A well-designed system of swales and ponds (aka dams) can handle enormous amounts of rain.
Geoff Lawton's Zaytuna farm has very large swales which fill with water in the rains, and allow it to gently soak in. I found looking at the map and
video of his property to be very helpful.
http://permaculturenews.org/2012/06/01/zaytuna-farm-video-tour-apr-may-2012-ten-years-of-revolutionary-design/
For smaller-scale solutions to flooding I recommend volume 2 of
Brad Lancaster's "Rainwater Harvesting for Drylands"
http://www.harvestingrainwater.com/
We're gradually implementing these structures on our 20 acres where we have two seasonal creeks which in flood provide about 39 million gallons (120
acre feet) of rainwater per hour over the bottom part of our
land. There are photos of the structures in my projects
thread.
The key is to plan for a safe overflow route for all rain harvesting structures as they will eventually overflow no matter how big. I have to direct water from uphill of our house and
workshop right through between the buildings and safely down to the creek. This area has flooded so badly that a
fence and a small tree were knocked over, so this is a challenge!