Massive potential for overspend building a 10 mile barrage

We cant build the Redland Green local secondary school to budget. Bath's spa project was three times over budget and six yrs late. Wembley Stadium was massively delayed and cost nothing like original estimates. Ok, each project had their own particular problems, but even so who thinks that we can build a 10 mile long Severn Barrage on time and within its estimated £15 billion (£15,000,000,000) budget? As with many big projects it wont just be private money that's involved - there will have to be local and national government involvement.

We really should ask whether investing in a huge barrage is the most effective and efficient way to spend money becoming greener. How many millions of houses could be made ultra energy efficient within months with spending on this scale, massively cutting carbon emissions? Private and public money going into the barrage isn't available to spend elsewhere, on what could be better projects.