It is well known that urbanization create increased runoff due to increasing impervious areas but the impact of quick water accumulation is not that well known. For example in a catchment where it took one hour for water to accumulate downstream it can happen in 20 minutes after urbanization.
This has a major impact on flood peaks, because if we experience 50 mm intensity one in 5 years with a duration of one hour, we may experience about 84 mm intensity for the same return period but with a duration of only 20 minutes.When combined with an increase of runoff coefficient from 0.3 to 0.7 this would produce flood peaks 4 times higher than the natural state.
