When rain falls on a road, a pavement or a shopping centre, it falls on tarmac or concrete. These materials do not let water seep in as would be the case for soil. So the water flows quickly over the surface.

If it was not carried away, it would become standing puddles. That would be unpleasant to walk through, and dangerous for cars to drive through (because of aquaplaning).

So drains are placed under roads. Gutters carry water to openings with grilles over them, and then large pipes carry water to rivers.

The trouble with this is that it takes water to rivers very quickly, and so can make river flooding more likely. In many river basins, floods are much more likely to occur now than they were a century ago, simply because of the drains.