Urban flooding is rarely caused by an unusual quantity of rain.
It is caused by an ordinary quantity arriving on a surface that cannot it.
Every square metre of concrete sends water somewhere else, and the system was sized for a city that no longer exists.
Enlarging pipes is the response that appears in most budgets and is the least effective per unit of money.
A larger pipe moves the problem faster and rarely removes it.
is the alternative and it requires space rather than engineering.
Parks designed to flood, ponds kept empty in the dry season and surfaces all hold water where it falls.
Cities that adopted this approach describe it as giving the water a place to wait.
The land required is the political obstacle, since the same land has a development value.
Filling a pond produces revenue this year and a cost that appears in somebody else's term of office.
Maintenance is the quieter failure and the more common one.
A network that is not cleared within a few seasons and performs below its design in every storm afterwards.
Waste in the is treated as a behaviour problem and is usually a collection problem.
Where collection is reliable, the volume in the drains falls without any campaign.
Flood maps published at street level change behaviour faster than any of this, because they reach the person buying a house.