[teisman]

I share your skepticism, but there's one thing you might be overlooking.

Canals can act as a buffer in case of heavy rainfall. This requires the canals not to be connected to open water. In anticipation of heavy rainfall you would pump water out of the canals, into open water (sea). The canals can then act as a temporary buffer for excess rainwater. True, this wouldn't help against dike-breaks.

Related to this: in Holland there are many farmlands in the proximity of rivers that are designated as buffers like these. When necessary, they let these farmlands run full of water in a controlled way to prevent downriver cities from flooding.

[vonmoltke]

> This requires the canals not to be connected to open water. In anticipation of heavy rainfall you would pump water out of the canals, into open water (sea).

Not necessarily. The South Florida Water Management District canals ultimately drain directly into either the ocean or the Everglades. The canal network has a series of flood locks and pumps, though, which can be used to manipulate water levels and force water to flow faster or slower than it would naturally.

Municipal drainage along the coast in southern Florida is also connected directly to the ocean. In that case as well the drainage system is augmented by high-capacity pumps to force the water out of the system and into the ocean. The municipal green spaces act as the buffers if the pumps cannot keep up with the rainfall.