An area of over 100 km2 of industrial and residential property was at increasing risk from flooding, including some 700,000 residents and the central section of the underground railway. Attention was drawn to this risk in a 1966 report by Sir Herman Bondi, and the GLC was empowered by the Thames Barrier Act of 1972 to construct a barrier at Woolwich Reach, 13 km downstream from London Bridge. The alternatives of a barrier and a barrage were examined, and the final design employs rising sector gates, providing four main navigation opening each of 61 m, and a further six of 31 m. Rotation of the gates is by radius arms, driven from hydraulic cylinders, with three 140 kW pump power packs on each of the principal piers. The site is provided with three independent sources of electric power, connected to the piers via duplicated access tunnels. For the construction there were three main and 20 other contractors, and site work began in 1975, erection of the gates in 1980, and the barrier was first completely closed in 1982. The projects passed through three stages: feasibility, detailed design and contract letting, and construction and commissioning. The project was controlled through monthly site meetings. Total cost escalated from £110 × 106 at 1973 prices to £440 × 106 on completion in 1984, which has to be set against the estimate of £5000 × 106 as the cost of a major flood disaster.