What is a low cost solution for flood control( for a river) inside a city?

there is two river inside the city of Sao Paulo,brazil. I want to know if there is a very low cost (afordable) solution which is more natural than mechanical to control the flooding inside the city.What is a low cost solution for flood control( for a river) inside a city?
a network of irrigation canals that take overflow to peoples gardens and to farms in the suburbs outside of the city, or to a huge container downhill at the end of a huge canal or a combo of all.What is a low cost solution for flood control( for a river) inside a city?
You want watergate the only real flood control product available. look at www.stormprotx.com

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Life jackets for everyone.
barricades on the river shore