The overarching view of Parsons Brinckerhoff, the US-headquartered engineering company charged with scoping out the range of tidal power schemes for the Severn Estuary, is that barrage-based concepts promise the "greatest degree of certainty in relation to energy yields, costs, timescales and technology". Tidal power barrages, Parsons Brinckerhoff points out, are known quantities, having been in operation in La Rance, France, and Canada's Bay of Fundy for decades, while no tidal lagoon has yet been built.