As I used to jab all the envirowhackos screaming about all the impervious surfaces created by the evils of malignant development... Saturated surfaces are also "impervious surfaces!"
Not really. There is substantial pulse-dissipation capacity in leaf, duff, and meander not found in pavement, even after it's already soaked. It's the thrity minute deluge that does the real damage around here. Further down it makes less difference.