A lot of planning for things like housing doesn't necessarily take notice of the affect downstream.
And even when the council etc have objections from virtually every involved party there is a lot of pressure on them to build as they're often expected by the government to build a certain number of homes.
We've got a proposed development near us that is getting the go ahead despite warnings from the EA re drainage and flooding, the water board and all the local services about the affect it would have on services
The likes of the government, the councils and the EA don't really seem to work well together with regards to planing and the requirements - it's a little like there was a thing on the news earlier about a massive shortage of housing suitable for the disabled, and yet at least one of the governments proposed major house building initiatives is using a suggested template house that would be utterly unsuitable and expensive to convert after the fact (things like multiple steps up to the front door in a terraced block), as the government suggests it would be up to the local council to make sure they met requirements (as opposed to the basic design meeting them from the word go). Basically the government's own designs and suggestions ignoring the governments own design criteria and regulation.
Which fits completely with "don't build on flood plains" and "why aren't you building houses over there *points to flood plain*"