Manasan Falls control structure design and construction aspects

Abstract

This paper describes the design, construction, and performance of the Manasan Falls control structure. It is located at Manasan Falls on the Burntwood River approximately 6 km (4 mi) upstream of Thompson in northern Manitoba. At this location the river is 210 m (700 ft) in width. The control structure consists of two rock- and earthfill groins creating a trapezoidal opening in the proximity of a bedrock outcrop at the falls. Its purpose is to increase the upstream water levels sufficiently to promote formation of a stable ice cover in the winter.Construction was carried out in the fall and winter of 1976, in the wet, on an overburden foundation with river flows averaging 170 m3/s (6000 cfs). Upstream of the bedrock lip of the falls, the channel floor consists of highly erodible varved silty clay. This area was protected with rockfill pavements. Two groins of about 9 m (30 ft) maximum height were constructed using random rockfill with upstream granular filter zones and seals. The ends of the groins were protected by 0.9–1.2 m (3–4 ft) nominal diameter armour rockfill.Performance to date has revealed that the groins and gap opening have provided the required stage–discharge relationships and have promoted a stable upstream ice cover in the winter. The fill materials have remained stable under flows of up to 850 m3/s (30 000 cfs) and average gap velocities exceeding 6 m/s (20 fps).