Flood defences and the Public Goods debate

It's a weather blog. Rain and flooding can be a cue into the public goods debate. Why is it likely that most spending on flood defences will have to come from the public purse? The picture I've used above is described as what London would look like without the Thames Barrier. Here are a few more links:

What are public goods, and what makes their existence a likely cause of market failure?

Some stimulus pictures of giant waves and floods.

The key issue at stake: should the government be investing more into flood defences?

There's a public goods revision quiz here too.

Martin Wolf's superb article from 2012 on public goods and the economy

Public_Goods_Martin_Wolf.pdf