What is residual flooding?

It is not uncommon that local flooding still occurs despite that drainage improvement measures have already been implemented in its vicinity. In some low-lying areas the ground level is lower than the water level in the nearby main drainage channel. As the channel collects stormwater from catchment by gravity, it is natural to follow that runoff from these low-lying areas can hardly be discharged into the channels during rainstorm. Such phenomenon is called “residual flooding”.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.