Impervious Surface Problem

Urbanisation creates impervious surfaces. If 100 units of rain fall over a vegetated landscape, a significant portion may infiltrate into soil, recharge groundwater or be intercepted by vegetation. If the same rainfall falls on concrete, much more water becomes immediate runoff.

Thus, cities often produce higher peak discharge within a much shorter time. This is why conventional drainage systems designed decades earlier may be overwhelmed even when their pipes are technically functional.