On the negative side, floods disrupt normal drainage systems in cities and typically overwhelm sewer systems.
Thus, raw or partially raw sewage spills are common in flooded area.
Additionally, if the flood is severe enough, destruction of buildings that can contain a large array of toxic materials (paints, pesticides, gasoline, etc..) can cause the release of these materials into the local environment, which is not good. I'm sure you could imagine other "negatives" similar to those I've just described.
Floods disrupt many people's lives each year and personal tragedies due to flooding occur frequently.
A wise society, in my opinion, would reap the benefits of flooding and avoid many of the negatives if they would choose to build cities in ways that can accomodate flooding without trying to avoid it.
Attempting to go against nature is almost certainly a loosing prospect in the end.
Flood prevention is really something left up to cities and towns.
If where you live isn't flood prone, maybe there is some other natural hazard that does affect your area that you could learn about and help with instead.
As individuals, if we all keep the minimum amount of toxic substances (paints, solvents, automotive fluids, etc..) around our homes, there will be less of these substances to "spill" when any sort of unfortunate natural dissaster befalls the communities in which we live.