River intake works for a hydroelectric plant in Papua New Guinea

The Ok Tedi mining development in Papua New Guinea is served by a 50 MW hydroelectric plant completed in 1988. The plant draws water from a run-of-river intake on the Ok Menga, a torrential mountain stream in tropical rain forest subject to frequent flash floods and carrying substantial sediment loads. Design and construction of a diversion weir and intake posed a number of severe problems, such as difficult access, the torrential and flashy nature of the river flows, severe constraints on location due to unstable banks and substrata, and uncertain loads of sediment and debris. Site investigations of a hydro technical nature included monitoring of precipitation, streamflow and flood levels, sampling of suspended sediment, and special measurements of bed material and bed load. On the basis of these investigations, preliminary concepts were developed for a diversion weir and intake chamber. A hydraulic model was then constructed to examine alternative forms of weir construction and to study the hydraulic behaviour of various intake chamber designs with special attention to exclusion and ejection of coarse sediment. The final design was based largely on the indications of the model study. Initial operating experience generally confirmed the validity of the design, although one or two problems arose that were not sufficiently appreciated at the design stage. These were mainly associated with organic debris and with excessive inflow of coarse sediment due to inexperienced operation. Relatively minor changes were made in operational procedures and equipment in order to remedy these problems. Key words: river, intake, sediment, diversion, weir, bedload, debris, torrent.