Do you have any idea or reliable source that suggests which percentage change of extreme rainfall is considered Significant (and meaningful) for putting drainage system at risk or ,aking drainage system problems includig lack of capacity for accomodating rainfall-induced runoff?
For example, if the drainage system is conveying 20 mm rainfall (as design rainfall) without any problem, is 20.05 mm of rainfall would be considered a threat for it? What is the exact threshold for considering the change in rainfall regarding the drainage system risk?