Project Description

Previous borehole investigations showed clayey silt overburden, weak red brown volcanic tuff, weathered brown andesitic rock and sound grey andesite with loose alluvial deposits of sand and gravel in the shallow river channel. No major faults were encountered. Earthquakes and seismic activity are anticipated.

On September 10, 1994, a major tropical storm Debbie hit the island of St. Lucia. The topography of the island resulted in high density rainfall over the centre of the island, in the headwaters of the major rivers, causing widespread flooding and damage. The partially constructed Roseau Dam was overtopped forming a breach 40 m wide by 15 m deep in the rockfill, delaying completion of the water supply dam by approximately six months. Two bulldozers, which had been parked on the dam crest, were carried approximately 0.5 km downstream during the overtopping and breaching event. Elsewhere on the island five bridges were lost and four damaged, the airport was flooded and closed temporarily, four deaths were attributed to the storm, 100 homes were destroyed, and approximately 1000 landslides occurred.

During the 1994 storm, the rainfall gauge nearest to the Roseau Dam catchment area recorded a maximum 1 hour rainfall intensity of 125 mm/h which was in excess of a 1000 year return period storm and well beyond the probable maximum precipitation for which the completed dam and spillway were designed.

The compacted rockfill dam and concrete spillway are founded on bedrock. The height is about 45 m, the crest length is 200 m, and the rockfill volume is 197,000 m . A temporary horseshoe shaped diversion tunnel with overhead rock anchor support was cut through andesitic rock on the west abutment. Compacted earthfill cofferdams were used to develop temporary upstream and downstream working areas. Earthworks involved access roads, slope stabilization, drainage ditches, downstream river channelization, rock quarry blasting and crushing, an upstream grout curtain beneath the concrete plinth, aggregate production for rockfill, drainage filters and cast-in-place concrete. The Roseau Dam reconstruction was completed in October 1995 and it is part of a larger water supply project on the island (C$56M).