When heavy summer rain hits green fields, it takes the water several hours to reach the nearest ditches and watercourses. Developed land (roofs and hard landscaping) is designed to get the water away quickly, so the time taken to reach the nearest watercourse is reduced from many hours to a few minutes.
The flow needs to be held back, or attenuated, so that the downstream system can cope. We apply the same lateral thinking to this problem to arrive at low cost solutions. In Aylesbury and in Ho Chi Minh City (yes, really!) we used above-ground storage tanks to save huge sums of money and many months of construction time. Low cost agricultural tanks were fed from high level roof gutters, and the water allowed to leave the tank at a very much reduced flow rate.