capacity has to be also verified for a much higher value, Q0.01%. The dam was provided with 3 bottom outlets, each equipped with trash racks and 2 "butterfly" valves. The advanced wear of the bottom outlets equipment determined malfunctions of the service valves, as well as deterioration of the thrash racks, metal fragments posing serious threat upon the safe operation of the valves and of the outlets. Rehabilitation of the bottom outlets equipment was not possible with the complete emptying of the reservoir since it represents the only source of water for a number of circa 150,000 inhabitants in about 7 localities in the neighbouring areas. However, the intervention was possible with partial emptying of the reservoir, using professional divers to perform repair works in very difficult conditions. The lower limit to which the reservoir was emptied was conditioned by a program of monitoring the water quality parameters, allowing the uninterrupted supply of water for population consumption.